| Abstract: | The pathogenic chytrid fungus Batrachochytrium dendrobatidis
(Bd) can cause precipitous population declines in its amphibian hosts. Responses
of individuals to infection vary greatly with the capacity of their immune
system to respond to the pathogen. We used a combination of comparative and
experimental approaches to identify major histocompatibility complex class II
(MHC-II) alleles encoding molecules that foster the survival of Bd-infected
amphibians. We found that Bd-resistant amphibians across four continents share
common amino acids in three binding pockets of the MHC-II antigen-binding
groove. Moreover, strong signals of selection acting on these specific sites
were evident among all species co-existing with the pathogen. In the laboratory,
we experimentally inoculated Australian tree frogs with Bd to test how each
binding pocket conformation influences disease resistance. Only the conformation
of MHC-II pocket 9 of surviving subjects matched those of Bd-resistant species.
This MHC-II conformation thus may determine amphibian resistance to Bd, although
other MHC-II binding pockets also may contribute to resistance. Rescuing
amphibian biodiversity will depend on our understanding of amphibian immune
defence mechanisms against Bd. The identification of adaptive genetic markers
for Bd resistance represents an important step forward towards that goal. |
