Larval fitness, microsatellite diversity and MHC class II diversity in common frog (Rana temporaria) populations

There is increasing interest in using functional rather than neutral loci to assess the genetic health of wild populations. We compared growth and survival of urban and rural frog larvae under three temperature regimes, to vary stress levels, and measured genetic diversities at nine microsatellites and at one major histocompatibility complex (MHC) class II locus. We found no significant differences between urban and rural frogs in larval survivorship, nor in microsatellite and MHC diversities. However, mean larval growth rates were significantly higher (by 4.5-18.9% according to temperature) in urban compared with rural populations. Microsatellite and MHC diversities were uncorrelated, though pairwise inter-site F(ST) estimates based on the two types of loci were correlated. MHC F(ST) estimates averaged about twice as high as those based on microsatellites. There was no evidence of isolation by distance with either marker. There were no correlations at the population level between any larval fitness attribute (growth rate or survival) and any diversity estimate (microsatellite or MHC). There was, however, a weak correlation at the sibship level between mean growth rate and microsatellite expected heterozygosity. MHC alleles varied among dying larvae and survivors, and in one case between urban and rural sites, with one allele being underrepresented in dying larvae and one allele only occurring in the rural sites. Drift was probably the primary cause of genetic population structure at both types of loci. The use of functional loci to assess population genetic health should focus on the roles of specific alleles as well as overall diversity.