Fixation of hybrid vigour: using Apomixis.

 

 

Apomixis is a unique genetically governed reproductive mechanism, capable of proving a maternal inheritance of characters and uniform offsprings. Heterosis could be maintained over generation of seed production. The apomictic hybrid display valuable pest resistance, environmental adaptability or other traits that are of immediate use for seed increase and replicated testing, while traditional breeding systems require four to eight generations of selfing or backcrossing of lines before advance trails (Hanna, 1995; Calzada et al., 1996).

Apomixis, or asexual reproduction through seeds (Nogler 1984), results in the formation of progeny that are genetic clones of the maternal parent, a trait of major importance for agriculture. In apomictic plants, offspring are produced from an unreduced cell that is either of sporophytic (sporophytic apomixis) or gametophytic (gametophytic apomixis) origin. Thus, the characteristic alternation of gametophytic and sporophytic generations of the plant life cycle is either bypassed, or occurs without the meiotic reduction of the somatic chromosome number.

Apomixes fixes a particular genotype because meiosis is not necessary to produce an embryo sac or egg like cell; therefore, there is no opportunity for recombination. The male gametophyte makes no contribution to the genetic makeup of the embryo, which ensures that the genotype is fixed and maternal in origin. Apomixis eliminates the need for events considered essential for the successful completion of reproduction by seed: meiosis is uncoupled from both female gametophyte development and egg cell formation, and double fertilization is uncoupled from embryo and endosperm development (Koltunow, 1993). Surprisingly, a viable seed is produced. Control of apomixes would enable the fixation of hybrid vigor and the development off true- breeding hybrids in a true breeding program.

Apomixis is rare in food crops and mainly exists in non-agronomic plants. However, the stable introgression of apomixis to crops by breeding has failed to date. The identification of apomixis genes in natural apomictic species has also been hampered by their association with large genomic regions where recombination is often repressed (Ozias-Akins et al., 1998). Thus a workable apomixis technology, combined with crop improvement programs using a wider germplasm pool, may aid in the increase of food yields and also assist the improvement and development of more locally adapted plant varieties that cope with changing climate pressures (Koltunow et al., 1995; Spillane et al., 2001).

 

References:

Calzada, I.P.V., Crane, C.F. and Stelly, D.M. (1996). Apomixis: the sexual revolution. Science, 247: 1322-1323.

Hanna, W.W. (1995). Use of apomixis in cultivar development. Adv. Agron., 54:333-350.

Koltunow, A. M. (1993). Apomixis: Embryo sac and embryo formed without meiosis or fertilization in ovules. Plant Cell, 5:1425-1437.

Koltunow, A.M., Bicknell, R.A. and Chaudhury, A.M. (1995). Apomixis: Molecular stratgegies for the generation of genetically identical seeds without fertilization; Plant Physiol. 108 :1345–1352.

Nogler, G.A. (1984). Gametophytic Apomixis. In: Johri BM (ed) Embryology of Angiosperms. Springer, Berlin Heidelberg New York, pp 475–518.

Ozias-Akins, P., Roche, D. and Hanna, W. W. (1998). Tight clustering and hemizygosity of apomixis-linked molecular markers in Pennisetum squamulatum implies genetic control of apospory by a divergent locus that may have no allelic form in sexual genotypes; Proc. Natl. Acad. Sci. USA 95: 5127–5132.

Spillane, C., Steimer, A. and Grossniklaus, U. (2001). Apomixis in agriculture: The quest for clonal seeds; Sex Plant Reprod.14: 179–187.