Boreal toad transcriptomic response to Bd infection

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. Overall design: To investigate the transcriptomic responses to Bd infection in tolerant and susceptible boreal toads, we reared toads from both poulations in the lab from wild-caught eggs of the same clutch (sibling biological replicates). A 34 day Bd challenge was conducted with sibling replicates for each site (Colorado and Utah) using Bd isolate JEL 275 from Colorado. Skin and liver tissue was collected from sets of six sibling biological replicates (3 female, 3 male) at day 34 post-Bd innoculation for control and Bd challenge groups, for each source site (24 skin and 24 liver). RNA-seq generated 150 bp paired end reads which were then used in a differential gene expression analysis using an assembled de novo transcriptome.