Abstract
Evolutionary theory predicts a negative correlation between the rate of self-fertilization and the magnitude of inbreeding depression in natural plant populations. We examined the mating system and inbreeding depression at four life stages in a diploid population of Epilobium angustifolium (Onagraceae). The multilocus rate of self-fertilization, inferred 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 selfed progeny prior to the seedling stage, the selfing rate was 0.45. The parental inbreeding coefficient (F) was -0.26 (SE ± 0.103) indicating more heterozygotes than in a randomly mating population. The magnitude of inbreeding depression at seed maturation, seed germination, juvenile 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 differed significantly among maternal plants and among stages. Inbreeding depression was largest during seed maturation (0.87), where seed-set in outcrossed flowers (65.7 per cent) was eight times higher than in selfed 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 survival (0.15). The magnitude of inbreeding depression for each of the 34 maternal parents was not significantly correlated across life stages, suggesting that its genetic basis varies across the life cycle. Our results indicate that E. angustifolium exhibits high levels of inbreeding depression, much of which is expressed early in the life cycle, during seed maturation, consistent with results for other long-lived, outcrossing species. However, given its moderate rate of self-fertilization, the total inbreeding depression (0.95) maintained in E. angustifolium is very high in comparison to other angiosperms.
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Acknowledgements
We thank S. Curtis, J. Ross, M. Conrath and D. Ewing for technical assistance. This study was supported by a Royalty Research Fund grant to D.S. and a Postdoctoral Fellowship from the Natural Sciences and Engineering Research Council of Canada to B.H.
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Husband, B., Schemske, D. Magnitude and timing of inbreeding depression in a diploid population of Epilobium angustifolium (Onagraceae). Heredity 75, 206–215 (1995). https://doi.org/10.1038/hdy.1995.125
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