Abstract: Global biodiversity is under threat from introductions of non-native fungal disease. The pathogenic chytrid fungus, Batrachochytrium dendrobatidis (Bd), causes chytridiomycosis – the infectious disease implicated in frog, toad, and salamander population declines and extinctions worldwide. Where this fungus has been introduced, a single hypervirulent variant (Bd-GPL) proliferates through host populations. In the southern Atlantic Forest of Brazil, recent human introduction brought the globally invasive Bd-GPL variant into secondary contact with a distantly related, endemic variant, Bd-Brazil. In most anthropogenically mediated secondary contact scenarios such as this one, the epidemiological and ecological consequences of variant interaction remain unknown. We found that Bd, long considered obligately asexual, is capable of second-generation hybridization following the human-induced contact of divergent lineages. Using whole-genome sequencing of fungal isolates cultured from wild-infected Brazilian frogs, we characterize the hereditary relationships among disease populations in this variant invasion zone. Our analyses reveal regions of the Bd genome that are potentially driving ecological adaptation among invasive and endemic variants. The patterns of hybrid inheritance we observe offer new insights into the genetic underpinnings of fungal reproductive isolation, the process which ultimately results in speciation of emerging fungal diseases. Finally, we compare our hybrid sequences to a newly discovered zone of variant hybridization in South Africa and reveal parallel patterns of second-generation introgression. These newly observed patterns of hybridization we describe are of particular ecological concern because they demonstrate the ability of anthropogenic change to drive novel recombinant genetic variation in a deadly pathogen. These findings show how humans are actively creating new ecological trajectories for emerging diseases, such as chytridiomycosis, by creating novel mating opportunities between previously allopatric variants in different regions of the globe.
