APOMIXIS

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By: raagh101 (7 month(s) ago)

hi arun, Asexually reproducing crop species are highly heterozygous?? i hope by oversight you might have mention, please correct it.

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APOMIXIS TYPES, SIGNIFICANCE & MAINTENANCE

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APOMIXIS ( TYPES, SIGNIFICANCE & MAINTENANCE ) DEFINITION Apomixis is a form of asexual reproduction that occurs via seeds in which embryos develop without fertilization. Apomixis is known in some 300 species belonging to about 40 families, it occurs most frequently in Compositae, Rosaceae & Graminae. Apomictic species are usually perennials derived from strongly out crossing ancestors, and apomixis is usually associated with interspecific hybridization and polyploidy. Facultative apomixis is more common in plants than obligate apomixis. Apomixis occurs in polyploidy species of Poa, Paspalum, Panicum, Pennisetum, Cenchrus, etc., in Citrus, Malus, Bubus and Mangifera and in some wild relatives of Zea mays and Beta vulgaris.

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TYPES (1.) Adventive Embryony In this case, embryos develop directly from vegetative cells of the ovule, such as, nacelles, integument, and chalazae. Development of embryo does not involve production of embryo sac. Polyembryony occurs in mango (Alangifera indica), Citrus, etc. (2.) Apospory Some vegetative cells of the ovule develop into unreduced embryo sacs without meiosis. The embryo may develop from the egg cell or some other cell of the embryo sac. Apospory occurs in some species of Hieraceunt, Maus, Grepis, Ranunculus, etc. (3.) Diplospory Embryo sac is produced from the megaspore, which may be haploid or, more generally, diploid. Generally the meiosis is so modified that the megaspore remains diploid. Diplo­spory leads to parthenogenesis or apogamy.

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(4.) Parthenogensis The embryo develops from the egg cell. Depending upon whether the embryo sac is haploid or diploid, parthenogenesis is termed as haploid or diploid parthenogenesis. Haploid arthenogenesis occurs accidently and has been reported in Solarwm nigrum, Nicotiana, Crepis and maize. Diploid parthenogenesis occurs in many grasses, e.g.. Taraxacum. In many species, e.g., Nicotiana, Datura, rice etc., pollen grains may be induced in vitro to produce haploid embryos or plantlets. For this, anthers or pollen grains are cultured on a suitable tissue culture medium. This technique offers several possibilities for crop improvement. (5.) Apogamy In apogamy, synergids or antipodal cells develop into an embryo. Like parthenogenesis, apogamy may be haploid or diploid depending upon the haploid or diploid state of the embryo sac. Diploid apogamy occurs in Antennaria, Alchemilla, Allium and many other plant species.

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The Ideal Apomictic System An ideal apomictic system, from the point of view of plant breeding, should have the following features. The apomictic genotype should preferably be fully male fertile and self-incompatible, and reproduce via pseudogamy. Pollen production is essential for the transfer of genes controlling apomixis to normal sexual genotypes. In case of diplospory, chromosomes should not pair or recombine during first meiotic division. This is because recombination would generate new gene combinations on the various chromosomes, which may give rise to variation among the progeny. In case of diplospory, restitution should occur at or just after first meiosis. If restitution occurs after the second division of meiosis, it is of little use in plant breeding, since progeny in such cases will be genetically variable and more comparable to sexual progeny than to asexual ones.

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Figure 1. Mechanisms of Sexual and Apomictic Reproduction in Hieracium . CCN:-central cell nucleus; EC:-egg cell; EM:- embryo; EN:-endosperm.

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Genetics of Apomixis In general terms, the following three sets of genes appear to be involved in female meiosis and reproduction, Genes controlling meiosis (2) Those concerned with the development of embryo sac from megaspore, i.e., megaspore mitosis (3) Those regulating embryo and endosperm development. Each of these sets is expected to involve scores or even hundreds of genes. Apomixis typically involves the following two developmental changes from the normal reproduction (1.) A suitable modification/failure of meiosis during megasporogenesis (2.) Initiation of mitosis in the unfertilized 2n egg cell or some other 2n cell of embryo sac. But in apospory and adventitious embryony , there is initiation of mitosis in vegetative cells leading to embryo sac/embryo development.

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Therefore, one may expect many different genes to be involved in apomixis. But in most cases, apomixis seems to be governed by one or two genes, which may be either dominant or recessive. In case of Pennisetum, Panicum and Citrus, apomixis is controlled by single gene or locus, apomixis being dominant in Panicum and Citrus. However, recent studies with Tripsacunz indicate that a single gene may, in fact, comprise several extremely tightly linked genes. While in Pennisetum ciliare (Cenchrus ciliaris ) obligate apomixis shows inhibitory gene interaction (the inhibitory gene specifies sexual reproduction). Development of Apomictic Lines Apomictic lines can be developed by the following three different approaches Gene transfer from wild species Induced mutations Selection of apomictic genotypes in the segregating generations of interspecific hybrids.

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(1.) Gene Transfer from wild species In this approach, genes controlling apomixis are transferred into a crop species from a related wild species, e.g., from Tripsacum dactyloides into maize, from Pennisetunt orientale and P. squantalum into pearl millet, from Beta trigyna, B. intermedia, etc. into sugarbect and from Elynzus sp. into wheat. (2.) Induced Mutations This approach aims at developing apomictic form in normally sexually reproducing species by utilising induced or even spontaneous mutations. These efforts have focussed primarily on sorghum, where two mutant lines showing facultative apospory have been isolated. These lines are being used in hybridisation programmes in an effort to develop obligate apomictic lines of this crop. (3.) Isolation of Apomictic Recombinants from Interspecific Crosses Sometimes apomictic recombinants can be recovered from segregating generations of crosses between two sexually reproducing species. For example, apomictic seed formation has been reported in the intergeneric hybrids between T. aestivunt and Avena sativa, H. vulgare and T. aestivunt or T. turgidunt, etc. However, the primary objective of these crosses is seldom the isolation of apomictic types; they are usually made for the transfer of disease resistance.

FIGURE : COMPARISION BETWEEN SEXUAL & APOMICTIC REPRODUCTION.: 

FIGURE : COMPARISION BETWEEN SEXUAL & APOMICTIC REPRODUCTION.

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Applications of Apomixis Apomixis can be used to achieve the following objectives fixation of heterosis production of homozygous lines development of phenotypically stable populations called vybrids. (1.) Fixation of Heterosis Apomixis (apospory and diplospery) leads of fixation of heterosis since apomictic progeny of hybrids have the same genotype as the hybrid. Obligate apomixis results in complete fixation of heterosis. In such cases where apomixis is dominant, a sexually reproducing line is used as the female parent and an obligate apomictic line is used as the male parent to produce the hybrid that is apomictic. The hybrid will produce only apomictic seeds, which are used for maintenance, multiplication and cultivation of the hybrid variety This has been achieved in Bahia grass (P. notatin) and in buffel grass (P. ciliare ), which are obligate apomicts. Sexual lines were isolated and used as female parent in crosses with apomictic lines to produce superior hybrids, which were apomictic.

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(2.) Production of Homozygous Lines Some types of apomixis involve parthenogenetic development of reduced egg cells, followed by reduplication or fusion, or development of embryos from secondary diploid cells of the imbryo sac derived through fusions. Such types to apomixis would give rise directly to homozygous diploids. Such apomixis is claimed to occur in Rubus and has been shown to occur in sorghum (S. bicolor); in sorghum true-breeding, sexually reproducing progenies were identified in F3 of the cross 8473 (apomictic line) x sexual lines (3 different lines) and the fusion of haploid nuclei of the embryo sac has been cytologically verified. (3.) Production of Vybrids A vybrid is the progeny obtained from a cross between two facultative apomicts; the vybrid itself reproduces through facultative apomixis and is maintained by harvesting the seeds of only Fr-like apomictic plants in every generation. Vybrids are expected to show consistent and superior yields over generations. In case of sorghum, vybrids were intermediate between hybrid and pureline varieties for most of the yield traits, including grain yield, but they were superior to hybrids and purelines in less faourable environments.

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Apomixis is nuisance when the breeder desires to obtain sexual progeny, sells or hybrids. But it is of great help when the breeder desires to maintain varieties. Thus in breeding of apomictic species, the breeder has to avoid apomictic progeny when he is making crosses or producing inbred lines. But once a desirable genotype has been selected, it would be multiplied and maintained by apomictic progeny. This would keep the genotype of the variety intact. Asexually reproducing crop species are highly heterozygous and show severe inbreeding depression. Therefore, breeding methods in such species must avoid inbreeding. Advantages of Apomixis Hybrid seed production using male sterility or self-incompatibility suffer from the following three main limitations The technologies are relatively sophisticated Farmers have to buy fresh hybrid seed every year The two parents of a hybrid must flower at the same time; this clearly restricts the range of parents that can be used for hybrid seed production. In contrast apomixis offers the following advantages.

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Other advantages (1.) Obligate apomixis permits fixation of heterosis in the hybrids. Therefore, farmers can resow the seeds produced by apomictic hybrids generation after generation . Apomictic hybrids can be produced by crossing an obligate apomictic line as male parent with a sexual line; apomixis should preferably be governed by dominant genes. (2.) The new hybrid variety could be multiplied from few hybrid seeds in a manner as arc purelines. This greatly simplifies hybrid seed production. (3.) Even such parents that flower at different times may be crossed in a greenhouse to obtain few hybrid seeds, which can be used to establish the new hybrid variety. (4.) The nucleus seed of hybrid varieties is conveniently maintained as the hybrid varieties themselves, and their parental stocks need not be maintained as nucleus seed. (5.) Hybrid seed fields require minimal (3 m) isolation to prevent mechanical mixtures. (6.) Pollen dispersal is not relevant during hybrid seed production.

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Problem in Utilization of Apomixis Utilisation of apomixis in plant breeding presents several problems which are outlined below. (1.) Apomixis is a very complicated phenomenon. Therefore, its utilization necessitates reorientation of breeding procedures as well as seed certification. (2.) Estimation of the level of apomixis, in cases of facultative apomixis, is tedious and time consuming. (3.) In case of facultative apomicts, the proportion of sexual progeny is affected by environmental factors like day-length & temperature, letter being more important. (4.) In the absence of morphological markers linked with apomictic development, maintenance of apomictic stocks becomes difficult. This is especially true for facultative apomicts. (5.) The genetic bases of apomixis is not clear in most cases.

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