Agriculture ditch management (i.e. removal of vegetation and/or sediments) may disturb native wildlife, such as amphibian bioindicator species. The objective of this thesis was to determine whether ditch management affected northern leopard frogs (Lithobates pipiens). Twelve embryo and tadpole health endpoints were compared in vegetated and managed (treeless or dredged) ditches using in situ cages in 2018 and 2019, while water quality was monitored. The managed treatment did not negatively affect frog health compared to the vegetated treatment. The significantly faster development and larger body size due to warmer water temperatures at the managed treatment were likely advantageous. The water quality was typical of the region regardless of the ditch treatment and the pesticide mixture detected had limited toxicity. The main effects of the studied ditch managements on resident L. pipiens (and potentially other amphibians) appeared sub-lethal, likely beneficial, and mainly the result of temperature differences. Although poorer embryo health was observed at the vegetated treatment, it was likely unrelated to ditch management and possibly due to high specific conductivity causing premature hatching and mortality. Ditch management may be less important for frog health than the surrounding land use, and efforts should be directed at increasing suitable habitat for amphibians in agroecosystems. La gestion des fossés agricoles (plus spécifiquement l'élimination de la végétation des bandes riveraines et /ou le dragage des sédiments) peut perturber la faune indigène, comme les espèces bio-indicateurs d'amphibiens. L'objectif de cette thèse était de déterminer si la gestion des fossés affectait la grenouille léopard du Nord (Lithobates pipiens). Douze paramètres d'évaluation de la santé des embryons et des têtards ont été comparés dans des fossés végétalisés et gérés (sans arbres ou dragués) à l'aide de cages expérimentales in situ en 2018 et 2019, pendant que la qualité de l'eau a été suivie. Le traitement géré n'a pas eu d'effets négatifs sur la santé des grenouilles par rapport au traitement végétalisé. Le développement beaucoup plus rapide et la taille corporelle plus grande en raison des températures plus chaudes de l'eau lors du traitement géré étaient probablement avantageux. La qualité de l'eau était typique de la région quel que soit le traitement du fossé et le mélange de pesticides détecté avait une toxicité limitée. Les principaux effets de la gestion des fossés étudiés sur L. pipiens résidents (et potentiellement d'autres amphibiens) semblaient sublétaux, probablement bénéfiques, et résultaient principalement de différences de température. Bien que la santé des embryons ait été moins bonne lors du traitement végétalisé, elle n'était probablement pas liée à la gestion des fossés et pourrait être due à une conductivité spécifique élevée entraînant une éclosion prématurée et la mortalité. La gestion des fossés peut être moins importante pour la santé des grenouilles que l'utilisation des terres environnantes, et les efforts de protection devraient être dirigés vers une augmentation générale de l'habitat approprié pour les amphibiens dans les agroécosystèmes.

Chytridiomycosis is an emerging cutaneous fungal disease that contributes to recent global declines and extinction of amphibian species, caused by infection of the skin with a fungus known as Batrachochytrium dendrobatidis (Bd). Many species of frogs secrete antimicrobial peptides onto their skin that are capable of killing Bd. This thesis is an investigation of the effect of corticosteroids on cutaneous innate immunity in frogs, in the context of infection with Bd. The general hypothesis was that injections of glucocorticoids would impair the cutaneous synthesis of these antimicrobial peptides, thereby increasing susceptibility to Bd infection. The objective of the first experiment was to measure and compare gene expression levels of cutaneous AMP’s in frogs treated with glucocorticoids with sham-treated controls. Wild-caught Lithobates pipiens were acclimatized and administered either the corticosteroid methylprednisolone or saline every 48 hours. Norepinephrine-elicited cutaneous secretions were collected prior to the first injection of corticosteroid or saline, and then every 8 days for 40 days. Gene expression of the AMP’s brevinin and ranatuerin in the cutaneous secretions was quantified relative to the reference genes EF1-α and RPL8 using reverse transcription quantitative polymerase chain reaction (RT-qPCR). Corticosteroid treatment was associated with a significant (P<0.027) increase in brevinin gene expression, which was most notable at 24-40 days of corticosteroid administration. Ranatuerin expression followed a similar but nonsignificant trend. The second experiment was a pilot study intended to establish a Bd challenge protocol in L. pipiens. Frogs were immersed in water containing 0, 104, 105 or 106 zoospores of Bd strain JEL 423. Cutaneous swabs were collected prior to challenge and tested for Bd by qPCR; unexpectedly, some tested positive, indicating pre-challenge infection. The analysis was complicated by an identified cross-reactivity of the assay with other fungi. The findings of the first experiment refuted the hypothesis, and suggested that corticosteroids promote rather than impair AMP gene expression in the skin of L. pipiens, under these experimental conditions. Further, the second study demonstrated that none of the frogs showed clinical abnormalities or died, despite exposure to Bd zoospores and despite molecular and histologic evidence of cutaneous Bd infection in some frogs. / NSERC Discovery Grant

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