Chapter 5 Genetic Analysis of Apomixis - cimmyt

Apooohis aod Ih......,.1 Gnelk DivenIty 21paruiglumis may be found in southwest Mexicoand is considered to be a very wild form, withalmost no link to modem maize. In the statesof Michoacan and Mexico, teosinte should beconsidered a weed. An incompatibility systemexhibited by these weedy teosintes, whichefficiently controls gene flow from maize toteosinte, has been detected and analyzed(Kermicle and Allen 1990). Moreover, asdescribed by Wilkes (1967), teosintes generallyhave a flowering period that is distinct frommaize. These mechanisms limit gene flowbetween this wild relative and maize.If we use model 1 to explain the transfer ofapomixis from apomictic plants to landraces,we can envisage the following process. Thefirst generation hybrid between teosinte(sexual, aa) and apomictic maize (AA) wouldbe apomictic (Aa), and BC Iplants with teosinteas female would produce Aa (apomictic) andaa (sexual) progeny. At each generation, theapomictic forms are fixed but they stillparticipate in the next generation from sexualplants through their pollen, which can transferthe apomixis allele to sexual plants. Therefore,a portion of each generation's progenybecomes apomictic. We can then deduce thatthe apomictic allele will diffuse into the wildpopulation. However, the assumptions madeto simplify the model may not prove accuratewhen applied to the relationship betweencultivated plants and wild relatives.Cultivated maize and its wild teosinte relativesare, morphologically, widely distinct.Apomictic maize x teosinte F] hybrids will beapomictic and will breed true. Sexual maize xteosinte Fls are known to have a low fitnessdue to their intermediate morphology andadaptation, and they are easily recognizedmorphologically. When they grow in a field,they are not harvested. However, if the hybridis apomictic, its pollen will transmit the A alleleat a rate of50%. Pollination efficiency dependson synchronization between flowering of thesehybrids and the wild relatives. As a lack ofsynchronization between the two types ofplants is anticipated, the gene flow betweenthem should be minimal. These observationsdeviate considerably from the assumptionsposited in the model in which apomictic plantsare expected to engage in pollination inproportion to their frequency in thepopulation. Moreover, in the long run, theapomictic intermediate forms should have alower fitness than the sexual forms, becausethe latter can take advantage of more newrecombinations and adapt faster toenvironmental changes. As noted earlier, astable polymorphism between sexual andapomictic forms is possible when fitness valuesof the two forms reach a certain ratio. We havealso observed that the speed of apomixisdiffusion is a function of the rate of residualsexuality-a high level of residual sexualitywill slow apomixis diffusion.Promoting Genetic Diversity and theRelease of Apomictic VarietiesWe base our models for apomixis diffusion onthe hypothesis that this mode of reproductionis under a simple genetic control. Currentknowledge about the mechanisms underlyingapomixis, however, is very incomplete,especially regarding the expression of anapomixis gene in a new genetic background,as would be the case with a Tripsacum apomixisgene transferred into a maize background. Ifgenetic control of apomixis in landraces andnew varieties involves several genes or a majorgene and modifiers, the dynamics of diffusionwill be more difficult to describe andtransformation ofcurrent varieties to apomicticvarieties would have to be carried out byprofessional breeders. In this instance,apomixis could be used as a genetic fixationtool and new varieties with a complex geneticstructure could be created and released. Suchvarieties would contribute to the maintenanceof diversity at the farmer's field level.