Inbreeding depression and partial selfing: evolutionary implications of mixed-mating in a coastal endemic, silene douglasii var. oraria (Caryophyllaceae)

Recent studies have found moderate to high levels of selfing in plants despite high inbreeding depression. Because both factors influence the evolution and persistence of rare plants, we conducted glasshouse and field studies of pollination and inbreeding in Silene douglasii var. oraria, a perennial tetraploid endemic to coastal prairies. We detected: (i) variation in reproduction or inbreeding depression among life stages, years and maternal families; (ii) partial selfing yet higher relative fitness in outcrossed than selfed progeny; (iii) differing values of selfing and inbreeding depression using population means vs. matched maternal families. Fruit and seed production varied significantly with pollination treatment and year in flowers manipulated in situ during three seasons of growth. Hand-pollinations providing pollen in excess of ovule production in 1996 yielded more seeds than marked, open-pollinated flowers, implying pollen limitation of seed production. However, among-year differences in reproductive success after open-pollination (i.e. values equivalent to autogamy, selfing or outcrossing) suggest that pollination levels also vary temporally. In pollinations matched by maternal family, selfing yielded significantly fewer seeds than outcrossing. Fitness differences between inbred and outbred progeny were significant (P < 0.05) for seed production, percentage germination, and biomass or fecundity, but not for survival. Maternal family data gave selfing rates intermediate between obligate outcrossers and predominant selfers (S = 0.34-0.51), but population-wide means gave unusually high values (S = 1.1-1.6). Cumulative inbreeding depression was 76% for maternal families, and 70-85% using population means; in all cases, inbreeding depression values were high in early and late life stages, and lowest for survival. Thus far, reproductive assurance offers the most cogent explanation for the coexistence of moderate selfing and high inbreeding depression in this strongly protandrous Silene once thought to be highly outcrossing. This possibility merits further study in other rare plants with mixed-mating systems, where inbreeding depression and pollinator scarcity may both compromise population persistence and raise the threshold below which selfing is favoured by evolution.