MAGNITUDE AND TIMING OF INBREEDING DEPRESSION IN A DIPLOID POPULATIONOF EPILOBIUM-ANGUSTIFOLIUM (ONAGRACEAE)

Evolutionary theory predicts a negative correlation between the rate o f self-fertilization and the magnitude of inbreeding depression in nat ural plant populations. We examined the mating system and inbreeding d epression at four life stages in a diploid population of Epilobium ang ustifolium (Onagraceae). The multilocus rate of self-fertilization, in ferred from the segregation of allozymes at two polymorphic loci (Pgi- 2, Mdh-2) among seedlings from 22 open-pollinated plants, was 0.06 (SE +/- 0.10). However, after adjusting for differential mortality of sel fed progeny prior to the seedling stage, the selfing rate was 0.45. Th e parental inbreeding coefficient (F) was - 0.26 (SE +/- 0.103) indica ting more heterozygotes than in a randomly mating population. The magn itude of inbreeding depression at seed maturation, seed germination, j uvenile survival and dry mass at 8 weeks was measured in a greenhouse experiment. Thirty-four plants, grown from open-pollinated seed, were each self-fertilized and randomly outcrossed. We observed significant inbreeding depression at all four stages, although its magnitude diffe red significantly among maternal plants and among stages. Inbreeding d epression was largest during seed maturation (0.87), where seed-set in outcrossed flowers (65.7 per cent) was eight times higher than in sel fed flowers (8.3 per cent). Inbreeding depression for dry mass after 8 weeks (0.50) was at least twice that for germination (0.25) and survi val 0.15). The magnitude of inbreeding depression for each of the 34 m aternal parents was not significantly correlated across life stages, s uggesting that its genetic basis varies across the life cycle. Our res ults indicate that E, angustifolium exhibits high levels of inbreeding depression, much of which is expressed early in the life cycle, durin g seed maturation, consistent with results for other long-lived, outcr ossing species. However, given its moderate rate of self-fertilization , the total inbreeding depression (0.95) maintained in E. angustifoliu m is very high in comparison to other angiosperms.