EVOLUTIONARY HISTORY OF THE MATING SYSTEM IN AMSINCKIA (BORAGINACEAE)

A survey of restriction site variation in the chloroplast genome of th e annual plant genus Amsinckia, together with estimation of outcrossin g rates, was conducted to analyze the evolutionary history of rbe mati ng system. Species, and in some cases populations within species, diff er markedly in their mating system. Five taxa are distylous and predom inantly outcrossing, or show mixed mating systems, while the remaining taxa are homostylous and predominantly self-fertilizing Reconstructio n of the molecular phylogeny of the group places different distylous a nd homostylous tars at four separate branch tips. When distyly is trea ted as ancestral in the group, or when the loss of distyly is assumed to be more common than its gain, the results of the phylogenetic analy sis support the hypothesis that the self-fertilizing taxa are of recen t origin from outcrossing relatives. These findings are discussed with respect to theory fur the evolution and breakdown of distyly and the probability of extinction of selling lineages.