Chapter 5 Genetic Analysis of Apomixis - cimmyt

72 R....11 T. SlMrwoodwe postulate that the tetraploids weretetrasomic with genotype AAoo, the observedratios fit ratios expected for the assumptionof random chromatid assortment (1:3.67) orthe assumption of a recessive lethal effect ofthe A allele (1:4) (Sherwood et al. 1994).Pennisetum ciliaTe (buffelgrass). A naturallyoccurring tetraploid sexual plant, designatedB-1S, was selfed and crossed with twoaposporous biotypes by Taliaferro andBashaw (1966). The S:A ratios (near 13:3 forSIS and 5:3 for Fls) suggest two disomicindependent genes with the dominant alleleof gene A being required for apospory, andthe dominant allele of gene B being epistaticto A and conferring sexuality. Sexual parentB-1S was assigned genotype AaBa. Furtherevaluation of Fls, F 2s, and a BC I from B-1S,identified true breeding sexual progeny ofapparent genotype aabb and apomictic plantsof putative genotypes Aabb and AAbb (Readand Bashaw 1969; Bashaw et al. 1970).Crane (1992) proposed a tetraploid Single genemodel to explain the Taliaferro and Bashawsegregations. Three alleles were postulated: a(wild type sexual), A (aposporous), and A+(super sexual). Only genotypes AAAA, AAAa,and AAaa would be apomictic. Chromosomalsegregation patterns were postulated to bepreferential and to differ duringmicrosporogenesis and megasporogenesis.Sherwood et al. (1994) studied inheritance ofembryo sac type of sexual tetraploid plant B­2S. From open pollinated B-25, five siblingsexual lines and five sibling aposporous lineswere selected as parents. Segregations weredetermined for crosses ofsexual x aposporouslines and sexual x sexual lines, and selfs ofsexual lines. Selfs and crosses ofsexual plantsgave only sexual progeny. F 1 s from sexual xaposporous combinations segregated for S:Aat ratios near 15:13 for four aposporous linesand 1:2.8 for the other line. Segregations didnot fit any one- or two-disomic gene models,nor any recessive gene models. Data werecompatible with a one-tetrasomic-gene modelwith apospory regulated by dominant allele A,under either of two assumptions:(i) randomassortment of chromatids, or (ii) A acting as arecessive lethal in gametes. Sexual plants wereassigned genotype aooa; apomicts were Aaaaand AAaa. Data on linkage of apospory inPennisetum with molecular markers (Gustine etal. 1997; see Grimanelli et al., Chap. 6) providesadditional evidence that a single major locusregulates apospory in Pennisetum.Panicum maximum (guineagrass). Hanna et al.(1973) reported that four naturally sexualtetraploid accessions produced 51 progeniessegregating in a combined ratio of 116S:54A.Crosses of sexuals x apomicts gave 21S:28A.They proposed a digenic, disomic additivemodel using the assignment of AaBb for thesexual plants and Aabb, aaBb, or aabb forapomicts (two dominant doses required forsexuality).Savidan (1981) crossed a colchicine-inducedautotetraploid sexual plant and a naturalapomictic tetraploid. Ten kinds of crosses weretested (Table 5.1). All the data fit perfectly withTable 5.1 Segregations for mode of reproduction in10 crosses of Panicum maximum (Savidan 1981;Savidan et a!. 1989)sexual x apomictic crosses sum apo sexFl hybrids,; SIx Al 133 71 623-way hybrids: IS1xAIlsex xA2 279 135 144Backcross: (S1xAIlsex xA1 26 14 12Backcross: sexual 3-way hybrid xA2 170 73 97sexual 3-way xapomictic 3-way 60 26 34Backcross: S1x(S1xAllapa 23 13 10total sex xapo crosses 691 332 359sexual x sexual aosses (or selfed)sexual Fl hybrids selfed 126 0 126sexual 3-way hybrids selfed 57 0 57sexual 3-way xsexual 3-way B2 0 B2total sex xsex crosses 265 0 265apomictic x apomictic crossesapomictic 3·way xapomictic 3-way' 71 53 18.. analysis made of off-types (maternal types nal counted)