Many amphibians are currently at risk of extinction due to abiotic and biotic environmental stress. Climate change promotes the development and survival of pathogens and enhances disease risk. A group of common fungal-like pathogens are those in the family Saprolegniaceae, so called water molds.
Effects and susceptibility to “Saprolegnia infections” vary greatly among amphibian species, individuals and populations, likely due to the amount of heritable variation and maternal effects. In this thesis, I studied extent and determinants of embryonic susceptibility to Saprolegniaceae in three experiments (EXPERIMENTS 1 to 3) and used molecular genetic tools to identify Saprolegniaceae strains occurring in amphibian breeding sites in the wild. In EXPERIMENT 1 and 2, to test lethal and sublethal effects of water molds on amphibian embryos and jelly mediated maternal effects, I exposed embryos from two populations within two ranid species (Rana temporaria, RT and R. arvalis, RA) to Saprolegniaceae and observed embryonic survival at four treatment combinations (i.e. Saprolegniaceae infection, no Saprolegniaceae, jelly intact and jelly removed). Most of all, one RA population exhibited lethal (increased mortality) and sublethal effects, such as earlier hatching at less developed stage.
To examine whether embryonic sensitivity to fungal infection depends on the development stage when infection occurs, in EXPERIMENT 2 I infected embryos with Saprolegniaceae within one RA population either one day or three days after fertilization (fertilization = day 0). I found that the time point of exposure to the pathogen was not important unless jelly layers were removed in which case embryos infected earlier had higher mortality than the later infected ones, indicating that jelly mediated maternal protection is especially important at the first days of embryonic development. In EXPERIMENT 3, I studied local adaptation of RA populations and their pathogens by exposing embryos of an acid and neutral RA population to either native or non-native Saprolegniaceae at two pH treatments (acid and neutral). The acid tolerant population tended to have highest mortality when exposed to native Saprolegniaceae at low pH. This might indicate an adaptation of the water molds to their hosts and/or local pH conditions.
To know which Saprolegniaceae species where involved in the infection experiments and to get a first insight to taxa occurring at RA and RT breeding ponds in the study area in Sweden, Saprolegniaceae species were identified by amplification of the variable internal transcribed spacer region (ITS4 and 6). I found various Saprolegnia and Achlya species and differences among sampling sites that indicated a higher prevalence of Aplanes androgynus in acidic habitats. Furthermore, S. ferax seemed to be less infectious than A. oligacantha and A. ambisexualis. The results indicate high species abundance in the field and give an idea of an occurrence pattern that varies seasonally and in relation to chemical parameters of the pond.
In general my results indicate a negative influence of Saprolegniaceae on embryonic fitness and a protective maternal effect mediated through the jelly, which is likely most important at early development stages. Furthermore, likely a range of Saprolegniaceae species occur at RA and RT breeding sites.