Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 2Apomixis and the Management ofGenetic DiversityJULIEN BERTHAUDIntroductionApomixis is a mode of reproduction (asexualpropagation through seeds) that exists inmany plants from different botanical families(review in Asker and Jerling 1992; Carman1997). It is most frequent in the dicots Rosaceaeand Asteraceae and in the moncot Poaceae.Some of these Poaceae genera are tropicalforages with wide colonizing ability, e.g.,Panicum maximum. From its center of origin inEast Africa, through human activities it hasexpanded to West Africa, where it can be foundcolonizing roadsides, and to tropical regionsof the Americas and Asia.Apomixis attracts considerable theoreticalinterest as it may help us better understandthe sexual mode of reproduction. It is also ofpractical interest to breeders as a means ofgenetic fixation, potentially offering thecapability of indefinite multiplication ofheterotic genetic combinations. In the case ofapomictic tropical forages (see Valle and Miles,Chap. 10), the problem faced by breeders ishow to overcome apomixis to take advantageofgenetic recombination in order to create newgenetic combinations to be maintainedthrough apomixis. Another challenge is totransfer apomixis into crops in which heterosishas been well documented. Research projectsfocused on this goal are underway for pearlmillet, Pennisetum glaucum (Hanna et al. 1993)maize (see Savidan, Chap. 11), and wheat(Carman 1992). Rice breeders are alsointerested as F] hybrids in rice show heterosis(sef~ Toenniessen, Chap. 1).Some scientists have solely pursued thesimplest model of apomixis, that with acomplete lack of sexuality, i.e., no possibilityof recombination and evolution. In this case,population genetics models show a diffusionof apomixis genes into natural populationswithout a need for some form of selectiveadvantage (Pemes 1971; Marshall and Brown1981). If this holds true, transferring apomixisto crops could ultimately decrease geneticdiversity in those crops and pose a threat tothe environment. From modern apomicticvarieties, the apomixis gene could move tolandraces and wild ancestors in their center oforigin. In a recent review of apomictic risk, vanDijk and van Damme (2000) based theirdiscussion almost entirely on this model.However, before overstating this possibility,one should know more precisely howapomixis functions, what diversity isconserved in wild populations where apomixisis the dominant mode of reproduction, andhow apomixis could be transferred tolandraces.To address these issues, this chapter discusses(i) genetic variation observed in progeny ofapomicts, (ii) diversity observed in wildapomictic populations, (iii) evolutionprocessesof agamic complexes, and (iv) thepossibility of transferring apomixis fromsynthetic apomictic crops to landraces andwild relatives.