Chapter 5 Genetic Analysis of Apomixis - cimmyt

118 Ron A. Mlltllstudies of apomixis used monocotyledonousspecies, principally relatives of the cereals, suchas ELymlls (Torabinejad and Mueller 1993),Tripsacum (Leblanc et al. 1995a), Panicum(Savidan 1982), and important forage grasses,such as Pennise/um (Dujardin and Hanna 1985;Ozias-Akins et al. 1998)), Brachiaria (Valle et al.1994), and Paspalum (Bonilla and Quarin 1997).The inheritance of apomixis has beenparticularly well characterized for Panicummaximum. Savidan (1990, 2000) partiallyascribed his succe~~with this system to the useof apomicJic-aria~exual forms from within thesqmespecies and at the same plOidy level. Thisimportant advantage is shared with only a verysmall number of studied apomictic taxa. Incontrast, most known apomictic species appearto have either evolved away from theirimmediate sexual progenitor(s) or theirprogenitor(s) no longer exists. As a result,apomicts have often been crossed wi th a relatedbut distinctly different sexual species. In suchstudies, the subsequent analysis of the progenymust consider the inheritance of the breedingsystem while also allowing for unrelated effectsresulting from interspecific hybridisation.Despite this caveat, however, mappingstrategies have led to the isolation of molecularmarkers linked to apospory in the grass genusPennise/um (Ozais-Akins et al. 1993, 1998). In asimilar approach, colinearity between grassgenomes has also been used to propose alinkage to apomeiosis in Tripsacum (Leblanc etal. 1995b) and Brachiaria (Pessino et al. 1997;see Grimanelli et a!., Chap. 6).Some dicotyledonous species have been usedpreviously as model systems, most notablyPo/en/ilia (Rutishauser 1948; Asker 1970, 1971);Taraxacum (Richards 1973; van Dijk et al. 1999);Hypericum (Noack 1939); Rammculus (Nogler1984a); and Hieracium (Bicknell et al. 2000). Theinheritance of apospory in Ramll1cultls has beenparticularly well studied (Nogler 1984b)through four generations of crosses andbackcrosses. The results indicate that aposporyis inherited as a simple dominant Mendeliantrait in this system, however, the alleleconferring apomixis could only be transferredin a diploid or polyploid gamete. Nogler notedthat this mechanism could explain the veryclose association observed betweenpolyploidy and apomixis in native systems.In contrast, recent results with thediplosporous genus Taraxacum (van Dijk et al.1999) indicate that a two loci model better fitsthe data obtained from controlled crosseswithin this taxon. Taraxacum is an attractiveexperimental system because it is a wellstudied ecological model (Richards 1986) andit has also been successfully transformed (Songand Chua 1991).One dicotyledonous taxon that appears to bewell suited for use as a model system isHieracium subgenus Pilosella (Koltunow et al.1995). The plants are small herbaceousperennial daisies that are easily propagatedand maintained in the greenhouse. Hieraciumis a long-day plant, flowering in response toextended photoperiods (Yeung 1989).Photoperiodicity is a very useful experimentaltool as it enables the programmed productionof flowers at any time during the year usingday-length extension lighting. Both H. pilosellaand H. auran/iacum set seed within 3-4 monthsof germination, allowing 3-4 generations perannum,(Bicknell 1994a).Apomictic biotypes of Hieracium subgenusPilosella develop seed by facultative aposporycoupled to autonomous endospermdevelopment. Pollination is therefore notrequired for the formation of maternal seed,and apomixis can be scored by quantifying theseeds that set after the excl usion of pollen. Aswith Taraxacum, the simplest method ofexcluding pollen is to decapitate the immaturebud, removing both the anthers and stigmas(Ostenfeld 1906; Richards 1986).