A manipulative experiment to estimate biparental inbreeding in monkeyflowers

Biparental inbreeding occurs when plants receive pollen from genetically related neighbors. The frequency of biparental inbreeding in natural populations is unknown but directly relevant to the evolution of plant mating systems. We suggest a simple manipulative experiment to distinguish the effects of biparental inbreeding from those of self-fertilization. The basis of the method is to compare the levels of inbreeding in plants with and without the potential to outcross with genetic relatives. We eliminate the potential for biparental inbreeding by transplanting seedlings to different locations within a population. The level of inbreeding is then estimated from homozygosity at molecular markers. This method is applied in a study of two natural populations of Mimulus guttatus (the yellow monkeyflower) using microsatellites as marker loci. In contrast to previous studies of this species, our study finds no evidence of biparental inbreeding in either population.