Chapter 5 Genetic Analysis of Apomixis - cimmyt

68 ••It T. Sllorwoodmegasporogenesis and megagametogenesistests whether the plant has a capacity forapomictic embryo-sac formation and howstrong that capacity may be relative to thesexual alternative, but it does not indicatewhether the unreduced sacs will formfunctional seeds. Cytological and whole plantmethods must be used in tandem tocharacterize parents (Savidan 1992).Comparisons of methods of classificationgenerally show good agreement. Cytologyand whole plant progeny testing give similarresults for most lines of Eragrostis curvula(Voigt and Burson 1983) and Panicummaximum (Nakajima and Mochizuki 1983). InBrachiaria, Miles and Valle (1991) foundcorrespondence between the two methods inclaSSifying 54 Fls as sexual and 37 Fls asapomictic, however, 10 plants that appearedsexual in progeny tests were facultativelyapomictic in embryo-sac analysis, i.e., progenytesting underestimated the genetic potentialfor apomixis. Cytological analyses generallyreveal higher sexual potential than is indicatedby whole plant progeny testing (Savidan1982a; Mazzucato et al. 1996).When apomixis is essentially obligate,progeny tests are considered as reliable ascytological analyses (Savidan 1982a). Infacultative lines with high levels ofapomeiosis, progeny testing in conjunctionwith a determination of chromosome levelsof off-type progeny may be efficient indetecting sexuality (Sherwood et al. 1980;Savidan 1982a). Progeny tests are unreliablein detecting low levels of apomixis in apredominantly sexual line (Savidan 1982a;Voigt and Burson 1992). Heterogeneity withina progeny cannot be considered proof of theabsence of apomixis (Yudin 1994).Early identification of nonmatemal plants inprogeny of facultatively apomictic Poapratellsis has been facilitated by isozyme andRAPD markers (Huff and Bara 1993;Mazzucato et al. 1995). Estimates of the degreeof apomixis or parthenogenesis in P pratensiswere higher with progeny testing than withembryo sac analysis (Mazzucato et al. 1996).The auxin test gave similar or higher estimatesthan embryo-sac analysis. It is necessary toexamine a large sample of ovules or seed(upwards of 100 individuals) from eachpotential parent to detect any tendenciestoward facultativeness.Controlled PollinationAccidental self- or cross-pollination candramatically influence genetic inferences,especially when wide ratios are being tested.Unintentional self-fertilization will skew ratiosin favor of the phenotype of the maternalparent. Markers to recognize hybrid or selfedprogeny should be used when available.Several approaches have been used to reduceunwanted fertilization in apomixis research:1) Protogyny. If the inflorescence exsertsreceptive stigmas before anthers are exsertedand dehisce, stigmas can be pollinated beforethe maternal floret sheds pollen (Voigt andBashaw 1972; Hanna and Powell 1973; Voigtand Burson 1983; Bashaw et al. 1992; Valle andMiles, Chap. 10). Extraneous pollen is excludedby covering the head with a paper or glassinebag prior to stigma exsertion and continuinguntil seeds are set.2) Suppressed anther dehiscence. Dehiscenceof exserted anthers can be suppressed bymaintaining high humidity. Humiditychambers (Taliaferro and Bashaw 1966),glassine bags (Hanna et al. 1973), and glassbottles lined with moist filter paper (Sherwoodet al. 1994) have been used for that purpose.3) Hand emasculation. Emasculation prior toopening of flowers, followed by bagging, hasbeen practiced for Potentilla (Asker 1970a) andRalllll1CltIlls (Nogler 1984b). Valle et al. (1991)showed that emasculation and bagging of