Chapter 5 Genetic Analysis of Apomixis - cimmyt

1'"""10••f ApomIxl. 10 S....I Plaot. by M.I......1s 219The above mutants of maize and barleyillustrate that mutations at a number of loci caninduce one element of apomixis-theformation of unreduced gametes. Furtherdevelopment is dependent on fertilization, andnormal endosperm development depends ona balanced maternal to paternal ratio.The preferential detection of unreduced eggsby crosses with tetraploids is a useful reminderof the importance of choosing a suitable pollenparent for mutagenesis studies involvingpollination. On one hand, mutants withunbalanced endosperm may be difficult toisolate initially because of low viability; on theother hand, the shrunken endospermphenotype could serve as a useful criterion inthe selection for meiotic mutants.2. Parthenogenetic mutants. The EMS-inducedhap mutant of barley was initially isolated inthe form of a chlorophyll deficient mutantcontaining at least three linked mutations, tig,let (pollen lethality), and hap (Nielsen 1974).Presence of the hap allele results in a lowfrequency of haploid progeny. After separationof hap from the other two mutations, Hagbergand Hagberg (1980) showed that hap isincompletely dominant over the wild typeallele: heterozygous (hap / +) plants produce3-6% haploid progeny, whereas homozygous(hap/hap) plants produce up to 40% haploidprogeny. Perhaps not surprisingly, crossesbetween a homozygous mutant (hap/hap )andwild type (+ / +) plants produce different resultsin the F I, depending on whether the mutant isthe male or the female parent: a hap/hap femaleplant pollinated by wild type (+/+) pollenresults in a high frequency of haploid F Iprogeny, whereas no haploids are producedwhen a wild type (+ / +) female plant is crossedby a hap/hap male. This indicates that the haplocus acts only through the maternal tissue,either to prevent fertilization of the egg cell orto stimulate the egg cell nucleus to divideprematurely. In this mutant, parthenogenesisis the only element of apomixis that has beeninduced. The formation of a perfectly welldevelopedendosperm, which supports theproduction of a viable seed, is presumablyfacilitated by normal fertilization events in thecentral cell that result in a genomicallybalanced endosperm.3. Aposporous mutants. In pearl millet, twomutants that produce aposporous embryo sacshave been reported. The first,Jemale sterile (js),is a recessive mutation induced by radiationtreatment (Hanna and Powell 1974; Arthur etal. 1993). Homozygous mutants are femalesterilebut produce normal viable pollen.Mutant ovules are small and immaturecompared with the wild type, and only abouthalf of them contain embryo sacs. Of these, themajority are multiple embryo sacs that appearto be aposporous, although sexual embryo sacsare observed in some ovules. Only a very smallproportion of ovaries display any endospermor pro-embryo development, and all ovulesdegenerate five days after pollination. Pollentube growth in the mutant is abnormal, andthe inhibition of fertilization has been proposedto explain the absence of seed set.The second, stubby head, was discovered inprogeny of seed treated with both thermalneutrons and diethyl sulfate (Hanna andPowell 1973). This recessive mutation causes apleiotropic phenotype, including twin ovules,shortened internodes, flattened stems, and astubby inflorescence. It produces both normalsexual embryo sacs in some ovules andmultiple embryo sacs, which arise fromnucellar cells, in others. Test crosses confirmedthat stubby head is a facultative apomict,producing maternal progeny at frequenciesranging between 23% and 77%.Mircrosporogenesis in this mutant is normal,however, seed set is low; this has beenattributed partly to nonfertilization because ofcompetition between the multiple embryo sacs.