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Meiosis is a specialized type of cell division that reduces the chromosome number by half. This process occurs in all sexually reproducing single-celled and multicellular eukaryotes, including animals, plants, and fungi Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent.
So it means that a haploid plant body will give rise to either a male sex organ or female sex organ that produces gametes to form zygote thus completing the alternation of generation. We also know that chara a monoecious plant can be dioecious, which means that haploid plant body is producing both antheridia and archegonia which produce gametes by mitosis. Coming to my question, "HOW CAN A PRODUCT OF MEIOSIS BEFORE FERTILISATION BE BISEXUAL?" Does this implies that reduction division is not responsible of sex differentiation? If yes then what is responsible for sex differentiation in plants like chara where a haploid produces both gametes ? Explanation will be appreciated
Meiosis does not determine sexual form. Eukaryotes use meiosis and fertilization to recombine genes to form new combinations. Meiosis does produce haploid cells from diploid cells, but that has nothing much to do with the sexual forms involved.
In the case of the algal genus Chara, the organism's life cycle is entirely haploid except for the single-celled zygote formed during fertilization; this is called a haplontic life cycle. Being haploid does not require an organism to be of one sex (or any sex), so there is no difficulty to being monoecious and having both male and female structures on a single organism. The reason for having two kinds of gametes, sperm and ova, is simply practical: the specialized job of sperm is to move to another gamete, adn the job of the ova is to provide a maximum amount of nutrition for the future zygote (which means larger size and hence limited mobility).
Puzzling out plant reproduction by haploid induction for innovations in plant breeding
Mixing maternal and paternal genomes in embryos is not only responsible for the evolutionary success of sexual reproduction, but is also a cornerstone of plant breeding. However, once an interesting gene combination is obtained, further genetic mixing is problematic. To rapidly fix genetic information, doubled haploid plants can be produced: haploid embryos having solely the genetic information from one parent are allowed to develop, and chromosome doubling generates fully homozygous plants. A powerful path to the production of doubled haploids is based on haploid inducer lines. A simple cross between a haploid inducer line and the line with gene combinations to be fixed will trigger haploid embryo development. However, the exact mechanism behind in planta haploid induction remains an enduring mystery. The recent discoveries of molecular actors triggering haploid induction in the maize crop and the model Arabidopsis thaliana pinpoint an essential role of processes related to gamete development, gamete interactions and genome stability. These findings enabled translation of haploid induction capacity to other crops as well as the use of haploid inducer lines to deliver genome editing machinery into various crop varieties. These recent advances not only hold promise for the next generations of plant breeding strategies, but they also provide a deeper insight into the fundamental bases of sexual reproduction in plants.
Methods of Haploid Production
There are two way for the production of haploid plants. They are: (1) In Vivo and (2) In Vitro. The process of apomixis or parthenogenesis is responsible for producing spontaneous natural haploids. Many techniques are followed both by in vivo and in vitro methods for haploid production. In vitro techniques are more applicable for much haploid production.
In Vivo Techniques to produce Haploids :
1. Androgenesis: In the process of androgenesis, haploid plants are mainly derived from male gamete. Development of a haploid plant from an egg cell containing male nucleus is referred to as androgenesis. Before fertilization this egg nucleus has to be inactivated or eliminated for successful haploid production in vivo techniques.
2. Gynogenesis: A haploid plant can be produced from an unfertilized egg cell as a result of delayed pollination. e.g. cross between Solanum tuberosum and Solanum phureja produced diploid potatoes.
3. Distant hybridization: Haploid hybrids can be produced by elimination. In this process, one of the parental genomes have to be eliminated to produce haploid hybrids. Distant hybridization may be continued by inter-specific or inter-generic crosses. Gersenson (1928) obtained 7 haploids out of 35 offspring by the crossing between Solanum nigrum and Solanum luteum.
4. Parthenogenesis : In plants, parthenogenesis is a component process of apomixis. Haploids may be developed from an unfertilized egg cell. In this case, no fertilization is needed.
5. Apogamy: In apogamy, haploids may be developed from other cells of the mega-gametophyte.
6. Semigamy: The nucleus of the egg-cell and the generative nucleus of the germinated pollen grains divide independently, resulting in a haploid chimera.
7. Shocks with high or low temperatures : Haploid plants may also be produced by applying high or low temperature. Blakesle et al (1927) got haploid plant by the application of high and low temperature (40 – 45°C).
8. Irradiation with X-rays or UV light: For producing haploids, x-rays or ultra violate rays may be used to induce chromosomal breakage and their subsequent elimination. Arenberger (1948) got haploid Antirrhinum majus and non-irradiated Antirrhinum majus.
9. Chromosome elimination : Chromosome elimination occurs in some intergeneric and interspecific crosses to produce haploids. e.g. cross between Hordeum vulgare and H. bulbosum. The process of chromosome elimination is briefly described below.
Production of Haploids by Chromosome Elimination
There are numerous examples of haploid production primarily achieved by wide crosses and embryo culture by chromosome elimination technique. e. g. barley, wheat etc.
Monoploid Barley Production by Chromosome Elimination
The barley example: The latest most important process in breeding methods of barley has been the recovery of high frequencies of doubled haploids (DH) i.e. homozygous lines in a single generation. Bulbosum method is more efficient than microspore culture in producing haploid barley. This was to be first reported in the early 1970s. In this method, haploid plantlets of Hordeum vulgare with seven chromosomes are produced as a result of gradual elimination of H. bulbosum chromosomes. This is achieved by an interspecific cross between barley (Hordeum vulgare 2n = 2x = 14, female) x Hordeum bulbosum (wild relative 2n = 2x = 14, male). The method was started with the fertilization of Hordeum vulgare by Hordeum bulbosum followed by complete elimination of the H. bulbosum genome.
In Vitro Techniques to Produce Haploids
1. Anther culture: Most research has been carried out on isolated anthers which has been isolated on to solid or on liquid nutrient media
2. Pollen grain culture: This is less used technique due to technical problems
3. Inflorescences: Useful with grasses and other plant species which have small flowers, e.g. Hordeum
4. Embryo culture: Hordeum vulgare is crossed with H. bulbosum. Soon after fertilization, the chromosome of H. bulbosum are eliminated. Haploid embryo of H. vulgare is produced with no endosperm and without embryo culture this embryo would abort.Another example is crossing between Triticum aestivum and H. bulbosum which resulted in the production of a haploid Triticum aestivum embryo.
5. Pseudo-fertilization: Hess and Wagner (1974) pollinated Mimulus luteus in vitro with alien pollen of Torenia fournieri by prickle pollination a haploid cell of Mimulus developed into a haploid plant.
6. Development of unfertilized ovules without pseudo-pollination: The main advantage of ovule culture appears to be that the majority of regenerants are green and are more genetically stable, e.g. rice, wheat, maize, Hordeum vulgare, etc.
7. Gametophytic irradiation and ovary culture: In Petunia, the in vitro ovary culture of plants pollinated with irradiated pollen is so far the most efficient techniques for haploid production. The irradiated pollen acts not only in stimulating gynogenesis but also takes part in fertilization.
Haploidy: Classification and Origin
According to their cytological features, haploids have been classified into different types (Fig. 17.1), a brief description of which is given below.
The chromosome number of such a haploid is an exact multiple of one of the basic numbers of the group. Euhaploids are of two types:
(i) Mono-haploid or monoploid:
It arises from a diploid species (2x) and possesses the basic chromosome number of the species it is designated as “x”.
Haploid individuals arising from a polyploid species are called polyhaploids they may be di-haploids, tri-haploids and so on. Based on their auto- or allo-ploid condition, polyhaploids have been divided into two groups:
(a) Auto-polyhaploid : Autopolyploid species give rise to auto-polyhaploids. For example,
Autotetraploid (Ax = AAAA)———-> Auto-di-haploid (2n = AA)
Auto-hexaploid (6x = AAAAAA)———–> Auto-ri-haploid (3x = AAA)
(b) Allopolyhaploid : Such individuals arise from allopolyploid species. For example,
Allohexaploid (6x = AABBDD)———–> Allotrihaploid (3x = ABD)
The chromosome number of such a haploid is not an exact multiple of the basic number of the group.
Aneuhaploids have been classified into five types:
(i) Disomic haploids (n + 1):
Such haploids have one chromosome in disomic condition, i.e., there is one extra chromosome belonging to the genome involved.
(ii) Addition haploids (n + few alien):
The extra chromosome(s) in the haploid is an alien chromosome.
(iii) Nullisomic haploids (n-1):
In such haploids, one chromosomes is missing from the haploid complement.
(iv) Substitution haploids (n-1 + 1 alien):
One or more chromosomes of the haploid complement are substituted by alien chromosomes.
(v) Misdivision haploids:
Such haploids contain telocentric or iso-chromosomes resulting from misdivision of centromere(s).
Origin of Haploids:
Haploids arise spontaneously, and can be experimentally induced. In nature, they arise through the development of egg cells without fertilization (parthenogenesis) or through pseudogamy. There are many animals where haploidy is common and is involved in sex determination.
For example, in insects of the order Hymenoptera (honey bees, wasps etc.), unfertilized eggs (haploid) develop into males, while fertilized eggs (diploids) develop into females. Spontaneous and induced haploidy has been reported in several animals such as Drosophila frog, mouse, chicken and newt.
Several plant species, such as, flax, cotton, rape, coconut, tomato, pearl millet, wheat, barley, are known to produce haploids spontaneously. In einkorn wheat (Triticum monococcum) Smith reported the occurrence of haploid plants at a rate of 1/1000.
The rate increased up to 20/1000 in interspecific pollination and up to 200/1000 when combined with delayed pollination. Some genotypes show very high frequency of haploid production.
The first report of induced haploidy in plant was that by Blakeslee et al. in 1922, through cold treatment of young buds of Datura stramonium. Later, haploids were induced in several other plant species through different techniques.
In general, there are three main approaches for the production of haploids:
(1) Parthenogenesis and apogamy,
(2) Somatic reduction and chromosome elimination, and (
3) Anther, pollen and ovule culture.
Parthenogenesis and Apogamy:
The term parthenogenesis is used for the development of embryo from ovum/egg cell without fertilization, whereas the term apogamy is used for the development of an embryo from a vegetative cell of the embryo sac without fertilization.
The haploids arising from the maternal cells in the embryo sac are called “gynogenetic haploids” whereas, those arising from the male (sperm) nucleus in the embryo sac are called “androgenetic haploids”.
The various methods used to produce parthenogenetic haploids are as follows:
(viii) Haploid initiator genes
(x) Selection of twin seedlings.
(i) Temperature treatments:
Parthenogenetic haploids may be produced by high or low temperature treatments. In Datura stamonium, Blakeslee and coworkers obtained haploids using cold treatment of young buds. Randolph obtained haploids in corn (Zea mays) through heat treatment just after pollination. Pollination with heat-treated pollen has also been employed for producing haploids.
Haploids may be produced through irradiation of the embryo sac or of the pollen. It is presumed that the irradiated pollen grains loose their ability to fertilize but they stimulate the egg to develop into embryo parthenogenetically.
(iii) Chemical treatments:
Certain chemical agents have been used for induction of haploidy in plants. The chemical toluidine blue, which inactivates the sperm nuclei, could induce haploidy in some plant species such as, Vinca tomato, maize and Populus. Nitrous oxide was found to stimulate embryogenesis in the cells of embryo sac resulting in haploid production in Capsicum annum.
(iv) Delayed pollination:
Haploids may be produced using delayed pollination. In Triticum monococcum, high frequency (200 per 1000 plants) of haploids were obtained through this method by Smith in 1946.
(v) Wide hybridization:
Interspecific and inter-generic hybridizations may lead to the production of haploids in various plant species. In potato, di-haploids are produced by crossing tetraploid Solatium tuberosum (♀) with diploid Solatium phureja (♂) in this cross the male gamete forms a restitution nucleus which fertilizes the fused polar nuclei, while the egg cell is stimulated to develop pertheno-genetically.
Solatium phureja (2x) has also been used as pollinator for haploid production in other tuber bearing Solanum species. The rate of haploid production is influenced by the genotypes of both the female and male parents.
(vi) Alien cytoplasm:
This method of haploid production was reported by Kihara and Tsunewaki in wheat. Haploid are produced when the nuclei of strain Salmon of bread wheat (T. aestivum) are placed into the cytoplasm of certain species of Aegilops, e.g., Ae. caudata. Along with haploids, diploid seedlings were also observed (n/2n twins). The 2n seedlings were produced from fertilization of the synergid cells by the male gamete, while the egg cell was stimulated to develop parthenogenetically into haploid embryo.
(vii) Inducing genes:
Certain genes are known to induce haploidy in plants. The gene ‘indeterminate gametiphyte’ (ig) induces hapioids of both female (gynogenetic) and male (androgenetic) origin in maize (Zea mays). There is an irregular development of the mega gametophyte in plants homozygous for this gene (ig ig)’, their embryo sacs contain 1-5 egg cells and 1-7 polar nuclei. This gene leads to the production of 3 per cent haploids among which the gynogenetic and androgenetic haploids occur in the ratio of about 2:1. Combination of the ig gene with certain marker genes, such as, r g (colourless scutellum and green plants) and R” j (purple pigmented kernel crown, scutellum, plumule and seedlings) enables the easy identification of androgenetic and gynogenetic haploids.
For example, in the cross ig ig r g r g ♀ x Ig Ig R nj R nj ♂, the haploids showing colourless scutellum and green seedlings will be gynogenetic, whereas in the cross ig ig R nj R nj ♀ x Ig Ig r 8 r g ♂, the haploids with colourless scutellum and green seedlings will be androgenetic.
(viii) Haploid initiator genes:
A gene for haploid induction was reported by Hagberg in barley this gene is called “haploid initiator gene” (hap). When plants homozygous for the hap gene are used in crosses as females or are selfed, they produce 15-40 per cent haploid progeny. However, when the hap plants are used as male parent, no haploid progeny are produced.
The heterozygous F1 plants (Hap hap) obtained from the cross hap hap ♀ X Hap Hap ♀ produce 3-6 per cent haploid progeny. In tiie F2 generation, the genotypes of diploid plants are Hap Hap, Hap hap and hap hap in the ratio 1 : 2 : 1 plants with Hap Hap genotype produce no haploid progeny, those having Hap hap genotype produce 3-6 per cent haploids, while hap hap plants produce 15-40 per cent haploids (Fig. 17.2). In contrast, 50% of the haploid F2 plants will possess hap allele, while the rest 50% will have the Hap allele the haploids possessing Hap allele are of value in breeding programmes.
In semi gamy, sperm nucleus enters the egg cell but nuclear fusion does not occur. Thus semi gamy falls between the two extremes, the syngamy (fusion of male and female gametes) and pseudogamy (development of embryo after pollination without involvement of male gamete). In semi gamy, the egg and sperm nuclei may begin to divide synchronously and the resulting embryo may produce plants that are chimeric in that they have distinct sectors containing either the sperm or the egg nucleus.
Occasionally, haploids of either maternal or paternal genotype may be produced. Although, semi-gametic production of haploids has been reported in several plant genera, it is mainly of interest in cotton (Gossypium) where extensive studies have been made. Semi gamy is genetically controlled, and in cotton it is caused by a dominant gene. Haploids have been produced in Gossypium hirsutum, G. barbadense, G. tomentosum and G. klatzschianum through the process of semi gamy.
(x) Selection of twin seedling:
Twin seedlings arise due to polyembrayony and may be x -x, 2x-x or 2x-2x twins. Twin seedlings are produced when a synergid cell is stimulated to divide in addition to the egg. Thus one embryo develops from the egg and the other from the synergid resulting in the production of twin embryos.
Lacadena in 1974 reported the occurrence of twin seedlings in 42 plant species. The occurrence of twin seedlings is also genetically influenced. In Capsicum frutescens, the frequency of twin seedlings is controlled by the genotype of the female parent. Frequencies of haploid-diploid (x-2x) twins vary in different species in Lilium, this frequency is 1%, while in pepper, it is as high as 30%.
Somatic Reduction and Chromosome Elimination:
Somatic reduction is the phenomenon which results in the reduction of somatic chromosome complements and involves the segregation of whole genome. Its mechanism may involve the abnormalities of spindle such as multipolar spindle formations. It was first described in insects and later it was found to occur spontaneously in plants and plant tissues.
Somatic reduction may give rise to haploid cells during the para-sexual cycle of certain fungi. It can be induced by certain chemicals in different plants e.g., by chloramphenicol in barley root tips, by parafluorophenylalanine (pFPA) in grape seedlings, fungi and yeast.
(b) Chromosome elimination:
In certain plants, interspecific or inter-generic hybridization leads to the gradual elimination of chromosomes of one of the parental genomes resulting in the production of haploids. The mechanism of elimination may involve failure of chromosome congression at metaphase leading to chromosome lagging at anaphase and formation of micronuclei. This process may also lead to the production of androgenetic haploids.
In 1970, Kasha and Kao obtained haploid barley plants when they crossed Hordeum vulgare with H. bulbosum, since there was a gradual elimination of the chromosomes of H. bulbosum from the young hybrid embryos few days after fertilization (Fig. 17.3). The chromosomes of H. bulbosum are eliminated irrespective of its use as female or male parent about 95% of the progeny are haploid.
However, in some of the progeny plants, all the bulbosum chromosomes are not eliminated and aneuploidy occurs. Subrahmanyam and Kasha in 1973 observed that there was gradual elimination of H. bulbosum chromosomes from the developing embryo. Subsequently, Subrahmanyam reported haploid production through chromosome elimination in various interspecific crosses in Hordeum.
Different species differ in the relative strength of chromosome elimination. Chromosome elimination is controlled by genetic factors present in both the species involved in a cross. The factors influencing the elimination of H. bulbosum chromosomes are located on chromosomes 2 and 3 of H. vulgare, the extent of elimination being also influenced by the genotype of H. bulbosum.
Inter-generic crosses involving Hordeum bulbosum can also produce haploids through selective chromosomes elimination. By crossing with H. bulbosum, haploids have been produced in Tridcum aestivum and Aegilops. In 1975, Barclay obtained haploids of T. aestivum var. Chinese Spring by crossing wheat with either 2x or 4x H. bulbosum.
Anther, Pollen and Ovule Culture:
Haploids have been produced by using anther, pollen and ovule cultures.
(a) Anther and pollen culture:
Haploids of several plant species are produced in very high frequencies by culturing anther or microspores on suitable culture media. In 1964, Guha and Maheshwari for the first time reported the production of Datura innoxia haploids through anther culture.
They noted the appearance of numerous embryoids inside the cultured anthers subsequently these embryoids developed into haploid plantlets. Haploid production through anther culture has been successfully extended to a large number of plant genera and species, e.g., barley, rice, Brassica and tobacco. The pollen present within anthers may either directly develop into embryoids or it may form a callus from which plantlets may differentiate.
Haploids can also be produced through the culture of isolated pollen (microspore), e.g., in Datura innoxia, Nicotiana tabacum, Petunia, Oryza sativa, barley, tomato, maize etc. The factors affecting haploid production through anther culture are: physiology of the donor parent the stage of development, temperature and culture medium etc.
Anthers of most plant species are cultured at the bi-nucleate stage of the microspore, but in some cases, the uninucleate stage is the most appropriate, while in some other species the best results are obtained at the tri-nucleate stage. In nature, mitotic division of the microspore nucleus produces two nuclei: one relatively larger vegetative nucleus and other smaller generative nucleus. The generative nucleus divides once more to yield two sperm nuclei.
When anthers/microspores are cultured, the development of pollen embryos or calli may follow one of the following four pathways:
(i) In this pathway, there is an equal division of the microspore which yields two similar cells that are not differentiated into vegetative and generative cells. Both the cells divide and contribute to embryo/callus development, e.g., in Datura innoxia.
(ii) Unequal division of the microspore leads to the formation of distinct vegetative and generative cells. The callus/embryo arises from the repeated division of the vegetative cell, while the generative cell usually degenerates, for example, in Hordeum vulgare and Nicotiana tabacum.
(iii) In this pathway also vegetative and generative cells are formed by unequal division of the microspore. However, the embryo/callus develops from the generative cell only, e.g., in Hyoscyamus niger.
(iv) Alternatively, both the vegetative and generative cells, produced by an unequal division of microspore contribute to the formation of callus/embryo.
(b) Ovule culture:
Haploids have been obtained by culturing unfertilized ovaries of a number of plants, such as, barley, wheat, tobacco and rice. In rice, the pro-embryos develop from the egg cell while in barley, they originate from the egg cell as well as other cells of the embryo sac.
Meiosis in Haploids:
Monoploid or Mono-haploid (x):
In a monoploid, all the chromosomes present in a nucleus are non-homologous. Therefore, only univalents are expected at MI. But chromosome pairing has been observed in monoploids of some plants, e.g., barley, maize, rice and tomato. Synaptinemal complexes have also been observed at pachytene.
Non-homologous chromosome pairing (of small chromosomal segments) has been assigned to small duplication and genetic redundancy. In barley monoploids (2n = x = 7), Sadasivaiah and Kasha in 1971 reported the occurrence of rod bivalents with a frequency of 0.03 to 0.05 per PMC. In 1983, Kasha and Seguin-Swartz made the following three main observations regarding chromosome pairing and chiasma formation in barley monoploids.
(i) Chiasma formation is mainly dependent on duplication of chromosomes or segments of chromosome rather than on the large content of highly repetitive DNA sequences (over 70%) in the barley genome.
(ii) Chiasma frequency is highly influenced by the duplicated regions and their location on the chromosomes.
(iii) The very low frequency of association, e.g., 0.03-0.05 bivalent per PMC, could be due to random breakage and crossing over.
In some species, all the chromosomes present at MI and AI may be included in a single restitution nucleus following the telophase I this nucleus undergoes a normal second meiotic division to produce two normal haploid spores. Therefore, after pollination by a diploid plant the monoploid may produce diploid progeny.
Chromosome pairing and crossing over in the duplicated region of non-homologous chromosomes of the monoploid will lead to the diploid progeny carrying a heterozygous translocation. Using this system, several interchanges between the chromosomes 6 and 7 and between 2 and 6 in Zea mays have been obtained it has been concluded that the repeated interchanges were produced due to crossing overs in the duplicated segments.
In mono-haploids, the distribution of chromosomes at AI may be random. In some cases, all the chromosomes divide at AI forming a dyad each cell of which possesses n chromosomes as was observed in Datura mono-haploids (2n = x = 12) by Belling and Blakeslee in 1927.
In the species where haploidy is a regular feature, e.g., in Hymenoptera, meiosis in the haploids differs from one species to the other. The first meiotic division is completely absent and the second division (meiotic mitosis) leads to the formation of haploid sperm cells.
In male honey bee, all the chromosomes are included in one cell and cytoplasmic bud (polar body first) is pinched off. In the second division, chromosomes divide mitotically and two daughter cells of unequal size are produced. The sperm is formed from the larger cells.
In auto-polyhaploids, two or more homologous chromosomes are present for the entire genome. Therefore, Homologous chromosome pairing is a common feature. In auto-di-haploids (2n = 2x = 24) of potato and those (2n = 2x = 16) of alfalfa, mostly bivalents with very few univalents are observed. However, fertility and vigour of these auto-polyhaploids are often lower than those of their polyploid counterparts, possibly due to inbreeding.
In case of allopolyploids, there should be no chromosome pairing since the chromosomes are not homologous. But a low frequency of bivalent formation occurs due to homoeologous pairing. In wheat (T. aestivum), 21-chromosome allotrihaploids (ABD) In = 3x = 21) showed 0.02 III and 1.69 II per cell.
The frequency of pairing increased in the nullisomic 5B-allotrihaploids (having 20 chromosomes) with the average frequency of 2.0 to 3.5 II percent this is expected since the gene ph present in 5B is known to suppress homoeologous chromosomes pairing. In polyhaploids from auto-allopolyploids, homologous chromosomes undergo pairing, while the homoeologous chromosomes generally remain unpaired e.g., in Solatium demissum polyhaploids (2n = 3x = 36).
Secondary association of univalents is also observed in haploids the association may be side-to side, end-to-end or end-to-side. The reasons for secondary association may be duplication, homoeology and the presence of highly repetitive DNA sequences.
Phenotypic Effect of Haploidy:
Mono-haploid plants are weaker, shorter and less vigorous than the respective diploids. They are highly sterile. Their leaves, flowers and other parts are smaller as compared to those of diploids. Stomatal size is smaller but the number of stomata per unit area is higher than that in the diploids. However, in certain plant species, such as, pepper, haploids may be comparable in size and vigour to diploids.
Possible Uses of Haploids:
Haploids may be utilized for various investigations of both fundamental and applied importance as briefly described below:
(1) Haploids are used to study the chromosome behaviour during meiosis. Study of chromosome pairing in mono-haploids indicates the presence of duplications in the chromosomes.
(2) Study of chromosome pairing in haploids indicates the origin of different species of a plant. For example, in Brassica, chromosome pairing in haploids indicated that the basic chromosome number in the genus is x = 6, and different species originated through dysploidy.
(3) Information on the ancestry of species can be obtained through the study of homoeologous chromosome pairing in the haploids of different allopolyploid species.
(4) One of the most important uses of mono-haploids and polyhaploids is the production of homozygous lines in the shortest possible time. This is achieved by extracting haploids from heterozygous plants, followed by chromosomes doubling of such haploids the resulting plants/lines are called doubled haploids or homodiploids. Chromosome doubling may occur naturally or may be induced using colchicine or some other suitable treatment. Doubled haploids may be used directly as cultivars. Cultivars derived from haploid systems have been produced in various crops such as wheat, rice, rapeseed, barley and tobacco.
(5) In cross-pollinated species, haploidy is an effective method for selecting viable combinations of genes which are then used as inbreeds after chromosome doubling.
(6) There is no segregation of genes in the homodiploids and therefore, it permits selection for quantitative characters thus selection efficiency increases.
(7) In cases of self-incompatibility, inbred lines are readily produced by doubling the chromosome number of haploids.
(8) Haploid tissues can be maintained in vitro in undifferentiated condition and they provide a source of suspension of haploid cells. Like micro-organisms, these haploid cells of higher plants can also be used to carry out new genetic researches such as mutational studies at physiological levels and biochemical analyses.
Monoploids can be used efficiently in mutational studies because they possess only a single set of genes. Therefore, both the dominant and recessive mutations are expressed in the M1 generation itself. Desirable mutants can be selected from among haploid cells cultured in vitro or from haploid plants and fertile homodiploids with all desirable mutations can be obtained through chromosome doubling.
(9) New genotypes can be incorporated into alien cytoplasm through androgenetic haploidy (androgenetic haploids may be produced by semi gamy and disruption of egg nucleus by irradiation). This method enables the transfer of a new genotype into the cytoplasm possessing factors for male sterility.
(10) Transfer of genes from wild diploid species to cultivated species can be done through di-haploids of polyploid species.
(11) Haploidy can be used in specific breeding schemes for dioecious plants, such as, Asparagus officinalis. Asparagus has the XX-XY system of sex determination, the male (XY) being more valuable commercially as they produce spears with a lower fibre content. An inbred population produced through sib-mating between pistillate (XX) and staminate (XY) plants consists of 50% males and 50% female plants. Androgenetic haploids, derived especially through anther culture, are used to produce homozygous female (XX) and super-male (YY) lines crossing of such female and super-male plants yields an all male population, which is commercially superior to the conventional % male + 50% female” populations.
(12) Monoploids obtained from di-haploids through parthenogenesis of androgenesis can be used to select and evaluate various genomes to be put together through protoplast fusion in asexually propagated crop like potato.
(13) Di-haploids may be used in selection or crossing at diploid level before chromosome doubling (autotetraploid) as suggested by Chase in 1963 in the “analytical breeding” scheme for potato (Solatium tuberosum).
(14) Through chromosome doubling in haploids, homozygous lines can be produced for various climatic regions in one laboratory.
(15) Haploids may be used to produce translocation stocks and aneuploid stocks which are of cytogenetic importance and can be used in improvement of crop plants.
The life cycle of plants proceeds via alternating generations of sporophytes and gametophytes. The dominant and most obvious life form of higher plants is the free-living sporophyte. The sporophyte is the product of fertilization of male and female gametes and contains a set of chromosomes from each parent its genomic constitution is 2n. Chromosome reduction at meiosis means cells of the gametophytes carry half the sporophytic complement of chromosomes (n). Plant haploid research began with the discovery that sporophytes can be produced in higher plants carrying the gametic chromosome number (n instead of 2n) and that their chromosome number can subsequently be doubled up by colchicine treatment. Recent technological innovations, greater understanding of underlying control mechanisms and an expansion of end-user applications has brought about a resurgence of interest in haploids in higher plants.
Life Cycles of Sexually Reproducing Organisms
Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant, in which the multicellular diploid stage is the most obvious life stage, such as with most animals including humans haploid-dominant, in which the multicellular haploid stage is the most obvious life stage, such as with all fungi and some algae and alternation of generations, in which the two stages are apparent to different degrees depending on the group, as with plants and some algae.
Diploid-Dominant Life Cycle
Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells, are produced within the gonads, such as the testes and ovaries. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 1).
Figure 1. In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote.
Haploid-Dominant Life Cycle
Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals, designated the (+) and (−) mating types, join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although haploid like the “parents,” these spores contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are conducive, the spores form multicellular haploid structures by many rounds of mitosis (Figure 2).
Figure 2. Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles. The haploid multicellular stage produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis. (credit “zygomycota” micrograph: modification of work by “Fanaberka”/Wikimedia Commons)
If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?
Alternation of Generations
The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes, because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes in this case, because the organism that produces the gametes is already a haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into the gametophytes (Figure 3).
Figure 3. Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes because they produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte. (credit “fern”: modification of work by Cory Zanker credit “sporangia”: modification of work by “Obsidian Soul”/Wikimedia Commons credit “gametophyte and sporophyte”: modification of work by “Vlmastra”/Wikimedia Commons)
Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly. In plants such as moss, the gametophyte organism is the free-living plant, and the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living however, the sporophyte is much larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte.
Sexual reproduction takes many forms in multicellular organisms. However, at some point in each type of life cycle, meiosis produces haploid cells that will fuse with the haploid cell of another organism. The mechanisms of variation—crossover, random assortment of homologous chromosomes, and random fertilization—are present in all versions of sexual reproduction. The fact that nearly every multicellular organism on Earth employs sexual reproduction is strong evidence for the benefits of producing offspring with unique gene combinations, though there are other possible benefits as well.
Haploid organisms are organisms that only contain haploid cells. Although most living things are made almost entirely of diploid cells, some multicellular organisms (like male insects of the order Hymenoptera) are haploid organisms. This is because they develop from unfertilized eggs, and therefore, only contain genetic information from a single parent (the mother). Despite this, the sole purpose of male ants, wasps, and bees is to reproduce. Male insects of this order produce haploid sperm using a type of aborted meiosis called spermatogenesis, which they use to fertilize the eggs of the queen ant.
57 Sexual Reproduction
By the end of this section, you will be able to do the following:
- Explain that meiosis and sexual reproduction are highly evolved traits
- Identify variation among offspring as a potential evolutionary advantage of sexual reproduction
- Describe the three different life-cycle types among sexually reproducing multicellular organisms.
Sexual reproduction was likely an early evolutionary innovation after the appearance of eukaryotic cells. It appears to have been very successful because most eukaryotes are able to reproduce sexually and, in many animals, it is the only mode of reproduction. And yet, scientists also recognize some real disadvantages to sexual reproduction. On the surface, creating offspring that are genetic clones of the parent appears to be a better system. If the parent organism is successfully occupying a habitat, offspring with the same traits should be similarly successful. There is also the obvious benefit to an organism that can produce offspring whenever circumstances are favorable by asexual budding, fragmentation, or by producing eggs asexually. These methods of reproduction do not require another organism of the opposite sex. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations, every individual is capable of reproduction. In sexual populations, the males are not producing the offspring themselves, so hypothetically an asexual population could grow twice as fast.
However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why are meiosis and sexual reproductive strategies so common? These are important (and as yet unanswered) questions in biology, even though they have been the focus of much research beginning in the latter half of the 20th century. There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population. Thus, on average, a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing population. The only source of variation in asexual organisms is mutation. Mutations that take place during the formation of germ cell lines are also the ultimate source of variation in sexually reproducing organisms. However, in contrast to mutation during asexual reproduction, the mutations during sexual reproduction can be continually reshuffled from one generation to the next when different parents combine their unique genomes and the genes are mixed into different combinations by crossovers during prophase I and random assortment at metaphase I.
The Red Queen Hypothesis Genetic variation is the outcome of sexual reproduction, but why are ongoing variations necessary, even under seemingly stable environmental conditions? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973. 1 The concept was named in reference to the Red Queen’s race in Lewis Carroll’s book, Through the Looking-Glass.
All species coevolve (evolve together) with other organisms. For example, predators evolve with their prey, and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species a reproductive edge over close competitors, predators, parasites, or even prey. However, survival of any given genotype or phenotype in a population is dependent on the reproductive fitness of other genotypes or phenotypes within a given species. The only method that will allow a coevolving species to maintain its own share of the resources is to also continually improve its fitness (the capacity of the members to produce more reproductively viable offspring relative to others within a species). As one species gains an advantage, this increases selection on the other species they must also develop an advantage or they will be outcompeted. No single species progresses too far ahead because genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.” This is an apt description of coevolution between competing species.
Life Cycles of Sexually Reproducing Organisms
Fertilization and meiosis alternate in sexual life cycles . What happens between these two events depends on the organism’s “reproductive strategy.” The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. Some organisms have a multicellular diploid stage that is most obvious and only produce haploid reproductive cells. Animals, including humans, have this type of life cycle. Other organisms, such as fungi, have a multicellular haploid stage that is most obvious. Plants and some algae have alternation of generations, in which they have multicellular diploid and haploid life stages that are apparent to different degrees depending on the group.
Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells , are produced within the gonads (such as the testes and ovaries). Germ cells are capable of mitosis to perpetuate the germ cell line and meiosis to produce haploid gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state ((Figure)).
Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals—designated the (+) and (−) mating types—join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although these spores are haploid like the “parents,” they contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are favorable, the spores form multicellular haploid structures through many rounds of mitosis ((Figure)).
If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?
The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes , because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes in this case, because the organism that produces the gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte . Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into the gametophytes ((Figure)).
Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly. In plants such as moss, the gametophyte organism is the free-living plant and the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living however, the sporophyte is much larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte.
Sexual reproduction takes many forms in multicellular organisms. The fact that nearly every multicellular organism on Earth employs sexual reproduction is strong evidence for the benefits of producing offspring with unique gene combinations, though there are other possible benefits as well.
Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis provides an important advantage that has made sexual reproduction evolutionarily successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. When two haploid gametes fuse, this restores the diploid condition in the new zygote. Thus, most sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly.
Visual Connection Questions
(Figure) If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?
(Figure) Yes, it will be able to reproduce asexually.
What is a likely evolutionary advantage of sexual reproduction over asexual reproduction?
- Sexual reproduction involves fewer steps.
- There is a lower chance of using up the resources in a given environment.
- Sexual reproduction results in variation in the offspring.
- Sexual reproduction is more cost-effective.
Which type of life cycle has both a haploid and diploid multicellular stage?
- asexual life cycles
- most animal life cycles
- most fungal life cycles
- alternation of generations
What is the ploidy of the most conspicuous form of most fungi?
A diploid, multicellular life-cycle stage that gives rise to haploid cells by meiosis is called a ________.
Hydras and jellyfish both live in a freshwater lake that is slowly being acidified by the runoff from a chemical plant built upstream. Which population is predicted to be better able to cope with the changing environment?
- The populations will be equally able to cope.
- Both populations will die.
Many farmers are worried about the decreasing genetic diversity of plants associated with generations of artificial selection and inbreeding. Why is limiting random sexual reproduction of food crops concerning?
- Mutations during asexual reproduction decrease plant fitness.
- Consumers do not trust identical-appearing produce.
- Larger portions of the plant populations are susceptible to the same diseases.
- Spores are not viable in an agricultural setting.
Critical Thinking Questions
List and briefly describe the three processes that lead to variation in offspring with the same parents.
a. Crossover occurs in prophase I between nonsister homologous chromosomes. Segments of DNA are exchanged between maternally derived and paternally derived chromosomes, and new gene combinations are formed. b. Random alignment during metaphase I leads to gametes that have a mixture of maternal and paternal chromosomes. c. Fertilization is random, in that any two gametes can fuse.
Animals and plants both have diploid and haploid cells. How does the animal life cycle differ from the alternation of generations exhibited by plants?
Nearly all animals employ a diploid-dominant life-cycle strategy only the gametes are haploid. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Plants, in contrast, have a blend of the haploid-dominant and diploid-dominant cycles — they have both haploid and diploid multicellular organisms as part of their life cycle. The diploid plant is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid plants that are called gametophytes because they produce gametes.
Explain why sexual reproduction is beneficial to a population but can be detrimental to an individual offspring.
Sexual reproduction increases the genetic variation within the population, because new individuals are made by randomly combining genetic material from two parents. Because only fit individuals reach sexual maturity and reproduce, the overall population tends toward increasing fitness in its environment. However, there is always a possibility that the random combination creating the offspring’s genome will actually produce an organism less fit for the environment than its parents were.
How does the role of meiosis in gamete production differ between organisms with a diploid-dominant life cycle and organisms with an alternation of generations life cycle?
Organisms with a diploid-dominant life cycle make haploid gametes by meiosis, while all their somatic cells are diploid. Organisms with an alternation of generations life cycle make gametes during their haploid life stage, so the chromosome number does not need to be reduced, and meiosis is not involved.
How do organisms with haploid-dominant life cycles ensure continued genetic diversification in offspring without using a meiotic process to make gametes?
Haploid-dominant organisms undergo sexual reproduction by making a diploid zygote. The cells that make the gametes are derived from haploid cells, but the + and – mating types that produce the zygote are randomly combined. The zygote also undergoes meiosis to return to the haploid stage, so multiple steps add genetic diversity to haploid-dominant organisms.
Life Cycles of Sexually Reproducing Organisms
In sexual reproduction, the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends upon the organism. The process of meiosis, the division of the contents of the nucleus that divides the chromosomes among gametes, reduces the chromosome number by half, while fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in eukaryotic organisms: diploid-dominant, haploid-dominant, and alternation of generations.
Plant Reproduction Bi
A life cycle includes all of the stages of an organism’s growth and development. A plant’s life cycle involves two alternating multicellular stages – a Diploid (2n) sporophyte stage and a Haploid (1n) gametophyte stage. This type of life cycle is called Alternation of Generations. The size of gametophytes and sporophytes varies among the plant groups.
OBJECTIVES: Describe the life cycle of a moss. Describe the life cycle of a typical fern. Describe the life cycle of a gymnosperm. Compare and contrast homospory and heterospory.
THE LIFE CYCLE OF MOSSES
1. A Moss is a Nonvascular Seedless Plant belonging to the Phylum Bryophyta.
2. Mosses are the best known and most common Bryophytes. The other Bryophytes are Liverworts and Hornworts. There are about 14,000 kinds of mosses.
CHARACTERISTICS OF MOSSES
1. Mosses grow on moist brick walls, in sidewalks, as thick mats on forest floors, and on the Shaded Side of Trees. Some are adapted to the Desert, or can survive periodic dry spells, reviving when Water becomes available.
2. ALL MOSSES NEED WATER TO COMPLETE THEIR LIFE CYCLE.
3. MOSSES SHARE SOME CHARACTERISTICS OF OTHER BRYOPHYTES:
A. They do not have complicated Vascular Systems. – Nonvascular Plants
B. Water passes from cell to cell by osmosis. They are only a few cells thick.
C. They do NOT have True Roots, Leaves, or Stems.
D. They Require Water for Fertilization.
E. They are small land plants.
THE LIFE CYCLE OF MOSSES
1. The Dominant form of a moss is a clump of leafy Green Gametophytes.
2. A typical moss Alternates between a HAPLOID GAMETOPHYTE and DIPLOID SPOROPHYTE Phases. (Figure 32-1)
3. Haploid and Diploid refer to the number of Chromosomes in the Cells of an Organism.
4. A Gametophyte is the Haploid (1n) generation that produces GAMETES.
5. The Sporophyte is the Diploid (2n) that produces SPORES by Meiosis.
6. The Gametophyte of a moss is usually the largest and longest-lived generation of the moss life cycle.
7. Gametophytes of Mosses have RHIZOIDS, slender, Rootlike Structures that Anchors the Moss in place.
8. The Gametophytes are the Photosynthetic Part of a Moss.
9. The Sporophyte of a Moss is usually smaller than the Gametophyte and is attached to and dependent on the Gametophyte.
10. Sporophytes lack Chlorophyll, they Depend on the Photosynthetic Gametophyte for Food.
11. The Sporophyte consists of a Foot that anchors it to the Gametophyte and a Stalk. The Stalk grows up from the Foot and resembles a Street Lamp.
12. Atop the long, slender Stalk is a CAPSULE.
13. A CAPSULE IS THE STRUCTURE OF A MOSS THAT FORMS HAPLOID SPORES.
1. Mosses, like most sexually reproducing organisms, produce TWO Kinds of GAMETES: EGGS AND SPERM.
2. GAMETES OF ALL BRYOPHYTES ARE SURRROUNDED BY A JACKET OF STERILE CELLS. The Sterile Cells are an important adaptation that protects the gametes from drying out and dying.
3. EGGS of Mosses are large, contain much Cytoplasm, and CANNOT Move.
4. SPERM are smaller and have FLAGELLA, enabling them to reach the Egg by swimming through Water.
5. THE EGG AND SPERM OF MOSSES FORM IN DIFFERENT REPRODUCTIVE STRUCTURES.
6. THE EGG-PRODUCING ORGAN OF A MOSS IS CALLED AN ARCHEGONIUM (ar-keh-GOH-nee-um). Each Flask-Shaped Archegonium forms ONE EGG by Mitosis. The Archegonia form on Branches of the Gametophyte.
7. THE SPERM-PRODUCING ORGAN OF A MOSS IS CALLED AN ANTHERIDIUM (an-theh-RIH-dee-um). Each Antheridium produces Many Sperm.
8. BOTH THE ARCHEGONIA AND ANTHERIDIA ARE PART OF THE GAMETOPHYTE.
9. Bryophytes such as Mosses are sometimes called the “Amphibians of the Plant Kingdom”. Mosses are Land Plants but they require Water for Sexual Reproduction.
10. For most Mosses, Fertilization can occur only during or soon after RAIN or after Flooding, when the Gametophyte is COVERED with Water.
11. The Sperm Swim to the Egg by following a Trail of Chemicals released by the Egg in the Water.
12. Fertilization produces a Zygote that undergoes Mitosis and becomes a Sporophyte.
13. When the Sporophyte matures cells inside the Capsule undergoes Meiosis and form Haploid Spores which are all the Same.
14. The production of One type of spores is called HOMOSPORY. The life cycle of Mosses is called HOMOSPOROUS ALTERNATION OF GENERATION.
15. THESE SPORES BEGIN THE GAMETOPHYTE GENERATION.
16. When spores are mature, the Capsule opens and Spores are carried off by Wind. If a spore lands in a Moist place, it Sprouts and forms a New Gametophyte.
1. Asexual Reproduction of most Mosses can occur in TWO WAYS:
A. FRAGMENTATION – Small pieces broken from a Gametophyte grow into a new plant.
B. GEMMAE – These are tiny pieces of Tissue that can form new Gametophytes.
2. When raindrops splash Gemmae from the Parent Plant, The Gemmae are carried to a new area where they can form Gametophytes.
THE LIFE CYCLE OF FERNS
1. Ferns are by far the LARGEST Group of Living Seedless Vascular Plants.
2. Ferns grow in a variety of places and are diverse in their appearance.
3. Like other Seedless Plants, Ferns usually live in Moist Habitats because they Need Water for Fertilization.
4. A TYPICAL FERN ALTERNATES BETWEEN HAPLOID GAMETOPHYE AND DIPLOID SPOROPHYTE PHASES. (Figure 32-2)
5. The Sporophyte Phase of the Fern’s Life Cycle is the Dominant Phase.
6. Fern Gametophytes are Tiny, Flat Plants that are Anchored to the soil by Rhizoids.
7. Both ANTERIDIA (Male) and ACHEGONIA (Female) form on the lower surface of a Fern Gametophyte.
8. When Water is present, Sperm released by Antheridia Swim to Archegonia.
9. One Sperm Fuses with the Egg in an Archegonium. Forming a Zygote, which is the First Cell of the Sporophyte.
10. In its Sporophyte Stage, a typical Fern has a Stem with True Roots and True Leaves. The Stem, Roots and Leaves are considered TRUE because they have special Water-Carrying Tissues.
12. Other Ferns form Spores in Special Structures on the UNDESIDE OF THE LEAVES. A SORUS (SORI) IS A GROUP OF SPORE-CONTAINING STRUCTURES (SPORANGIA) CLUSTERED ON THE UNDERSIDE OF A FERN LEAF (Figure 32-2).
A. The BLADE is the broad, flat, photosynthetic surface of the Frond. The Blade contains the Chloroplast. The Blade also contains Vascular Tissue that brings water and minerals from roots.
B. On most ferns, a Blade does not attach directly to a Stem. Instead, a Stalk attaches the Blade to the Stem. The PETIOLE is the Stalk that attaches the Frond’s Blade to the Stem. The Petiole contains vascular tissue that carries Water and nutrients through the Plant.
LIFE CYCLES OF CONIFERS – GYMNOSPERMS – NAKED SEEDS – CONES
1. The oldest surviving Seed Plants on Earth are Gymnosperms. In Seed Plants the Sporophyte Phase is the Dominant Phase.
2. Gymnosperms are referred to as Naked Seeds, because they develop on the Scales of Female Cones and NOT inside a Fruit.
3. Gymnosperms are adapted to live in cold climates there are extensive forests of gymnosperms in most of the colder zones of northern temperate regions.
4. There are about 700 species of gymnosperms, such as pine, fir, and spruce, which are also called Evergreen Trees.
5. Gymnosperms include one of the largest and some of the oldest organisms on Earth. The Giant Redwood is one of the Earth’s largest organisms. The Bristlecone Pines is among the oldest, some more than 5000 years old.
6. Unlike mosses, and most Ferns, Gymnosperms produce TWO Types of spores – MALE MICROSPORES AMD FEMALE MEGASPORES.
7. Microspores grow in into Male Gametophytes, while Megaspores grow into Female Gametophytes.
8. The Production of different types of Spores is called HETERSPORY. The Gymnosperm Life Cycle is called HETEROSPOROUS ALTERNATION OF GENERATION.
9. Heterospory ensures that a Sperm will fertilize an Egg from Different Gametophyte and increase the chance that New Combinations of Genes will occur among Offspring.
10. Gymnosperms are Plants (Trees) that reproduce by way of CONES. (Figure 32-3)
11. The Pine Tree is a typical Gymnosperm. The Large, Familiar Cones known as Pinecones are actually the FEMALE Cones of a Pine Tree.
12. Pine trees also have MALE Cones, which are SMALLER than Female Cones. Male and Female Cones have a vital roles in the reproductive cycle of pine trees and other Gymnosperms.
13. THE LIFE CYCLE OF A PINE TAKES TWO OR THREE YEARS FROM THE TIME THE CONES FORM UNTIL SEEDS ARE RELEASED.
14. Male and Female Gametes are made by the Male and Female Cones, which are on the SAME Tree.
15. The Female Cones consist of spirally arranged Scales and Secrete a STICKY RESIN.
16. At the Base of each scale are TWO EGG-CONTAINING OVULES.
17. AN OVULE IS A STRUCTURE, CONSISTING OF AN EGG INSIDE PROTECTIVE CELLS, THAT DEVELOPS INTO A SEED.
19. POLLINATION IS THE TRANSFER OF POLLEN FROM THE MALE TO THE FEMALE PART OF A PLANT.
20. When a Pollen Grain reaches a Female Cone, it sticks to the RESIN of the cone. As the Resin dries, the Pollen Grain begins to grow a structure called a POLLEN TUBE that extends to an Ovule near the base of a Scale, it enables the sperm to reach an egg. The Pollen Tube takes about a year to grow and reach the Egg.
21. A Sperm Cell released from the Pollen Tube Fertilizes an Egg in the Ovule forming a Zygote. Pine sperm Do Not have Flagella and they do Not Swim to an Egg.
22. A Zygote forms and grows into an Embryo surrounded by a SEED.
23. As the Embryo Matures, the Pinecone enlarges and the scales Separate releasing the Seed from the Female Cone.
24. If the seed lands in an environment with the proper conditions for growth, it will sprout and form a New Sporophyte Pine Plant.
25. Seed Plants Do Not Require Water for Reproduction, Sexual Reproduction in Seed Plants can therefore take place independent of seasonal rains or other periods of moisture.
SECTION 32-2 SEXUAL REPRODUCTION IN FLOWERING PLANTS – ANGIOSPERMS – FLOWERS & FRUITS
You have probably admired flowers for their bright colors, attractive shapes, and pleasing aromas. These characteristics are adaptations that help ensure sexual reproduction by attracting animal pollinators. But some flowers are not so colorful, large, or fragrant. Such flowers rely on wind or water for pollination.
OBJECTIVES: Identify the four main flower parts, and state the function of each. Describe ovule formation and pollen formation in angiosperms. Relate flower structure to methods of pollination. Describe fertilization in flowering plants. Compare and contrast the gymnosperm and angiosperm life cycles.
ANGIOSPERMS REPRODUCTION (FLOWERS & FRUITS)
1. The importance of a Flower is NOT in the way it LOOKS or SMELLS, but in WHAT IT DOES.
3. FLOWERS are MODIFIED STEMS with SPECIALIZED LEAVES and other structures for REPRODUCTION. All of these specialized leaves from on the Swollen Tip of a floral “Branch” which is called the RECEPTACLE.
4. FLOWERS HAVE THREE BASIC COMPONENTS: MALE, FEMALE, AND STERILE PARTS.
5. The Male and Female Parts Produce the GAMETES. Sterile Parts ATTRACT POLLINATORS (The Birds and The Bees) and Protect the Female Gametes.
6. Flowers that produce BOTH Male and Female Gametes in the SAME Flower are called PERFECT FLOWERS.
7. IMPERFECT FLOWERS are EITHER a Male or a Female Flower.
8. Some Angiosperms have separate Male and Female Flowers, but BOTH SEXES are on the SAME Plant. Others, the entire plant is Male or Female.
FEMALE STRUCTURES OF FLOWERS
1. The Female Structures of flowers produce EGGS.
2. THE FEMALE, OR EGG-PRODUCING, PART OF A FLOWER IS CALLED THE CARPELS.
3. ONE OR MORE CARPELS FUSED TOGETHER MAKE UP THE STRUCTURE CALLED THE PISTIL. Pistils form at the CENTER of the Flower and usually have THREE PARTS: STIGMA, STYLE, AND OVARY, EACH PART HAS A DIFFERENT FUNCTION:
A. THE STIGMA IS THE STRUCTURE ON WHICH POLLEN LANDS AND GERMINATES. IT IS USUALLY STICKY OR HAS HAIRS TO HOLD POLLEN GRAINS. THE TIP OF THE STYLE.
B. THE STYLE IS THE STALK-LIKE STRUCTURE CONNECTING THE STIGMA TO THE OVARY.
C. THE OVARY IS THE ENLARGED BASE OF A PISTIL, IT IS THE STRUCTURE THAT CONTAINS OVULES AND DEVELOPS INTO A FRUIT. OVULES FORM IN THE OVARY, AND EACH OVULE CONTAINS AN EGG.
MALE STRUCTURES OF FLOWERS
1. THE MALE STRUCTURES OF FLOWERS PRODUCE POLLEN.
2. THE MALE, OR POLLEN-PRODUCING PART OF A FLOWER, IS CALLED THE STAMEN.
3. STAMENS USUALLY HAVE TWO PARTS: ANTHER AND FILAMENT. EACH PART HAS A DIFFERENT FUNCTION:
A. THE ANTHER IS THE STRUCTURE THAT CONTAINS MICROSPORANGIA, WHICH PRODUCE MICROSPORES THAT DEVELOP INTO POLLEN GRAINS. POLLEN GRAINS CONTAIN SPERM CELLS.
B. THE FILAMENT IS THE STRUCTURE THAT HOLDS UP AND SUPPORTS THE ANTHER.
STERILE PARTS OF A FLOWER (ATTRACT/PROTECT)
1. THE STERILE PARTS OF A FLOWER ARE THE PETALS AND SEPALS.
2. PETALS are usually Colorful, Leaflike appendages on a Flower. Their Function is to ATTRACT Pollinators.
3. ALL THE PETALS IN A FLOWER ARE COLLECTIVELY CALLED THE COROLLA.
4. The Protective Leaves at the Base of a Flower are SEPALS. Sepals are often Green, cover the BUD of a Flower and Protect the developing Flower parts as they Grow.
5. ALL THE SEPALS ARE COLLECTIVELY CALLED THE CALYX.
6. Monocots and Dicots can often be distinguished by their Flowers. MONOCOT Floral Parts are arranged in multiples of THREE, The Floral Parts of DICOTS are arranged in multiples of FOUR OR FIVE.
7. FLOWER PARTS ARE USUALLY FOUND IN FOUR CONCENTRIC WHORLS, OR RINGS. (Figure 32-5)
A. OUTERMOST WHORL – THE SEPALS (Calyx) (#1)
B. THE PETALS (Corolla) MAKE UP THE NEXT WHORL. (#2)
C. THE TWO INNERMOST WHORLS OF FLOWER PARTS CONTAIN THE REPODUCTIVE STRUCTURES. FIRST THE MALE (STAMENS, #3) AND THE INNERMOST WHORL CONTAINS THE FEMALE (CARPELS, #4). ONE OR MORE CARPELS FUSED TOGETHER MAKE UP THE PISTIL.
LIFE CYCLE OF ANGIOSPERMS
1. An Angiosperm undergoes Alternation of Generations. The Sporophyte undergoes meiosis to form spores, which then divide mitotically to form Gametophytes.
2. The Gametophytes form the GAMETES: EGG AND SPERM.
3. Sexual Reproduction BEGINS WHEN MICROSOPORE MOTHER CELLS undergo Meiosis in the ANTHER to become Pollen Grains, which is a two-celled or three-celled Male Gametophyte. (Figure 32-7) Notice: Each of the four Microspores will form a Pollen Grain that consists of Two Cells a Tube Cell and a Generative Cell, The Male Gametophyte.
4. During the same time, MEGASPORE MOTHER CELLS undergo Meiosis in Ovules, forming four megaspores in each Ovule, One will become an EGG. (Figure 32-6) Notice: Of the four Megaspores, Three of the Megaspores Degenerate, and the Fourth forms the structures of the Embryo Sac, The Female Gametophyte.
5. Because the Ovule of a flower contains the egg, the ovule contains the Female Gametophyte.
6. The next step is Pollination, the transfer of Pollen from the Anther to the Stigma.
7. When a pollen Grain lands on a Stigma, it sends out a POLLEN TUBE that grows through the Style to the Ovary. Inside the Ovary it enters and Ovule which contains an Egg.
8. Fertilization occurs when a Sperm Nucleus from the Pollen Tube FUSES with the Egg and forms a Zygote.
9. While one sperm fertilizes and Egg, a Second Sperm Nucleus from the pollen tube fertilizes TWO Polar Nuclei.
10. The Second Fertilization forms a Food-Storing Tissue in the Seed called ENDOSPERM.
11. The process in plants that involves TWO Fertilizations is called DOUBLE FERTILIZATION. ONLY ANGIOSPERMS HAVE DOUBLE FERTILIZATION.