The genetics of mirror-image flowers

Abstract

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 flowers on a plant have styles that are consistently deflected either to the left or the right side of the floral axis. Using Heteranthera multiflora (Pontederiaceae), a shortlived herb, we conducted crosses within and between left and right–styled plants and scored progeny ratios of the style morphs in F⊂1 F⊂2 and F⊂3 generations. Crosses conducted in the parental generation between morphs or rightndash;styled plants resulted in rightndash;styled progeny, whereas crosses between leftndash;styled plants resulted in leftndash;styled progeny. When putative heterozygous F⊂1 plants were selfed, the resulting F⊂2 segregation ratios were not significantly different from a 3 : 1 ratio for right– and left–styled plants. Crosses between left– and right–styled plants in the F⊂2 generation yielded F⊂3 progeny with either a 1 : 1 ratio of leftndash; and rightndash;styled plants or rightndash;styled progeny. Our results are consistent with a model in which a single Mendelian locus with two alleles, with the rightndash;styled allele (R) dominant to the leftndash;styled allele (r), governs stylar deflection. The simple inheritance of dimorphic enantiostyly has implications for the evolution and maintenance of this unusual sexual polymorphism.