Chapter 5 Genetic Analysis of Apomixis - cimmyt

76 Rokrt T. SHtwoodPerhaps the most significant aspect of thehypothesis is.that Mogie deduced that the wildtype (a) allele of the apomixis locus has anessential function in the plant. He suggests thatit codes for meiotic reduction and that it is alsoinvolved in the control of mitosis, whichwould be disrupted by the expression of themutant allele in somatic cells.Multicellular ArchesporiaBeta (beet, Chenopodiaceae). At least twospecies in the section Corollinae formmulticellular archesporia that show bothdiplosporous and aposporous development.Jassem (1990) conducted extensive analyses ofcrosses involving sexual and apomicticspecies, across ploidy levels, including F 2andBC lgenerations. Bill hybridization andaneuploidy complicated the results. Althoughunable to draw unequivocal conclusions, shebelieved that apomixis genes were partlydominant and acted in a complementaryfashion.Sorbus (mountain ash, Rosaceae). Liljefors(1955) used leaf morphology and chromosomepairing to deduce genomic formulas ofaposporous polyploids in the agamic complexSorbllS. He assigned the genomic formula BBto totally sexual S. aucuparea. Species assignedgenomic formulae AAAA, AAAB, and AABwere fully aposporous, and AABB wasfacultative. He postulated that a gene or genesfor apomixis were associated with genome A,and that expression was dosage/balancerelated. However, species with genomicformulae ABBB and ABB were also highlyapomictic, and he had to postulate exchangeof the gene into the B genome. Liljefors'observations may be equally well explainedusing a one major-gene model with facultativeexpression, and postulating that genotypesAAnn, Annn, and Aan are aposporous.Towards a ComprehensiveModel of InheritanceInheritance of apomixis has been explored inrelatively few species, yet several geneticmodels have been put forward that differwidely in postulated number of loci andnature of gene action (Bashaw and Hanna1990; Asker and JerJing 1992; den Nijs andvan Dijk 1993). Is the seemingly capriciousoccurrence and regulation of apomixis indifferent taxa to be attributed to independent,random mutations at various reproductiveloci leading to similar phenotypicconsequences? Is there a single apomixislocus or linkage group shared by allapomicts? The reality lies somewherebetween these extremes.Regulation of Monopolar AposporyMonopolar (Panicurn-type) apospory occurscommonly throughout Panicoideae andArundinaceae and is found nowhere else.This indicates a common genetic basis. Brownand Emery (1958) postulated that coding forthe monopolar pattern arose early in theevolution of the group. Inheritance data forall the Panicoid species studied have beeninterpreted as indicating that expression ofapomixis requires a major locus, withapomixis behaving as a dominant trait.Common gene action and phenotype inrelated species indicate the same linkagegrouP. may be involved. The molecularevidence for one linkage group in Pennisetumis impressive. However, C. F. Crane (personalcomm.) cautions that this does not necessarilyimply that the monopolar type evolved onlyonce and spread laterally among relatedgenera that are well populated with sexualspecies. He considers it more likely that theancestor of the A-a locus hi,ls becomeWidespread in the panicoids and chloridoids,and that apospory has emerged repeatedlyby mutation of the wild type locus.