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 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 1 : 1 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.

Darwin's legacy: the forms, function and sexual diversity of flowers

“…In contrast, populations of dimorphically enantiostylous species are composed of left-and right-styled plants, often in equal frequencies, as in Wachendorfia (Jesson & Barrett 2002a). Here the condition is a true genetic polymorphism and inheritance studies of stylar bending in Heteranthera multiflora indicate single-locus control with right deflection dominant to left ( Jesson & Barrett 2002b).…”

Left–right patterning from the inside out: Widespread evidence for intracellular control

“…6B). (109) As in fiddler crabs, however, genes controlling direction of floral asymmetry clearly evolved after such asymmetries already existed. Within one branch of the Haemodoraceae Figure 5.…”

What determines direction of asymmetry: genes, environment or chance?

“…In nearly all cases of random asymmetries where crossing has been tried, direction of asymmetry is not inherited [3]. The one known exception is direction of style bend in Heteranthera flowers, where direction of bending is controlled by a single locus with right-bending dominant [50]. This remarkable exception nonetheless provides a powerful test of whether genes are leaders or followers in the evolutionary origin of left-right asymmetry and could offer valuable insights into how genes somehow 'capture' pre-existing phenotypic variation [3] (see §6).…”

“…Two facts render this system highly attractive: (i) in most Heteranthera species, direction of style bend varies at random within a single individual [134], so direction is clearly not inherited. (ii) In Heteranthera multiflora two alleles at a single locus control direction of style bending [50]. Heteranthera multiflora most probably evolved from an ancestor that exhibited monomorphic enantiostyly (styles on flowers of an individual plant bend both right and left), as it has in other monocot clades where dimorphic enantiostyly has evolved [16,134].…”