Chapter 5 Genetic Analysis of Apomixis - cimmyt

Apooni.~ aod I~. Maoogo....'.f Geoeti< D1ve"ity 17In the breeding of apomictic forage grasses,sexuality is involved at different steps andpermits genetic recombination (Valle and Miles1992; see Valle and Miles, Chap. 10). Releasedvarieties are apomictic and have beendistributed mainly outside their centers ofdiversity. In this instance, breeding activity isgenerating new genetic diversity.Because projects are now underway to transferapomixis to pearl millet, maize, wheat, andrice, we must consider the consequences ofapomixis on the diversity management oflandraces and that apomixis drasticallyreduces the recombination rate. It is importantto remember that these landraces and theirwild ancestors represent our current reservoirof genetic diversity. Thought should also begiven to conserving the diversity of wildancestors that grow near fields planted withapomictic varieties, which could be recipientsof apomixis genes through naturally occurringgene flow.Projects to transfer apomixis to pearl millet andmaize have reached an intermediate stage:advanced generations of interspecific hybridsbetween apomictic forms and cultivatedspecies have been produced that retain theapomictic trait. In the case of rice, possiblesources of apomixis are yet to be identified. Forwheat, F} and BC lhybrids between Triticumand Elymus have been produced (Peel et al.1997; Savidan et al., Chap. 11). Pearl millet andmaize are allogamous crops and so methodsmust be developed to maintain geneticallyadaptative processes once this new mode ofreproduction is introduced. In its currentdeSign, the Penl1isetum project considers thecreation of tetraploid apomictic varieties ofpearl millet (Dujardin and Hanna 1989). Uponrelease, the distinct ploidy levels of currentlycultivated millet and the tetraploid apomicticnew varieties will act as a genetic barrierbetween them. Dissemination of apomixisgene(s) from the tetraploid to the diploid levelwould involve production of triploid plants,which are usually male sterile; sodissemination through triploids should benegligible. However, in agamic complexes,apomixis seldom occurs at the diploid level.Some mechanism may suppress the expressionof apomixis or impeach transmission to thediploid level. In the pearl millet program, thereis no clear evidence that apomixis can beexpressed at the diploid level. In contrast, afew BC 2diploid-like hybrids in the maize­Tripsacum program were found to expressapomixis (Leblanc et al. 1996). These plants are211 =28 with x =10 from maize and x =18 fromTripsacum. Furthermore, triploid Tripsacum aremale and female fertile. Thus, tetraploidapomictic varieties of maize will probably notrestrict diffusion of apomixis gene(s) to othermaize lines or its wild ancestor, teosinte.Therefore, the models of diffusion of apomixisdiscussed below are based on diploidy.Apomixis fixes heterosis, thereby presentingtwo options for its use: (i) to produce apomicticF 1hybrids through breeding programs andrelease them to farmers as end products; and(ii) to release to farmers apomictic varieties thatwould be used to transfer (diffuse) gene(s) tolandraces, which would eventually becomeapomictic. In the latter case, breeding forapomixis would be a local activi ty. Infact, thesetwo options are complementary and relatedas they pertain to the diffusion of apomixisgene(s). F1apomictic hybrids could be releasedin an area where landraces and wild relativesstill exist. The transfer of the gene to theselandraces and wild relatives will depend onthe parameters cited above in option 2.Transfer of Apomixis Gene(s) andEvolution of landracesWe deduce from Sherwood (see Chap. 5), thatapomixis is probably initiated by onedominant gene (see also Valle and Savidan1996). The active A allele of this "apomixisgene" would be found mostly in the