
	The evolution of plant mating systems The evolutionary causes and consequences of inbreeding depression Small and/or inbred populations face multiple hazards including the accumulation of deleterious mutations (increasing the genetic load) and the increased expression of these mutations upon inbreeding (inbreeding depression). The genetic load and inbreeding depression are thus central to mating system evolution and many issues in conservation biology. While inbred populations may eliminate some of their load via selection against deleterious recessive alleles, such purging may be inefficient in small populations with a history of inbreeding. In addition, the efficiency of purging depends upon the genetic architecture of the load. In lab experiments with the fast-cycling annual Brassica rapa, we are attempting to track how the genetic load shifts in response to differences in population size and levels of inbreeding. Evaluating how much purging occurs in small, inbred populations compliments existing theory and enhances our understanding of the short-term dynamics of mating system evolution and the genetic hazards faced by small populations. Return to top. The coevolution of inbreeding and inbreeding depression Inbreeding depression critically influences both mating system evolution and the persistence of small populations prone to accumulate mutations. Under some circumstances, however, inbreeding will tend to purge populations of enough deleterious recessive mutations to reduce inbreeding depression (ID). The extent of purging depends on many population and genetic factors, making it impossible to make universal predictions. We review 52 studies that compare levels of ID among species, populations and lineages inferred to differ in inbreeding history. Fourteen of 34 studies comparing ID among populations and species found significant evidence for purging. Within populations, many studies report among family variation in ID and 6 of 18 studies found evidence for purging among lineages. Regression analyses suggest that purging is most likely to ameliorate ID for early traits but these declines are typically modest (5-10%). Meta-analyses that combine results over a somewhat different set of studies reveal no significant overall evidence for purging but rather the opposite tendency for more selfing populations to experience higher ID for early traits. The likelihood of finding purging does not vary systematically with experimental design or whether early or late traits are considered. Perennials are less likely to show purging than annuals (2 of 10 vs. 7 of 14). Although these results doubtless reflect variation in population and genetic parameters, they also suggest that purging is an inconsistent force within populations. Such results also imply that attempts to deliberately reduce the load via inbreeding in captive rearing programs may be misguided. Inbreeding depression also strongly affects the evolution of plant mating systems by selecting against direct or geitonogamous self-fertilization and biparental inbreeding. We therefore investigate levels of inbreeding depression in both cleistogamous species (which regularly self via closed flowers, like Impatiens) and those that regularly outcross (because of self-incompatibility or dioecy). We have demonstrated that even species that regularly self-fertilize often maintain moderate to high levels of inbreeding depression and that inbreeding depression varies considerably among individuals and populations (Byers & Waller 1999). This variation is important in that it provides insights into how inbreeding and inbreeding depression coevolve. We are particularly interested in how associations between inbreeding depression and mating system loci affect mating system evolution (see Uyenoyama et al. 1993). NSF Fellow Janet Steven has recently completed her PhD thesis, where she explored how inflorescence form and levels of pollinator service affect inbreeding depression and other fitness components led to the evolution of dioecy in the genus Thalictrum. Recent Publications Steven, J.C., T.P. Rooney, O.D. Boyle, and D.M. Waller. 2003. Density-dependent pollinator visitation and self-incompatibility in upper Great Lakes populations of Trillium grandiflorum. J. Torrey Bot. Soc. 130: 23-29. Keller, L. and D.M. Waller. 2002. Inbreeding effects in wild populations. Trends in Ecology & Evolution 17: 230-241. Byers, D., and D.M. Waller. 1999. Do plant populations purge their genetic load? Effects of population size and mating history on inbreeding depression. Ann. Rev. Ecol. Syst. 30:479-513. Uyenoyama, M.K., K.E. Holsinger, and D.M. Waller. 1993. Ecological and genetic factors directing the evolution of self-fertilization. Oxford Surveys in Evolutionary Biology 9: 327-381. See entire list of lab publications here Return to top.
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The evolution of plant mating systems

The evolutionary causes and consequences of inbreeding depression

Small and/or inbred populations face multiple hazards including the accumulation of deleterious mutations (increasing the genetic load) and the increased expression of these mutations upon inbreeding (inbreeding depression). The genetic load and inbreeding depression are thus central to mating system evolution and many issues in conservation biology. While inbred populations may eliminate some of their load via selection against deleterious recessive alleles, such purging may be inefficient in small populations with a history of inbreeding. In addition, the efficiency of purging depends upon the genetic architecture of the load. In lab experiments with the fast-cycling annual Brassica rapa, we are attempting to track how the genetic load shifts in response to differences in population size and levels of inbreeding. Evaluating how much purging occurs in small, inbred populations compliments existing theory and enhances our understanding of the short-term dynamics of mating system evolution and the genetic hazards faced by small populations.

Return to top.

The coevolution of inbreeding and inbreeding depression

Inbreeding depression critically influences both mating system evolution and the persistence of small populations prone to accumulate mutations. Under some circumstances, however, inbreeding will tend to purge populations of enough deleterious recessive mutations to reduce inbreeding depression (ID). The extent of purging depends on many population and genetic factors, making it impossible to make universal predictions. We review 52 studies that compare levels of ID among species, populations and lineages inferred to differ in inbreeding history. Fourteen of 34 studies comparing ID among populations and species found significant evidence for purging. Within populations, many studies report among family variation in ID and 6 of 18 studies found evidence for purging among lineages. Regression analyses suggest that purging is most likely to ameliorate ID for early traits but these declines are typically modest (5-10%). Meta-analyses that combine results over a somewhat different set of studies reveal no significant overall evidence for purging but rather the opposite tendency for more selfing populations to experience higher ID for early traits. The likelihood of finding purging does not vary systematically with experimental design or whether early or late traits are considered. Perennials are less likely to show purging than annuals (2 of 10 vs. 7 of 14). Although these results doubtless reflect variation in population and genetic parameters, they also suggest that purging is an inconsistent force within populations. Such results also imply that attempts to deliberately reduce the load via inbreeding in captive rearing programs may be misguided.

Inbreeding depression also strongly affects the evolution of plant mating systems by selecting against direct or geitonogamous self-fertilization and biparental inbreeding. We therefore investigate levels of inbreeding depression in both cleistogamous species (which regularly self via closed flowers, like Impatiens) and those that regularly outcross (because of self-incompatibility or dioecy). We have demonstrated that even species that regularly self-fertilize often maintain moderate to high levels of inbreeding depression and that inbreeding depression varies considerably among individuals and populations (Byers & Waller 1999). This variation is important in that it provides insights into how inbreeding and inbreeding depression coevolve. We are particularly interested in how associations between inbreeding depression and mating system loci affect mating system evolution (see Uyenoyama et al. 1993). NSF Fellow Janet Steven has recently completed her PhD thesis, where she explored how inflorescence form and levels of pollinator service affect inbreeding depression and other fitness components led to the evolution of dioecy in the genus Thalictrum.

Recent Publications
Steven, J.C., T.P. Rooney, O.D. Boyle, and D.M. Waller. 2003. Density-dependent pollinator visitation and self-incompatibility in upper Great Lakes populations of Trillium grandiflorum. J. Torrey Bot. Soc. 130: 23-29.

Keller, L. and D.M. Waller. 2002. Inbreeding effects in wild populations. Trends in Ecology & Evolution 17: 230-241.

Byers, D., and D.M. Waller. 1999. Do plant populations purge their genetic load? Effects of population size and mating history on inbreeding depression. Ann. Rev. Ecol. Syst. 30:479-513.

Uyenoyama, M.K., K.E. Holsinger, and D.M. Waller. 1993. Ecological and genetic factors directing the evolution of self-fertilization. Oxford Surveys in Evolutionary Biology 9: 327-381.

See entire list of lab publications here

Return to top.

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