Investigating Associations Between Skin Microbiome and Disease in the Common Frog (Rana temporaria)

Abstract

Habitat loss and disease are causing global amphibian population declines. Commensal bacterial species on amphibian skin are crucial for host-pathogen defence, prompting an outpouring of studies of skin microbiome diversity for pathogen mitigation. However, microbiome diversity understanding is limited by the focus on fungal pathogens, studies in captive settings, or on restricted spatiotemporal scales, with few exploring individual-level diversity. This thesis aimed to investigate host microbiome dynamics at landscape scale, within wild individual hosts over time, using the widespread and accessible common frog (Rana temporaria) as a model system in South West England. Frog skin and water samples from spatial surveys at nine ponds revealed bacterial community composition variation by geographic location. Skin microbiome diversity reflected pond water, but not exactly, likely due to host regulation. Temporal surveys utilised dorsal pigmentation to track individuals across one breeding season at three ponds, and two seasons at two others. Findings exemplified population-specific skin microbiome composition, and variation also occurred over time at population and individual level. This is the first study to investigate symptoms indicative of ranid herpesvirus infection upon skin microbiome diversity. Symptoms were common across surveys, however evidence of infection altering skin bacterial composition was limited. Common frogs are ubiquitous in garden ponds, but knowledge of their ecology and management there is limited. To address this, a citizen science survey was distributed across the United Kingdom and Ireland. From >2000 participants, it uncovered frogs occurring more readily in complex garden habitats, managed for wildlife, and with high wildlife diversity. The majority of participants reported calming benefits from garden ponds, most widely felt in urban areas, and where wildlife diversity was high. These findings support the inclusion of ponds in conservation policy, and demonstrate the need to study host microbiomes on spatiotemporal and individual scales to fully understand natural microbiome variation, and further amphibian conservation.