Chapter 5 Genetic Analysis of Apomixis - cimmyt

138 udda lorge. do Val. aod Joh W. MilesCameron (1983) posed a Shakespeareanquestion: "To breed or not to breed," inreference to Australian investment in breedingtropical forage plants utilizing limited geneticresources. To quote Harlan (1983): "It is fruitlessto engage in plant-breeding programs withinadequate germplasm collections...."Representative collections for most of thetropical apomictic grasses are limited,therefore, a key prerequisite for effectivetropical forage breeding projects is to acquirediverse germplasm from the centers of originof the target genus/species.Panicum maximum was extensively collected byFrench and Japanese geneticists (Combes andPemes 1970; Nakajima et aI. 1978); the resultinggermplasm collections are representative of thenatural variation (Savidan et aI. 1989). Anextensive collection of Brachiaria wasundertaken by the Centro Internacional deAgricultura Tropical (CIAT) in 1984-85 (CIAT1986). Other important apomictic tropicalforage genera (Hyparrhenia, Melinis, Urochloa,Cenchrus, and Penniselum) still need to becollected to broaden variability and to identifysexual accessions to facilitate breeding.Surveys of closely related species are relevantwhen sexual plants are not available in theapomictic species of interest or when otherdesirable traits cannot be found in the primarygene pool. In sexual crops, the search forapomixis may involve other species or generain order to find cross-compatible wild relatives,as seen in the Zea x TrypsaCllm transfer program(Savidan, Chap. 11). To accomplishhybridization, research is needed to establishphylogenetic relationships and to overcomedifferences in ploidy level, genomerelationships, and gene pools (Hanna andBashaw 1987). If the species relationship issufficiently close, and given that apomixistends to restore fertility, one should be able toproduce useful obligate apomictic, interspecifichybrids with good seed set.An extensive species relationship survey wascarried out on Paspalum, a large grass genuswith tropical and subtropical adaptations(Burson 1983; Quarin and Norrmann 1987;Burson 1989; Quarin 1992). Two species areparticularly important as forage grasses, P.nolalum and P. dilalatum, and several othershave shown promising results in agronomicand grazing trials (Grof et al. 1989b; Fernandeset aI. 1992; Pizarro and Carvalho 1992; Batistaand Godoy 2000). Approximately 400 specieshave been described taxonomically, and about80% of these are polyploids. The genus hassexual diploids and apomictic and sexualpolyploids, which range from triploids to 16x(Quarin 1992). Diploid species reproducesexually and have regular meiosis (bivalentchromosome pairing and normal distribution).Polyploidy, apomixis, and irregular meioticchromosome associations are highly correlated(Quarin and Norrmann 1987). Valuableinformation has been gathered about thisgenus, leading to more effective interspecifichybridization that may result in superiorapomictic genotypes (Quarin 1987).A second fundamental prerequisite forbreeding is adequate knowledge of biology,cytology, and reproduction of the material athand (Asker and Jerling 1992). Breeders havelong been challenged by the problems ofreproductive isolation resulting from apomixisand polyploidy. Efforts directed atdetermining the genetic basis of apomixis inseveral species have generally shown it to beunder simple genetic control (see Sherwood,Chap. 5), e.g., Bothrioc"loa-Dichanthium(Harlan et aI. 1964), Panicum (Savidan 1982),CenchYlls (Sherwood et aI. 1994), Paspalum(Burton and Forbes 1960), Brachiaria(Ndikumana 1985; Valle et aI. 1993b; Valle andSavidan 1996), TripsaCllnl (Leblanc et aI. 1995b),and possibly Eragrostis (Voigt and Burson1992). Hence it should be possible tomanipulate apomixis in a breeding programonce cross-compatible sexual or highly sexual