Partial self-fertilization and inbreeding depression in two populations of Mimulus guttatus

Abstract

Inbreeding depression is likely to be a common selective force opposing the automatic selective advantage of self-fertilization in self-compatible plants and animals, yet relatively few studies have measured both the breeding system and inbreeding depression in natural populations. In this study, I estimated the frequency of selfing, using data obtained by gel electrophoresis, in two annual populations of the monkeyflower Mimulus guttatus for 2 years, and measured the relative performance of selfed and outcrossed progeny over several stages of the life cycle in both the field and the greenhouse. Rates of outcrossing were not significantly different from 1.0 in either population in 1989, but both populations exhibited significant and moderate amounts of selfing in 1990. Outcrossing rates were significantly different between years for the Cone Peak population but not for the Iron Mountain population. Significant inbreeding depression was detected for almost every component of fitness measured, including germination success, survival to flowering, and flower, fruit and seed production in the native field environments for both populations. The final cumulative value of inbreeding depression, calculated as one minus the relative total seed production of selfed to outcrossed progeny, was 0.69 for Iron Mountain and 0.64 for Cone Peak. Inbreeding depression was also severe in the greenhouse experiments, even though fitness components only up to flower production were measured: 0.52 for Iron Mountain and 0.48 for Cone Peak. These results are consistent with theoretical predictions of the magnitude of inbreeding depression in primarily outcrossing populations, and indicate that inbreeding depression is an important factor in the maintenance of outcrossing in these populations.