Reproductive Isolation in the Mimulus guttatus M. nasutus Complex

Abstract

M. guttatus is an outcrossing perennial and reproduces by seeds and vegetative runners. M. nasutus, a daughter species of M. guttatus, is a self-pollinated annual. Altough previous studies have indicated that postmating isolation between the 2 spp. is weak, few hybrids are found in sites where the 2 spp. are adjacent. Ecological studies and field observation indicate that M. guttatus prefers areas with a dependable moisture supply while M. nasutus is restricted to dry summer habitats. M. nasutus flowers earlier than M. guttatus in the growing season, and has a pseudocleistogamous mode of reproduction. The differences in flowering time and mode of reproduction, when combined with the partial habitat isolation and partial postmating barriers, effectively retard the exchange of genes between the 2 spp.