Chapter 5 Genetic Analysis of Apomixis - cimmyt

Sueeolog P........,.. r. Idootily ood Qoaotily ApMixIs 131obviously provides insights only aboutapomeiotic or meiotic events, not about thecomplete process of apomixis. Data on the nextgeneration (progeny test) must be collected tostudy fertilization and parthenogenesis eventsas well as the degree of apomixis. The choiceof the level of analysis (apomeiosis /parthenogenesis / apomixis) depends on theobjectives of the research, i.e., whether onewishes to determine only cytologicalprocesses, study parthenogenesis, orinvestigate apomixis in its entirety.2. Comparing results. Limited information isavailable on diplosporous development, butcytological analyses ofparent plantscomparedwith progeny tests generally show goodagreement between apomeiosis and apomixisscreenings in Eragrostis curvula (Voigt andBurson 1981), Allium tuberosum (Kojima andNagato 1992b), and Tripsacum spp. (Leblanc1995). By contrast, the situation in aposporousspecies appears more complex:cytoembryological analyses generallyrevealed higher sexual potential than didmorphology-based progeny tests in Panicummaximum (Savidan 1982b), Poa pratensis(Nygren 1951), and Bothriochloa-Diclumthium(Harlan et al. 1964). The same tendency wasalso observed by Mazzucato et al. (1996) in Poapratensis, when auxin tests and field data werecompared. However, using progeny tests onmore than 100 Brachiaria F1s, Miles and doValle (1991) classified ten plants that werehighly facultative apomicts as sexual,according to cytoembryological tests. Sexualpotential in aposporous tropical grasses hasgenerally been scored according to theformation of 8-nucleate ESs that may developconcomitantly with several apomeiotic (4­nucleate) ESs. The competition among ESsmorefavorable to apomeiotics (Savidan1982a)--and the possible weakness of certainhybrids that are eliminated early, may explainthe overestimation of sexuality in facultativeapospore as measured using cytoembryology(Clausen et al. 1947; Kojima and Nagato 1992b).This was confirmed by Savidan (1982a) in onePanicum maximum accession: sexual potentialwas estimated using a clearing procedure at22.5%, but only3% of the open pollinated adultprogeny, were off-type5. Elimination of hybridoffspring occurred at germination (-7%) oraftertransferring plants to the field (-12.5%), becauseof their inbred nature (resulting from selfingor hydridization with genetically closegenotypes in the collection). On the other hand,after self- or sib-pollination, the lack ofheterozygous loci in segregation may cause anoverestimation of apomixis, with progeny testsshowing the presence of "apparent apomixis"(Bayer et al. 1990).Screening Procedures:Advantages and ConstraintsUntil recently, screening tools for mode ofreproduction were limited to easy-but-latemorphological progeny tests or skilldemandingand time-eonsuming cytologicalsectioning methods (see Table 9.3). During thepast 15 years, new tools in molecular and cellbiology have made screening for mode ofreproduction more efficient, rapid, and reliable.These techniques include ovary progenytesting, flow cytometry for determining ploidylevel, auxin test, and molecular markers thatcosegregate with reproductive mode. Themajor disadvantage of the new methods is theirexpense. In addition, though the methods seemto agree with cytological and/or fieldobservations, additional data are needed toconfirm their reliability.1. Apomixis identification andcharacterization. As mentioned, apomixis maybe detected in various ways, butcytoembryological observations are ultimatelyneeded to confirm the origin of the ES and todetermine the type of apomixis. Clearingtechniques are now quick and easy but requirethe use of phase-contrast or differential