Magnitude and timing of inbreeding depression in a diploid population of Epilobium angustifolium (Onagraceae)

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.

INBREEDING DEPRESSION IN GYNODIOECIOUSLOBELIA SIPHILITICA: AMONG-FAMILY DIFFERENCES OVERRIDE BETWEEN-MORPH DIFFERENCES

“…Thus, we had six families representing the female lineage and 12 representing the hermaphrodite lineage. The number of families we used is thus comparable to previous studies on amongfamily differences in inbreeding depression, where the number of families has varied from six to 34 (Sakai et al 1989;Agren and Schemske 1993;Husband and Schemske 1995;Carr et al 1997;Dudash et al 1997).…”

“…Hence, one possibility for why hermaphrodite lineages showed a trend toward higher values of inbreeding depression than female lineages is that one or more of the loci affecting fitness were acting in an overdominant manner. However, this suggestion goes against the view that most inbreeding depression is caused by partially dominant rather than overdominant loci (Charlesworth and Charlesworth 1987;Husband and Schemske 1995;Johnston and Schoen 1996).…”

“…Our results contradict this prediction: The lineages did not differ in their levels of inbreeding depression. In fact, for the self/near comparisons we observed slightly, but nonsignificantly, higher inbreeding depression in hermaphrodite lineages than in female lineages, Although Husband and Schemske's (1996) review of inbreeding depression in 54 species revealed a significant negative correlation between inbreeding and inbreeding depression, several studies in addition to ours have shown a weak correlation or no correlation, suggesting that the relationship is a complicated one. For example, a significant correlation between prior inbreeding and inbreeding depression in several taxa of the Mimulus guttatus species complex only existed for one of the five fitness traits measured (Latta and Ritland 1994).…”

“…The prediction of a negative correlation between the level of inbreeding depression and the degree of inbreeding has been tested by comparing plant species with different mating systems or comparing populations within a species with different degrees of selfing. The majority of these studies provide support for the prediction (Holtsford and Ellstrand 1990;Dole and Ritland 1993;Husband andSchemske 1995, 1996;Carr and Dudash 1996;Johnston and Schoen 1996;Affre and Thompson 1997).Recent theoretical studies have emphasized that familylevel variation in inbreeding depression is not equal to the inbreeding depression in the population as a whole (Holsinger 1988(Holsinger , 1991 Uyenoyama and Waller 1991a,b,c;Uyenoyama et al 1993). According to these models, variation in inbreeding depression at the family level is produced by associations that develop between loci determining the mating system and loci determining the probability of survival (inbreeding depression).…”

“…The prediction of a negative correlation between the level of inbreeding depression and the degree of inbreeding has been tested by comparing plant species with different mating systems or comparing populations within a species with different degrees of selfing. The majority of these studies provide support for the prediction (Holtsford and Ellstrand 1990;Dole and Ritland 1993;Husband andSchemske 1995, 1996;Carr and Dudash 1996;Johnston and Schoen 1996;Affre and Thompson 1997).…”