Do Plant Populations Purge Their Genetic Load? Effects of Population Size and Mating History on Inbreeding Depression

Abstract

▪ Abstract 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 (6 studies), but these declines are typically modest (5–10%). Meta-analyses of results from 45 populations in 11 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 somewhat less likely to show purging than annuals (2 of 10 vs. 7 of 14). We conclude that 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. Future studies should examine male and female fitness traits over the entire life cycle, estimate mating histories at all levels (i.e. population and families within populations), report data necessary for meta-analysis, and statistically test for purging of genetic loads.

An investigation of inbreeding depression and purging in captive pedigreed populations

“…Byers and Waller, 1999;Hedrick and Kalinowski, 2000;Keller and Waller, 2002) and threatens the survival of small populations, both wild and captive. However, since inbreeding increases the frequency of the deleterious homozygous genotype, selection against deleterious alleles is also increased during inbreeding (Crow and Kimura, 1970), thereby providing an opportunity for a population to be 'purged' of its mutational load (Lande and Schemske, 1985;Barrett and Charlesworth, 1991).…”

“…Saccheri et al, 1996;Lacy and Ballou, 1998;Byers and Waller, 1999;Swindell and Bouzat, 2006), selection in these studies was fairly intense and it would be inappropriate to extrapolate directly to populations of conservation concern. For example, zoo conditions are often benign and hence the selection they exert may not be sufficient to cause purging (but see King, 1939;Gilligan et al, 2003).…”

The Bletia catenulata ornamental orchid is self-compatible but pollinator-dependent for reproduction

“…The endogamic depression observed in P. corcovadensis is usually found in predominantly allogamous plant populations (Wiens et al 1987, Husband & Schemske 1996, Byers & Waller 1999. Here, despite the intense presence of unviable seeds or infertile ovules, higher germination percentages were obtained by self-pollination and geitonogamy originated seeds.…”

Variation of selfing rate and inbreeding depression among individuals and across generations within an admixed Cedrus population

“…Indeed, we detected no purge of ID at this short time scale. The effectiveness of purging is not frequently observed in plant populations, even when inbreeding occurs (Byers and Waller, 1999). Comparing selfed and crossed progenies obtained in a natural population of P. sylvestris, Koelewijn et al (1999) observed an effect of ID on survival and seed set up to an age of 23 years, but no significant differences in height and flowering, indicating that high levels of ID both for early stage (seed production) and later stages of the life cycle were maintained in the population in the long run.…”

“…Effective purging of inbreeding depression (ID) depends not only on the mating system that generates inbred individuals but also on the genetic make-up of ID (Byers and Waller, 1999). ID that is caused by a small number of recessive genes that have major deleterious effects on fitness is expected to vary among individuals depending on their genotype at these few major genes and, therefore, to respond rapidly to selection.…”