Project description:Boreal toads (Anaxyrus boreas boreas) of the Southern Rocky Mountain population are declining due to the introduction of the chytrid fungus Batrachochytrium dendrobatidis (Bd). Boreal toads in Colorado are generally susceptible to Bd infection, but some Bd-tolerant populations persist in parts of the Southern Rocky Mountain and broader Eastern boreal toad population. We conducted a Bd challenge with lab-reared sibling toads from Bd-susceptible Colorado and purportedly Bd-tolerant Utah populations and report on transcriptomic responses to Bd during late infection in skin and liver tissue. Fewer immune genes were expressed in response to Bd in Colorado toads, but with greater upregulation compared to Utah toads, indicating a dysregulated immune response. Signatures of Bd-tolerance in Utah toads included more moderate upregulation in immune gene expression and a significantly enriched suite of gene functions related to innate and adaptive immune responses. Our transcriptomic results support the notion that Utah toads are tolerant to Bd, rather than resistant, carrying Bd loads similar to Colorado yet having a unique transcriptomic profile and presenting minimal clinical signs of chytridiomycosis. We conclude that closely related populations have divergent transcriptomic responses to Bd with a dysregulated immune response in Bd-susceptible toads.
Project description:The amphibian skin pathogen Batrachochytrium dendrobatidis (Bd) has caused an ongoing biodiversity crisis, including in the locally endangered Colorado boreal toad (Anaxyrus boreas boreas). Although researchers have investigated the bacteria living on amphibian skin and how they interact with Bd, there is less information about fungal community members. This study describes (1) the diversity of culturable fungi from boreal toad skin, (2) which subset of these isolates is Bd-inhibitory, and (3) how Bd affects these isolates' growth and morphology. Most isolates were from the orders Capnodiales, Helotiales, and Pleosporales. Of 16 isolates tested for Bd-inhibition, two from the genus Neobulgaria and three from Pseudeurotium inhibited Bd. Fungal growth in co-culture with Bd varied with weak statistical support for Neobulgaria sp. (isolate BTF_36) and cf Psychrophila (isolate BTF_60) (p-values = 0.076 and 0.092, respectively). Fungal morphology remained unchanged in co-culture with Bd, however, these results could be attributed to low replication per isolate. Nonetheless, two fungal isolates' growth may have been affected by Bd, implying that fungal growth changes in Bd co-culture could be a variable worth measuring in the future (with higher replication). These findings add to the sparse but growing literature on amphibian-associated fungi and suggest further study may uncover the relevance of fungi to amphibian health and Bd infection.