On the evolution of distyly and morph ratio distortion in Lithospermum caroliniense

Abstract

Two conflicting models presently exist describing the evolutionary history of heterostyly, a condition in which there presents two morphs -- different plant sexual organ structure that some individuals within the same species exhibit as a means of promoting outcrossing. The charlesworth model (1979) and the Lloyd-Webb model (1992) differ in that the latter assumes the ancestral character state was one of approach herkogamy, whereas the former assumes it was homostyly. One such heterostylic species, Lithospermum caroliniense (Walt.) MacMill. (Boraginaceae), is unique in that it additionally exhibits morph ratio distortion, that is, in natural populations, one morph is disproportionately represented over the other morph, which is of particular interest because inverse frequency dependent selection should presumably shape a 1:1 morph ratio. In this paper, we discuss the Levin (1968) model of morph ratio distortion in L. caroliniense, and propose the superthrum model, in which we hypothesize the existence of superthrums, a genetically unique form of the thrum morph that, after 5 generations, produces distortion to the degree recorded by several field studies (Cartmill and Murray 2011; Levin 1968; Levin 1972).