User:Jesham/sandbox

Evolution of heterostyly
Evolution of Heterostyly

Heterostyly has evolved independently in over 25 different plant families, including the Oxalidaceae, Primulaceae, Pontederiaceae, and the Boraginaceae families. It should be noted that these families do not exhibit heterostyly across all species, and some families can exhibit both mating systems, such as the Eichhornia genus (in the Pontederiaceae family). Eichhornia azurea exhibits distyly, whereas another species in the same family, Eichhornia crassipes, demonstrates tristyly. The main reason for heterostyly to have evolved is to promote outcrossing. Several hypotheses have been proposed to explain the repeated independent evolution of heterostyly: 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 sexually monomorphic animal-pollinated species.

Heterostyly is most often seen in actinomorphic flowers presumably because zygomorphic flowers are effective in cross- pollination.

Models for the Evolution of Heterostyly

Current models for evolution include the Pollen Transfer Model and the Selfing Avoidance Model.

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. 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.

The alternative model- the Selfing Avoidance Model- was introduced by Charlesworth and Charlesworth in 1979 using a population genetic approach. The Selfing Avoidance model assumes that the self-incompatibility system was the first trait to evolve and that the physical attribute of reciprocal herkogamy evolved as a response to the former.

Genetic Determination of Heterostyly

The Supergene Model describes how the distinctive floral traits present in distylous flowers can be inherited. This model was first introduced by Ernst in 1955 and was further elaborated by Charlesworth and Charlesworth in 1979. Lewis and Jones in 1992 demonstrated that the supergene is comprised of three linked diallelic loci. The G locus is responsible for determining the characteristic of the gynoecium which includes the style length and incompatibility responses, the P locus determines the pollen size and the pollen’s incompatibility responses, and finally the A locus determines the anther height. These three diallelic loci comprise the S allele and the s alleles segregating at the supergene S locus, which is notated as GPA and gpa, respectively. 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.

Additionally, supergene control is implied for tristyly, but there is no genetic evidence available to support it. A supergene model for tristyly would require the occurrence of two supergenes at the S and M  loci.