Chapter 5 Genetic Analysis of Apomixis - cimmyt

130 Ofjyier LH......AodrH Mom","Markers lor hybrid dete(tion. Traits undersimple genetic control are ideal for progenytesting by crossing recessive maternalgenotypes with homozygous dominant testers(Hanna et al. 1970; Bashaw and Hanna 1990).Models for estimating levels of apomixis byfollowing marker segregation have beendeveloped (Marshall and Brown 1974),however, recombination can occur withoutfertilization, and the presence of dominanttraits in progeny tells nothing about the originof the off-types (n + n or 2n + n) in the absenceof cytological data. Moreover, identification ofsuch "ideal" markers in apomictic species oragamic complexes is not necessarily easy,because traits in polyploid apomicts aredifficult to analyze genetically.Morphological descriptors are the easiestmeans for conducting progeny tests. If thetester (pollen donor) differs significantly fromthe progeny-tested plant, hybrids will varysufficiently from the maternal type to allowdetection. In the case of selfing, becauseapomicts are generally highly heterozygous,offspring arising through sexuality will varysufficiently from the mother plant to be scoredas off-types. In most species, (poly)haploidsare easily detected because of their pa,:ticularphenotypes and the low vigor they exhibit(Asker and Jerling 1992). However, whenusing morphological descriptors, it is often notpo.ssible to distinguish between sexuality(n+n) and genomic accumulation (2n + n). Butwhen morphological and cytological(chromosome number) data are combined, theidentification of all classes is theoreticallypossible. Analysis of seedlings has the majoradvantages of timeliness and saving space, butthe most informative descriptors for screeningpurposes are usually expressed at maturity.There are few reports of successful progenytesting for morphology on seedlings afterinterspecific crosses (Williamson 1981).Isozymes or molecular markers can be usedto assess variation in progenies (fingerprintinganalyses; Nybom 1996). Finding goodpolymorphic isozyme systems, RFLP probes,or primers for peR as candidates forfingerprint experiments is not a major obstacle.Although genetic analysis is still hindered bypolyploidy, any variation in isozyme or DNApatterns indicates off-type production,provided that somatic recombination does notoccur frequently in the material under study.Esterase and peroxydase were the first systemsused to isolate sexual plants from Panicummaximum (Smith 1972). Apomixis expressionwas also confirmed or quantified usingisozymes in Taraxacum (Ford and Richard1985), Arabis holboellii (Roy and Rieseberg1989), Allium tuberosum (Kojima et al. 1991b),Poa pratensis (Wu et al. 1984; Barcaccia et al.1994), Tripsaatm spp. (Leblanc 1995), and Malussp. (Ur-Rahman et al. 1997).Mazzucato et a1. (1995) showed a slightlyhigher capacity of RAPD markers indiscriminating off-types in progenies from thesame species, when compared with threepolymorphic isozyme systems or with analysisof traditional morphological traits. Althoughstill seldom used, molecular markers havebeen successfully used for progenyfingerprinting (e.g., Poa pratensis: Huff andBara 1993; Barcaccia et al. 1997; Paspalumnotatum: Ortiz et al. 1997).rChoosing Suitable ProceduresAnalyses at the Plant Level versusProgeny Tests1. Nature of the information obtained.Apomixis results from apomeiotic processes(apospory or diplospory) that produceunreduced ESs, and parthenogenetic embryodevelopment from unreduced eggs. Althoughnonreduction and parthenogenesis arethought to be closely linked in apomicts,observations and/or analyses of the plant itself