Heterostyly

In a heterostylous species, two or three morphological types of flowers, termed "morphs", exist in the population.

Heterostyly has evolved independently in over 25 different plant families, including the Oxalidaceae, Primulaceae, Pontederiaceae, and the Boraginaceae.

Several hypotheses have been proposed to explain the repeated independent evolution of heterostyly as opposed to homostylous self-incompatibility: 1) that heterostyly has evolved as a mechanism to reduce male gamete wastage on incompatible stigmas and to increase fitness through male function through reciprocal herkogamy; 2) heterostyly evolved as a consequence of selection for heteromorphic self-incompatibility between floral morphs in distylous and tristylous species; and, 3) that the presence of heterostyly in plants reduces the conflict that might occur between the pollen dispersal and pollen receipt functions of the flower in a homomorphic animal-pollinated species.

The pollen transfer model proposed by Lloyd and Webb in 1992 is based on the efficacy of cross-pollen transfer, and suggests that the physical trait of reciprocal herkogamy evolved first, and then the diallelic incompatibility arose afterwards as a response to the evolution of the reciprocal herkogamy.

[6] This model is similar to Darwin's 1877 idea that reciprocal herkogamy evolved as a direct response to the selective forces that increase accuracy of pollen transfer.

[11] Genetic determination The supergene model describes how the distinctive floral traits present in distylous flowers can be inherited.

There have been other propositions that there are possibly 9 loci responsible for the distyly supergene in Primula, but there has been no convincing genetic data to support this.