The role of sloughing on the pathophysiology of chytridiomycosis in amphibians

Abstract

Amphibians periodically remove the dead skin layer through a process known as sloughing.Because of their thin semi-permeable skin, sloughing is important to maintain skin integrity and biological functions such as ionic and osmotic regulation. However, sloughing has been shown to affect cutaneous ion and water movements, temporarily disrupting osmotic homeostasis. Since sloughing occurs regularly in amphibians, every 1-14 days depending on the species and temperature, it is important to assess the potential magnitude of osmotic/ionic disturbances that may accompany normal sloughing regimes and the mechanisms underpinning the temporary loss of homeostasis. Thus, the first aim of this thesis (Chapter 2) was to investigate the effects of sloughing on the expression, abundance and activities of cutaneous ion transport proteins in cane toads (Rhinella marina). Sodium efflux and skin permeability significantly increased during sloughing. However, internal electrolyte balance remained stable after the sloughing period, via increase in epithelial transport protein abundance and activity in the skin, which lead to a commensurate increase in active sodium uptake back into the skin. This suggests amphibians actively maintain osmotic homeostasis despite having a 'leaky' skin.Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has been linked to worldwide amphibian population declines. Bd disrupts the skin's ionic and osmotic function, and blood electrolyte levels. Infected amphibians increase the rate of sloughing in an I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis and have sought permission from co-authors for any jointly authored works included in the thesis.

Mechanistic basis for loss of water balance in green tree frogs infected with a fungal pathogen

“…Sloughing has been shown to significantly increase the permeability of the skin, leading to increased ion efflux (79), increased water uptake rates when in water (22,34), and increased rates of water loss in dry environments (62). Although the ionic and osmotic disruption to the skin during sloughing can be substantial, we have shown that in healthy amphibians there is no evidence of a loss of internal ion homeostasis (77,79). We hypothesize that sloughing frogs may upregulate the transcellular and/or paracellular movement of water to compensate for the increase in skin permeability after sloughing.…”

“…However, increased sloughing frequency can be detrimental for some frog species that are unable to remove the fungus completely or quickly enough (50). In these frogs, the combined effect of increased sloughing frequency and damage from Bd further exacerbates the loss of cutaneous functions (77,78).…”

Pathogen load predicts host functional disruption: A meta‐analysis of an amphibian fungal panzootic

“…A naïve Boolean search string [(Batrachochytrium OR chytrid*) AND (frog OR toad OR amphibia* OR anur* OR Caudat* OR Urodela OR salamand*)] was first performed on Web of Science on the October 28, 2021, resulting in 2113 records. The full record from the naïve search and citations in Table S1.1 from Wu (2019) were exported to R 4.1.0 (R Core Team, 2021) for keyword co‐occurrence network analysis (Supplementary Figure S1) using the litsearchr package (Grames et al, 2019). This was used to create a comprehensive Boolean search string for both Web of Science and Scopus detailed in the Supplementary Figure S2.…”

“…I focused on post-metamorph amphibians as Bd infection in tadpoles does not cause mortality directly (Rachowicz & Vredenburg, 2004). S1.1 from Wu (2019) were exported to R 4.1.0 (R Core Team, 2021) for keyword co-occurrence network analysis (Supplementary Figure S1) using the litsearchr package (Grames et al, 2019). This was used to create a comprehensive Boolean search string for both Web of Science and Scopus detailed in the Supplementary Figure S2.…”