Do plant populations purge their genetic load? Effects of population size and mating history on inbreeding depression

Inbreeding depression critically influences both mating system evolution an d 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, mak ing it impossible to make universal predictions. We review 52 studies that compare levels of ID among species, populations, and lineages inferred to d iffer in inbreeding history. Fourteen of 34 studies comparing ID among popu lations and species found significant evidence for purging. Within populati ons, 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 pur ging, but rather the opposite tendency, for more selfing populations to exp erience higher ID for early traits. The likelihood of finding purging does not vary systematically with experimental design or whether early or late t raits are considered. Perennials are somewhat less likely to show purging t han 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 res ults also imply that attempts to deliberately reduce the load via inbreedin g in captive rearing programs may be misguided. Future studies should exami ne male and female fitness traits over the entire life cycle, estimate mati ng histories at all levels (i.e. population and families within populations ), report data necessary for meta-analysis, and statistically test for purg ing of genetic loads.