Abstract

Plant populations of many species are reproductively subdivided into morphologically distinct mating groups. These polymorphic sexual systems have been of considerable interest to evolutionary biologists since Darwin’s classic work “Different Forms of Flowers on Plants of the Same Species” (1877). Indeed, many fundamental concepts in genetics, including linkage, supergenes, epistasis and frequency-dependent selection were developed, in part, from early studies of plants with sexual polymorphisms. In this talk I will review our recent studies on two of the most attractive sexual polymorphisms for evolutionary analysis – dioecy and heterostyly – and address several outstanding issues concerned with their evolution and maintenance. For dioecy, I discuss the demographic and genetic mechanisms governing biased sex ratios in sexually dimorphic populations, and recent insights into the evolution of sex chromosomes, particularly Y-chromosome degeneration and its potential as a mechanism causing female-biased sex ratios in Rumex. For heterostyly, I examine the ecological conditions favouring the evolutionary breakdown of this complex floral polymorphism, and the demographic and genomic consequences of the transition from outcrossing to selfing in Eichhornia. I also report on our recent efforts to map the genetic architecture of tristyly and the selfing variants causing transitions to autogamy. Throughout, I stress the value of intra-specific variation for studies in ecological genetics and evolutionary genomics.