Evolution and Phylogeny of Self-Incompatibility Systems in Angiosperms

Abstract

The evolution of self-incompatibility (SI) has been debated since its genetic bases were first fully described over half a century ago. The diversity of SI systems and their scattered distribution among different flowering plant lineages suggest that SI has evolved independently on many different occasions. Recent advances in our understanding of the evolutionary relationships among angiosperms together with a growing understanding of the genetic, cellular, and molecular control of SI systems have now allowed objective insights to be made into the evolution of SI. Mapping the presence of the various SI systems within robust angiosperm phylogenies allows us to view the phylogenetic distribution of SI as a whole, and compare the distribution and phylogenetic relationships of the different forms of SI. Here we discuss the diversity of SI systems in flowering plants and current hypotheses on their evolution and phylogenetic relationships. Using the APG II tree we map the distribution of SI and self-sterility and show: (1) Self-sterility and SI are well represented among basal angiosperm lineages, suggesting that self-sterility/SI rather than self-compatibility (SC) was the ancestral angiosperm mating system; (2) late-acting ovarian SI is the basal SI state; (3) gametophytic SI systems evolved independently in the earliest diverging angiosperm lineages; (4) heteromorphic sporophytic SI evolved on numerous different occasions in eudicots and probably only once in monocots; and (5) different forms of homomorphic sporophytic SI evolved relatively recently in eudicots.