Heterostylous Genetic Polymorphisms: Model Systems for Evolutionary Analysis

Abstract

Heterostyly is a genetic polymorphism in which plant populations are composed of two (distyly) or three (tristyly) morphs that differ reciprocally in the heights of stigmas and anthers in flowers (Fig. 1). The style-stamen polymorphism is usually accompanied by a sporophytically controlled, diallelic self-incompatibility system that prevents self- and intramorph fertilizations, and a suite of ancillary morphologi- cal polymorph isms, particularly of the stigmas and pollen of floral morphs. Heteros- tyly is reported from approximately 25 angiosperm families and has usually been viewed as a floral device that promotes outcrossing, hence reducing the harmful effects of close inbreeding in plant populations.