PARTIAL SELF-FERTILIZATION AND INBREEDING DEPRESSION IN 2 POPULATIONSOF MIMULUS-GUTTATUS

Inbreeding depression is likely to be a common selective force opposin g the automatic selective advantage of self-fertilization in self-comp atible plants and animals, yet relatively few studies have measured bo th the breeding system and inbreeding depression in natural population s. In this study, I estimated the frequency of selfing, using data obt ained by gel electrophoresis, in two annual populations of the monkeyf lower Mimulus guttatus for 2 years, and measured the relative performa nce of selfed and outcrossed progeny over several stages of the life c ycle in both the field and the greenhouse. Rates of outcrossing were n ot 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 comp onent of fitness measured, including germination success, survival to flowering, and flower, fruit and seed production in the native field e nvironments for both populations. The final cumulative value of inbree ding depression, calculated as one minus the relative total seed produ ction of selfed to outcrossed progeny, was 0.69 for Iron Mountain and 0.64 for Cone Peak. Inbreeding depression was also severe in the green house experiments, even though fitness components only up to flower pr oduction were measured: 0.52 for Iron Mountain and 0.48 for Cone Peak. These results are consistent with theoretical predictions of the magn itude of inbreeding depression in primarily outcrossing populations, a nd indicate that inbreeding depression is an important factor in the m aintenance of outcrossing in these populations.