Larval fitness and immunogenetic diversity in chytrid-infected and uninfected natterjack toad (Bufo calamita) populations

Chytrid fungus Batrachochytrium dendrobatidis (Bd), an emerging disease, has been decimating amphibian populations around the world for several decades. We quantified aspects of larval fitness, adaptive (major histocompatibility complex) diversity and neutral (microsatellite) diversity in natterjack toad (Bufo calamita) populations in two regions of north-west England. Toads in region one had no evidence of chytrid infection, whereas in region two there was a substantial prevalence of Bd. Larval fitness (growth rate, time to metamorphosis and survival) of B. calamita did not differ between the regions. Genetic diversity at microsatellite loci was much higher in the infected than in the uninfected region, but the converse was true of MHC diversity indicating that genetic drift was unlikely to explain the differences in MHC between the regions. Furthermore, MHC allele frequencies varied significantly between Bd-infected and uninfected populations. Microsatellite diversity was not a robust indicator of larval fitness in these toad populations while MHC genotype frequencies varied in a way that was consistent with directional selection in response to pathogen prevalence. The acquired immune defences may therefore play an important role in determining the susceptibility of amphibian species to chytridiomycosis.