Chapter 5 Genetic Analysis of Apomixis - cimmyt

G...1i< loaly,i, .f Apomixi, 71generation SI plants of sexual parents wereanalyzed. Information from Fls and SIS hasonly limited power for testing alternativegenetic models. Interpretations can bestrengthened using advanced generations, asin studies by Burton and Forbes (1960); Nogler(1984b); Savidan (1981); and Valle and Miles(Chap. 10).Classification and GroupingHow should facultative progeny displayingvarious degrees of apomixis be grouped whentesting segregation ratios? Current practiceplaces all plants showing any apomixis intoone group (deemed apomictic) and all plantsdevoid of apomixis into a second group(deemed sexual) (Savidan 1981; Voigt andBurson 1983; Miles and Valle 1991; Sherwoodet al. 1994). Most facultative plants produce farmore apomictic sacs and progeny than sexualsacs or nonmatemal progeny, and classificationis relatively straightforward. With only twogroups being recognized, it is inevitable thatthe genetic models are for Simple gene actionwith Mendelian interpretations. Unimodaldistributions of reproductive types indicativeof quantitative or polygenic inheritance havenot been tested.Testing Genetic ModelsAll reasonable genetic models andpermutations of genotypes should be tested.Predicted ratios should consider the effects ofnumber of loci, dominance, epistasis, andlethali ty. If the degree of alloploidy versusautoploidy of tetraploids is unknown, bothdisomic and tetrasomic inheritance and bothpartial and complete random assortment ofchromosomes or chromatids should beconsidered (Sherwood et al. 1994).Inheritance of ApomixisLiterature on inheritance of apomixis has beenre\'iewed by Stebbins (1941), Gustafsson (1946­1947), Asker (1980), Nogler (1984a), Bashawand Hanna (1990), and Asker and Jerling(1992). Reports issued since 1950 aresummarized here, with a reinterpretation ofsome of the results. Symbol "AU is used todenote the dominant allele of the putativeapomixis gene regardless of the symbols usedin the original reports.Monopolar Apospory (Gramineae­Panicoideae)Paspaillm notatum (bahiagrass). Burton andForbes (1960) crossed colchicine-ind ucedsexual autotetraploid lines of Pensacolabahiagrass with an apomictic tetraploid line.Progeny were classified only by whole plantprogeny testing. The crosses producedsexual:apomictic (S:A) segregation ratios near3:1. Selfing the sexual progeny gave an F 2witha ratio near 35: 1. Selfing the apomictic progenygave only apomictic F 2progeny. Burton (1992)postulated an A gene dominant for apomixisand an independent 5 gene dominant forsexuaJity. He aSSigned genotype Aaaassss toMH, and genotype 111111115555 to the sexualparents. Some progeny from selfing sexualsappeared apomictic based on their uniformitybut actually may have been sexual; this couldgive rise to the 35:1 ratios. In the crosses giving3:1 ratios, some facultatively apomictic hybridF I progeny may have been inadvertentlyclassified sexual, skewing the ratios in favorof high numbers of sexual progeny. Morerecent data support the view that apomixis iscoded for by dominant genes (Burton andHanna '1992). The bahiagrass system deservesreexamination using modern classificationmethods.Dichanthium-Bothriochloa (bl uestem).Harlan et al. (1964) crossed sexual tetraploidsof D. 1I11111t1l1tl/m and D. grahnmii with apomictictetraploid pollen parents. Crosses of sexual xsexual provided only sexual Fls. Crosses ofsexuals x apomicts gave a S:A of 1:4.1. Theypostu lated random assortment of two disomicgenes and assigned genotype A ll A z II tol 1 Ztetraploid apomicts, and II 11 11 11 1 1 Z 2to sexuals. If