Susceptibility of frog (Rana temporaria) and toad (Bufo bufo) eggs to invasion by Saprolegnia

Infections of amphibian eggs sometimes cause catastrophic losses of reproductive effort, but susceptibility to infection in different species is poorly understood. Using laboratory trials we showed that direct hyphal invasion of adjacent eggs by Saprolegnia caused a higher incidence of infection than invasion by zoospores. Moreover, we observed that dead eggs were much more readily colonized than live eggs when challenged with zoospores from two strains of Saprolegnia. The two strains were equally effective in causing infections of Rana teinporaria eggs, but differed in their ability to infect eggs of Bufo bufo. In live R. temporaria eggs, early stages (pre-tailbud) were more frequently infected by hyphal invasion than later stages by the same strains, suggesting that susceptibility to infection decreases as development proceeds.