Spatial variation in species composition of Saprolegnia, a parasitic oomycete of amphibian eggs, in Scotland

Parasitic water moulds in the genus Saprolegnia cause mortality of amphibian embryos and reduced size at metamorphosis, leading to increased adult mortality. Most studies of virulence have focused on only a single Saprolegnia species, but the Saprolegnia species associated with amphibian eggs and their distributions are not well known. This study aimed to investigate the distribution of amphibian-associated water moulds in Scotland. In particular, we asked the questions: i) Does Saprolegnia species composition vary between sites?; and ii) Is presence of Saprolegnia related to environmental parameters? Common frog (Rana temporaria) eggs with evidence of Saprolegnia infection were sampled from ten sites, cultured, and the 28S region of the rDNA array sequenced. Thirteen samples isolated from four sites were identified as members of the Saprolegniaceae and the ITS region of these samples were subsequently sequenced to further resolve species identification. Four species of Saprolegnia were found in total, with one or two species of Saprolegnia present in each of four sites. S. diclina was the most common species identified and was found at three of the four sites. Acidity was significantly lower and altitude significantly higher at sites where Saprolegniaceae were present. Therefore, R. temporaria eggs in different pools are subject to infection by different, and in some instances more than one, species of Saprolegnia. Overall, our findings suggest that future studies of virulence need to consider the effect of multiple Saprolegnia species within a site as well as the population of origin of the amphibian host.