Chapter 5 Genetic Analysis of Apomixis - cimmyt

S"...iog Pro...,.. '0 Idnllfy ood a.aotlfy Apomixis 129segregation after selfing and when there issignificant diversity in the surrounding fieldcollection, as is the case for most apomictic species,which are generally polyploid, polymorphic, andhighly heterozygous.Identifying or quantifying apomixis does notrequire the same number of progeny. To detectapomixis, a relatively small number ofprogeny (15­25) can be analyzed. Aberrant rates typically are a:nratios with 'n' the progeny size and 'a' the numberof aberrants observed in the progeny. Statistically,such samplings are binomial; 'p' (aberrant rate) isthe ratio to be estimated for a given value of n(progeny size) on the basis of an observed value fora (number of aberrants detected within theprogeny). Confidence limits for p in a binomialsampling are given in Figure 9.4 for various valuesof n (a = 0.025). Note that for n>30, confidence limitscan be estimated using formulas for the normalFigure 9.4 (onfidence limits (a=0.025) for pin binomialsampling, given a sample fraction a/n. The numbersprinted along the curves indicate the sample size, n. For agiven value of aln (abscissa), limits for p (PA and PB) arethe ordinates read from the appropriate lower and uppercurves (Pr{PA ~ p ~ PB} ~ 1-2a).distribution. Curves shown in Figure 9.4clearly indicate that, up to n = 100, theinformation obtained is poorly signjficantregardless of the value of a. Finally, toobtain good estimations of aberrant rates(i.e., less than 10% confidence limits), itappears that a high number ofindividuals is required.Chromosome number determinationwithin progenies. The sexual or ;:lsexualorigin of offspring is not detectable fromcrosses made at the same level of ploidy,but 2n + nand n + 0 off-types are easilydetected even at the seedling stage.lnterploidy-Ievel crosses could be usedto detect all classes of offspring, butinformation can be biased bydisturbances caused by unstable ploidylevels or ploidy barriers. Chromosomecounts can be made from root tips,microspores, or any somatic tissue usingflow cytometry (Hignight et al. 1991).Detection of seed production in absenceof pollination. Species carrying nonhermaphroditeflowers obviouslyrepresent the easiest situation, the onlyprecaution required being to avoid pollencontamination. Contrarily, emasculationwill be required unless an appropriategenetic system that ensures male sterilitycan be developed. Such systems requirethorough knowledge of the genetics andgenetic stocks of the material understudy, making their application verylimited in natural populations. They havebeen exclusively developed Inexperimental mutagenic populations ofsexual model species (Aradidopsis tha/ulna,Petllnia hybrida), with the aim ofidentifying mutants that reproducethrough autonomous apomixis(Koltunow et al. 1995b; Chaudhury et al.1997; Ramulu et al. 1997).