Chapter 5 Genetic Analysis of Apomixis - cimmyt

Model Sy" ...... s..dy til. Geoetk, aN Dew...p......1llo!ovY o. Apaonllls 113For an experimental system using a nativeapomict, it is convenient to use a facultativespecies in which both maternal andnonmaternal ("aberrant") progeny can beharvested from the same plant. Rutishauser(1948) identified three aberrant types amongthe progeny of these plants: B lIhybrids,derived from the fusion of a reduced egg celland sperm nucleus; Bill hybrids, derived fromthe fusion of an unreduced egg cell and spermnucleus; and polyhaploids, which arisethrough parthenogenesis from a red uced eggcell. Aberrant progeny are often very usefulfor studying the genetics of apomixis. Hybridprogeny permit the evaluation of cytoplasmicinheritanceand the hybridization of pollensterilebiotypes such as interspecific hybrids(Savidan et al. 1994). Similarly polyhaploidshave been used to study the expression ofapomixis in diploids (Nogler 1982; Bicknell1997).Competence to form both meiotic andapomeiotic seed is also invaluable for mutationscreening. Dominant inheritance (see below)suggests that the selective inactivation ofeitherdevelopmental pathway by a mutation willlead to the exclusive expression of the other.This dual competency provides a usefulinternal control. The continued formation ofat least one class of seed indicates that themutation(s) is specific to an event(s) associatedwith apomixis and that related requirementsfor floral development, megagametogenesis,and embryogenesis remain intact. Similarly, asfacultative mechanisms incorporate a balancebetween the utilization of paralleldevelopmental pathways, they can be used tostudy factors that influence that balance, suchas the impact of physiological stress and ofinteractions between genetic modifiers.Finally, as one clear goal of this research is toincorporate apomixis into crop populations, afacultative mechanism is likely to provide themaximum flexibility for farmers and plantbreeders alike. Fortunately, most gametophyticapomicts of all types appear to be facultative,although they differ in the relative importanceof the meiotic and apomeiotic pathways ofseed formation.The formation of the endosperm may also bean important consideration in the choice of anexperimental system. The endosperm mayeither form spontaneously, as in"autonomous" apomicts, or require thefertilization of the polar nuclei by a spermnucleus (pseudogamy). As pseudogamy onlyrequires spontaneous embryogenesis and notspontaneous endosperm formation,pseudogamous species may be consideredpotentially simpler models to study. Thispossible advantage, however, is offset byexperimental constraints associated withpseudogamy. In these plants, pollination isrequired for seed formation, so it becomesnecessary to demonstrate that the appliedpollen led only to the fertilization of the polarnuclei and not of the egg. This difficulty hasbeen largely overcome, however, in speciesthat can be assessed for apomixis using acorrelated cytological character, such as callosedeposition in diplosporous species ofTripsacum (Leblanc et al. 1995a) and four-