Body size mediates latitudinal population differences in the response to chytrid fungus infection in two amphibians

Abstract

Factors behind intraspecific variation in sensitivity to pathogens remain poorly understood. We investigated how geographical origin in two North European amphibians affects tolerance to infection by the chytrid fungus Batrachochytrium dendrobatidis (Bd), a generalist pathogen which has caused amphibian population declines worldwide. We exposed newly metamorphosed individuals of moor frog Rana arvalis and common toad Bufo bufo from two latitudinal regions to two different BdGPL strains. We measured survival and growth as infections may cause sub-lethal effects in fitness components even in the absence of mortality. Infection loads were higher in B. bufo than in R. arvalis, and smaller individuals had generally higher infection loads. B. bufo had high mortality in response to Bd infection, whereas there was little mortality in R. arvalis. Bd-mediated mortality was size-dependent and high-latitude individuals were smaller leading to high mortality in the northern B. bufo. Bd exposure led to sub-lethal effects in terms of reduced growth suggesting that individuals surviving the infection may have reduced fitness mediated by smaller body size. In both host species, the Swedish Bd strain caused stronger sublethal effects than the British strain. We suggest that high-latitude populations can be more vulnerable to chytrids than those from lower latitudes and discuss the possible mechanisms how body size and host geographical origin contribute to the present results.

Batrachochytrium dendrobatidis strain affects transcriptomic response in liver but not skin in latitudinal populations of the common toad (Bufo bufo)

“…We sequenced RNA samples from 106 individuals infected with a Swedish or UK Bd strain or sham-infected with Bd medium. All animal samples were collected from a previous experiment by our group 37 . The samples consisted of paired liver and skin samples from 75 individuals, plus 11 unique liver and 20 unique skin samples (N = 181 samples, Table 1 , Supplementary Table S1 ).…”

“…The lower genetic diversity of the northern populations is reflected in the reduced diversity of MHC class IIB haplotypes in the north as compared to the south of Sweden 22 . The two populations also differ in developmental strategies, with northern populations reaching metamorphosis at a smaller mass 37 . Meurling et al 37 challenged newly metamorphosed common toads with two different strains of Bd- GPL, a 2008 UK isolate (UKMal 01) from a wild alpine newt ( Icthyosaura alpestris ) and a 2015 Swedish isolate (SWED-40-5) from a wild green toad ( Bufotes viridis , see O’ Hanlon et al 7 for information on the isolates).…”

“…The two populations also differ in developmental strategies, with northern populations reaching metamorphosis at a smaller mass 37 . Meurling et al 37 challenged newly metamorphosed common toads with two different strains of Bd- GPL, a 2008 UK isolate (UKMal 01) from a wild alpine newt ( Icthyosaura alpestris ) and a 2015 Swedish isolate (SWED-40-5) from a wild green toad ( Bufotes viridis , see O’ Hanlon et al 7 for information on the isolates). They demonstrated that survival was size-mediated, with smaller individuals (primarily the ones from the North) suffering higher mortality and the UK strain causing higher mortality overall.…”

“…The present study aims to elucidate the mechanisms behind the population and treatment ( Bd strain) differences observed in the previous infection experiment 37 , by examining the transcriptional responses of the metamorphs’ skin and liver to infection by Bd . As the skin is the primary site of fungal infection, the transcriptional consequences of Bd infection in the skin are of great interest.…”

“…Additionally, the liver is a site of macrophage production, making it a relevant organ for analysis in the context of disease pathophysiology 38 . Based on the survival results of Meurling et al 37 , we hypothesized to see a stronger immune response in southern individuals as well as in those infected with the Swedish strain, as those two groups exhibited higher survival during the infection experiment.…”