Apomixis

A game changer in plant breeding
,powerful breeding tool ,genetics ,asexual reproduction ,apomixis technology ,food ,agriculture research ,agriculture ,apomixis

1. Apomictic Seed Production By : E. Chethan Swamy (12004648) To : Dr. Sanjeet Singh Sandal

2. Introduction: Seed production in higher plants that are sexually propagated species normally occurs after a sexual union in which male and female gametes fuse to form a zygote, which then develops into an embryo. However, some species have the natural ability to develop seed without fertilization, a phenomenon called “ Apomixis ” . The consequence of this event is that apomictically produced seeds are clones of the mother plant. That is, Apomixis is a form of asexual reproduction in plants through which the seeds are formed without fertilization . Unlike sexual reproduction, there is no opportunity in apomixis for new recombination to occur to produce diversity in the offspring.

3. Occurrence in nature: Apomixis is widespread in nature and occurs in several unrelated plant families. About 10% of the estimated 400 plant families and a mere 1% of the estimated 40 000 plant species they comprise exhibit apomixis. The plant families with the highest frequency of apomixis are Gramineae (Poaceae), Compositae, Rosaceae, and Asteraceae. Many species of Citrus, mango, perennial forage grasses, and guayule reproduce apomictically. There are several indicators of apomixis. When the progeny from a cross in a cross-pollinated species fails to segregate, appearing uniform and identical to the mother plant, this could indicate apomixis.

4. Similarly, when plants expected to exhibit high sterility (e.g., aneuploids, triploids) instead show significantly high fertility, apomixis could be the cause. Facultative apomixis may be suspected if the progeny of a cross shows an unusually high number of identical homozygous individuals that resemble the mother plant in addition to the presence of individuals that are clearly different (hybrid products). The indicators suggested are by no means conclusive evidence of apomixis. To confirm the occurrence of apomixis and to discover its mechanisms require additional progeny test as well as cytological tests of megasporogenesis and embryo sac development.

5. Based on the Occurrence of the Sexual Reproduction: Facultative Apomixis : In this the Sexual reproduction occurs along with the Apomixis. And the percent of the progeny identical to female parent gives the “ degree of Apomixis". Also known as “Partial Apomict”. Obligate Apomixis : In this the Only Apomixis Occurs and there is no sexual reproduction taken place. And the progeny from the cross completely resemble the female parent. Also known as “Complete Apomict”. Further, Facultative apomixis is more common than the Obligate apomixis

6. Gametophytic Apomixis Sporophytic Apomixis If unreduced cells give rise to an megagametophyte or embyo sac. If unreduced cells directly give rise to embryo. Recurrent Apomixis Non Recurrent Apomixis Where embryos arise from diploid cells of embryo sac. Where embryos develop from haploid cells of embryo sac. progeny can be maintained further. progeny cannot be maintained further.

7. Gametophytic Apomixis: If the unfertilized cells give rise to a mega gametophyte or embryo sac. Diplospory: The embryo sac originates from the megaspore mother cell either directly by mitosis and/or after interrupted meiosis . The embryos in such embryo sacs may arise by either parthenogenesis or pseudogamy. Parthenogenesis: Embryos develop from the embryo sac without pollination. Gonial parthenogenesis Somatic parthenogenesis

8. Gonial parthenogenesis : Embryos develop from the egg cell Somatic parthenogenesis : Embryos develop from some other cells of embryo sac Pseudogamy (Apogamy): Pollination is necessary for embryo development but fertilization of egg cell does not takes place. Embryo develops from synergids or antipodal cells. 2.Apospory: If the embryo sac originates from nucellus or integument cells, the process is called Apospory. e.g.:Hieraceum, Malus, Orchids etc. 3 . Androgenesis: The embryo sac originates from generative nucleus of pollen tub e cells when pollen grains are cultured in vitro. e.g.: Nicotiana, Datura, Rice etc.

9. Sporophytic Apomixis If the unfertilized cells give rise directly to an embryo. Adventitious embryony ( sporophytic budding ) : Embryo develops directly from nucellus or integuments ( no production of embryo sac) e.g. mango , citrus.

10. The ideal apomictic system: All the progeny of plants should be apomictic. Apomictic genotype should be fully male fertile and self incompatible. Chromosomes should not pair in first meiotic division in case of diplospory. Because this would generate new gene combination and variation may arise. Apomixis should be dominant over sexual reproduction. Expression of apomixis should be little a ffected by the environment.

11. DEVELOPMENT OF APOMICTIC LINES Apomictic lines can be developed by three different approaches. In addition, molecular biologist are beginning to explore possibilities of engineering apomixis in crop plants. Gene transfer from wild species: Ge nes controlling apomixis can be transferred into crop species from relative wild species. e.g.: Tripsacum dactyloides into maize from Pennisteum orientale. Pennisteum squamalum into pearl millet from Beta trigyna .

12. 2.Induced mutations : It aims at developing apomictic forms in normally sexually reproducing species by utilizing induced and even spontaneous mutations. e.g. : in sorghum mutant line R473 has been isolated. 3.Isolation of Apomictic Recombinants from Interspecific crosses: Sometimes Apomictic recombinant can be recovered from seggregationg generations of crosses between two sexually reproducing species. e.g.. intergeneric hybrids between T.aestivum and Avena sativa or H.vulgare … ………………. and T.aestivum

13. Importance of Apomictic Cultivars in Plant Breeding Biotechnologists are making important strides toward creating a new and profound enabling technology for the agricultural, horticultural and forestry industries. The new technology induces and controls the expression of asexual or clonal seed formation that is apomixis. Because seeds form asexually, progeny of apomictic plants are clones of the mother plant. Apomixis is perhaps the most important trait agricultural biotechnology could confer to crops in the foreseeable future. With apomixis a 30% increase in world rice yields, currently 700 million tons per year, could be achieved, and this could feed an additional billion people.

14. World wheat yields, currently 770 million tons per year, could be increased by 15%.5 The market value for the increased yield from these two crops alone could exceed $17 billion annually, yet this is only the beginning of the potential benefits. Apomixis may soon be readily conferred to essentially all of the world's major food, feed, and fiber crops, as well as to horticulture and forestry crops. In many of these cases, yield enhancements comparable to those for wheat and rice may be expected.

15. Seed companies in the United States currently spend an estimated $435 million per year in field costs alone (field production and detasseling costs) to produce hybrid corn seed. Apomixis could eliminate about 84% of these costs, or $366 million annually. Likewise, nearly 50% of China's rice land is planted with hybrid rice seed, which is produced using labor and facility intensive processes. Conferring apomixis to rice would eliminate most of these intensive activities and greatly facilitate the worldwide spread of superior-yielding hybrids

16. Apomixis may also revolutionize plant breeding practices. Currently, many generations of inbreeding are required to produce new varietal crops as well as the parent lines used to produce hybrid crops. Incorporating apomixis into breeding programs will relax this intensive inbreeding requirement and allow breeders to explore alternative approaches to maximizing hybrid vigor by crossing non-inbred lines and fixing heterosis in the highest yielding progeny

17. For many crops, significant yield and quality enhancements are possible simply by doubling the chromosome number of the plant, i.e. raising its "ploidy level", and reproductively stabilizing the new ploidy level through apomixis. Similarly, apomixis technology may be useful in achieving similar yield and quality enhancements in other diploid crops, including: rice, corn, barley, oats, soybeans, sugar beets, millet, alfalfa, other forage crops, as well as many vegetable, horticulture, and forestry crops.

18. The genetic and molecular control of apomixis is intriguing from both developmental and evolutionary perspectives, and it attracts significant interest because of its potential value to agriculture if it can be harnessed for plant breeding. Studies involving natural apomicts and sexual mutants displaying apomixis like phenotypes have led to the identification of apomixis loci and candidate genes that may be responsible for superimposing apomixis on the sexual pathway. The possibility of apomixis being epigenetically regulated is an attractive hypothesis with growing support from studies in sexual plants where mutations in epigenetic pathways lead to apomixis-like phenotypes. Epigenetic regulation will continue to be a key focus of future studies in natural apomicts.

19. APPLICATIONS OF APOMIXIS 1.Fixation of Heterosis: Apomixis leads to fixation of heterosis since apomictic progeny of hybrids have the same genotype as the hybrid. Obligate apomixis results in complete fixation of heterosis. s e x ua l ly r ep r od u ci n g l i n e × (female) Apomictic line (male) Apomictic Hybrid Maintained and multiplied by seed

20. Production of Homozygous lines: Some types of apomixis involve parthenogenetic development of reduced egg cells, followed by reduplication or cell fusion, or development f embryos for secondary diploid cells of embryo sac derived through fusions. 3. Production of Vybrids: Vybrid is a 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 F1- like apomictic plants in every generation

21. ADVANTAGES OF APOMIXIS Obligate apomixis permits fixation of heterosis in the hybrids. Therefore, farmers can resow the seeds produced by apomictic hybrids generation after generation. The new hybrid variety could be multiplied from few hybrid seeds in the same manner as purelines. This greatly simplifies hybrid seed production. 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. The nucleus seed of hybrid varieties can be conveniently maintained as hybrid varieties. Hybrid seed fields minimum isolation (3m) to prevent mechanical mixtures.

22. PROBLEMS IN UTILIZATION OF APOMIXIS Apomixis is a very complicated phenomenon. Estimation of the level of facultative apomixis, is tedious and time consuming. In case of facultative apomicts, the proportion of sexual progeny is affected by environmental factors like daylength and temperature. In the absence of morphological markers linked with apomictic development, maintenance of apomictic stock becomes difficult. The genetic basis of apomixis is not clear in most cases.

23. Thank You By : Emmadishetty Chethan Swamy (12004648)