Mating patterns in self-compatible distylous populations of Amsinckia (Boraginaceae)

Abstract

Progeny tests of naturally pollinated pin- and thrum-form plants of two species of Amsinckia were conducted to determine whether net selfing, random mating, or net disassortative mating occurred in natural populations. The locus determining flower form was used as a marker gene. Amsinckia spectabilis experienced close to random mating in a dense population, but in a small, diffuse population a high rate of selfing was indicated. Pin plants of Amsinckia vernicosa var. furcata experienced 44.3% net disassortative mating, and thrum plants in this species experienced complete (100%) disassortative mating.Compared with A. spectabilis, stigmas and anthers are about twice as far apart in A. vernicosa var. furcata; yet the pollinators are essentially similar, suggesting that greater separation of stigmas and anthers greatly improves the efficiency of distyly at promoting disassortative pollination. The two populations of A. spectabilis studied indicate that autogamy and (or) geitonogamy is much higher in diffuse populations. The results support previous studies that disassortative pollination is greater in thrum-form flowers than in pin flowers. Distyly without self-incompatibility can be an effective outbreeding system but is much more sensitive to variations in floral structure, population density, and pollinator behavior than are self-incompatible breeding systems.