Transgenerational plasticity alters parasite fitness in changing environments.

Transgenerational plasticity can help organisms respond rapidly to changing environments. Most prior studies of transgenerational plasticity in host­parasite interactions have focused on the host, leaving us with a limited understanding of transgenerational plasticity of parasites. We tested whether exposure to elevated temperatures while spores are developing can modify the ability of those spores to infect new hosts, as well as the growth and virulence of the next generation of parasites in the new host. We exposed Daphnia dentifera to its naturally co-occurring fungal parasite Metschnikowia bicuspidata, rearing the parasite at cooler (20°C) or warmer (24°C) temperatures and then, factorially, using those spores to infect at 20 and 24°C. Infections by parasites reared at warmer past temperatures produced more mature spores, but only when the current infections were at cooler temperatures. Moreover, the percentage of mature spores was impacted by both rearing and current temperatures, and was highest for infections with spores reared in a warmer environment that infected hosts in a cooler environment. In contrast, virulence was influenced only by current temperatures. These results demonstrate transgenerational plasticity of parasites in response to temperature changes, with fitness impacts that are dependent on both past and current environments.