Environmental triggers of resting spore germination in the chytrid parasite Staurastromyces oculus

Abstract Many parasites rely on a resting stage to endure across adverse environmental conditions or over periods of host scarcity. However, maturation conditions of the resting stage during dormancy and the proximate cues that trigger germination remain unknown for most parasite species, limiting our ability to predict when parasites reinitiate infection. Here, we designed an experiment in a desmid host—chytrid parasite system to decouple environmental conditions required during parasite dormancy from the immediate cues that trigger subsequent activation. We experimentally investigated whether resting spores require a mandatory cold dormancy period (with or without light), and how subsequent germination is affected by a temperature increase, light exposure and host addition. To evaluate the combined and potentially interactive effects of abiotic and biotic factors on subsequent germination, we fitted a structural equation model that accounted for variation in host and resting stage abundance. Overall, cold conditions during dormancy followed by a temperature upshift resulted in resting spore germination. Light enhanced germination but was not a mandatory germination cue, whereas we found no evidence that host presence or abundance had a strong effect on germinability. Neither dormancy at warm conditions or incubation at cold conditions resulted in synchronised germinations. The requirement for cold conditions during dormancy and subsequent germination upon a warming cue aligns with the parasite´s seasonal ecology, with epidemics emerging presumably from deeper, cool water layers in early summer when host populations increase, and the parasite disappearing in autumn as host populations collapse.