Chapter 5 Genetic Analysis of Apomixis - cimmyt

78 Robe" T. S~erwoodEnvironment plays a role in expression(Nogler 1984a; den Nijs and van Dijk 1993).A short photoperiod increases the frequencyof aposporous vs. sexual embryo sacs inDichal1thil/m aristattlm (Knox 1967) andPaspall/ni chromyorrhizol1 (Quarin 1986). Saltstress affects facultative expression in P ciliare(Gounaris et a!. 1991).Harlan et a!. (1964) accurately assessed therelation between sexual and aposporousreproduction. "Apomixis (read apospory)and sexual reproduction are not alternativemodes of reproduction, either genetically oroperationally, but are simultaneous andindependent phenomena. The genescontrolling normal sexual reproduction arenot allelic to those controlling apomixis in theconventional sense." This accounts forfacultative expression of apospory. Inaposporous lines, meiotic reduction of thearchesporial nucleus and apomeioticinduction of apospory in nucellar cells goforward at about the same time. Aposporousini tials and umed uced embryo sacs normallycrowd out the red uced megaspores and sacs.Facultatively displosporous plants alsopossess all of the genetic information requiredfor completion of both meiotic andapomeiotic embryo sacs. However, in contrastto facultative apospory, the two eventscannotproceed simultaneously in a facultativelydiplosporous ovule, for they compete for thesame site in the ovule. Events beginning inthe megaspore mother cell can proceed onlytowards normal meiosis or apomeiosis, butnot both. Variability within facultativediplosporous or aposporous types indicatesthat the entire apomictic developmentalprocess cannot be explained on the basis of asingle gene (Grimanelli et a!. 1995; Savidanet a!. 1995).The lethal Gene as the Basis forHeterozygosityNogler (1984b) concluded that functionalgametes of Rammcl/lus contain a copy of thewild type a allele. Noirot (1993) reviewedevidence that the A allele may act adverselyin Pal1iCllm maximum; his report focused onmale and female sterility. Male and femalesterility is encountered in spontaneousdihaploids and trihaploids of Pal1icl/n1 ofputative genotypes Aa and Aaa, respectively(Combes 1975). Mogie (1988) proposed that thewild typea allele has a function that is essentialto normal plant processes. If gametophytesand gametes bearing dominant allele A mustalso bear the wild type a allele to remainfunctional, this would account for theobservation (H arlan et aI. 1964) tha taposporous apomicts invariably areheterozygous at the apomixis locus. Noapomict has ever transmitted an exclusivecapacity for apomixis to the offspring; acapacity for sexual reproduction is always alsotransmitted, al though it may not surface untillater generations. Heterozygous Aa gametescan be formed by dihaploids, triploids, ortetraploids. In the case of the dihaploid, theAa gamete'is from an unreduced (apomictic)sac, and the progeny are either maternal (nofertilization) or Bill hybrids (fertilization), asshown for the dihaploids of Ralll/I1Cl1Il/s(Figure 5.1). In the case of triploids andtetraploids, the Aa gamete occurs in sexuallyreduced (meiotic) sacs, and the egg is usuallyfertilized (B IIhybridization), but occasionallymay parthenogenetically form an Aadihaploid as part of a diploidtetraploid-dihaploidcycle (de Wet and Harlan1970; Savidan and Pernes 1982). Insofar assexual transmission of A is concerned, theparent must be polyploid and heterozygous;for asexual transmission, the parent must beheterozygous.