The genetics of mirror-image owers

Conspicuous asymmetries in forms that are polymorphic within a species can be genetically or environmentally determined. Here, we present a genetic analysis of the inheritance of dimorphic enantiostyly, a sexual polymorphism in which all  owers on a plant have styles that are consistently de ected either to the left or the right side of the  oral axis. Using Heteranthera multi ora (Pontederiaceae), a short-lived herb, we conducted crosses within and between leftand right-styled plants and scored progeny ratios of the style morphs in F1, F2 and F3 generations. Crosses conducted in the parental generation between morphs or right-styled plants resulted in right-styled progeny, whereas crosses between left-styled plants resulted in left-styled progeny. When putative heterozygous F1 plants were selfed, the resulting F2 segregation ratios were not signiŽ cantly different from a 3 : 1 ratio for rightand left-styled plants. Crosses between leftand right-styled plants in the F2 generation yielded F3 progeny with either a 1 : 1 ratio of leftand right-styled plants or right-styled progeny. Our results are consistent with a model in which a single Mendelian locus with two alleles, with the right-styled allele (R) dominant to the left-styled allele (r), governs stylar de ection. The simple inheritance of dimorphic enantiostyly has implications for the evolution and maintenance of this unusual sexual polymorphism.