Rapid selection against inbreeding in a wild population of a rare frog

Abstract

Populations that are small and isolated can be threatened through loss of fitness due to inbreeding. Nevertheless, an increased frequency of recessive homozygotes could increase the efficiency of selection against deleterious mutants, thus reducing inbreeding depression. In wild populations, observations of evolutionary changes determined by selection against inbreeding are few. We used microsatellite DNA markers to compare the genetic features of tadpoles immediately after hatch with those of metamorphosing froglets belonging to the same cohort in a small, isolated population of the threatened frog Rana latastei. Within a generation, the inbreeding coefficient (F IS) decreased: at hatch, F IS was significantly >0, whereas F IS was <0 after metamorphosis. Furthermore, heterozygosity increased and allelic frequencies changed over time, resulting in the loss of genotypes at metamorphosis that were present in hatchlings. One microsatellite locus exhibited atypically large F ST values, suggesting it might be linked to a locus under selection. These results support the hypothesis that strong selection against the most inbred genotypes occurred among early life-history stages in our population. Selective forces can promote changes that can affect population dynamics and should be considered in conservation planning.

Keywords: Rana latastei; genetic purging; heterozygosity; microsatellites; temporal variation.

Figure 1

Inbreeding coefficient (F_IS) of hatchling (empty diamonds) and metamorphic Rana latastei (solid diamonds) derived from variation at three polymorphic microsatellite loci (RlatCa9, RlatCa27 and RlatCa41). Error bars are 95% CI.

Figure 2

Distribution of simulated F_IS values. The two bold vertical lines indicate the 1% percentiles; the arrow indicates the F_IS value observed in metamorphs. See text for more details on the simulations.

Figure 3

Individuation of loci under selection using Fdist2: plot of F_ST values against heterozygosity estimates (Beaumont and Nichols 1996) for the comparison tadpoles/metamorphs. Each diamond represents a microsatellite marker; the intermediate line is the expected median; upper and lower lines are the limits of the 99% envelope.

Figure 4

Observed frequencies of genotypes at locus RlatCa27 for the two life-history stages. Alleles are enumerated by length in base pairs. Open bars: hatchlings; solid bars: metamorphs.

Figure 5

Box-plots representing the distribution of genotypes of hatchlings and metamorphs along the first component extracted by Correspondence Analysis.