The genetics of mirror-image flowers

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 short-lived 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 right-styled plants resulted in right-styled progeny, whereas crosses between left-styled plants resulted in left-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 I : I ratio of left- and 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 deflection. The simple inheritance of dimorphic enantiostyly has implications for the evolution and maintenance of this unusual sexual polymorphism.