Globally invasive genotypes of the amphibian chytrid outcompete an enzootic lineage in coinfections

Competition between genotypes is likely to be a key driver of pathogen evolution, particularly following a geographical invasion by distant strains. Theory predicts that competition between disease strains will result in the most virulent strain persisting. Despite its evolutionary implications, the role of strain competition in shaping populations remains untested for most pathogens. We experimentally investigated the in vivo competitive differences between two divergent lineages of the amphibian-killing chytrid fungus (Batrachochytrium dendrobatidis, Bd). These Bd lineages are hypothesized to have diverged in allopatry but been recently brought back into secondary contact by human introduction. Prior studies indicate that a panzootically-distributed, global lineage of Bd was recently introduced into southern Brazil, and is competitively excluding enzootic lineages in the southern Atlantic Forest. To test for differences in competitive ability between invasive and enzootic Brazilian Bd isolates, we coinfected a model host frog system which we developed for this study (Hymenochirus curtipes). We tracked isolate-specific zoospore production over the course of the coinfection experiment with chip-based digital PCR (dPCR). The globally invasive panzootic lineage had a competitive advantage in spore production especially during the first one to four weeks of infection, and on frogs that eventually succumbed to Bd infection. Our study provides new evidence that competitive pressure resulting from the human movement of pathogen strains can rapidly alter the genetics, community dynamics and spatial epidemiology of pathogens in the wild.