Transgenerational virulence: Maternal pathogen exposure reduces offspring fitness

There is intense interest in understanding pathogen virulence – that is, the degree of damage caused to a host by a pathogen. However, despite abundant evidence that effects of stressors can carry across generations, studies of pathogen virulence have focused almost exclusively on a single generation. Here, we describe transgenerational virulence, where parental exposure to a pathogen reduces offspring fitness. Daphnia host exposure to a microsporidian pathogen significantly reduced lifespan and fecundity of offspring. The trans-generational virulent effects of maternal exposure to the microsporidian pathogen matched or even exceeded the within generation effects of a fungal pathogen that is considered highly virulent. Moreover, they did not result simply from immune activation in the host, as the transgenerational effect was seen for infections by the microsporidian but not for infec-tions by two other pathogens. The transgenerational effects of the microsporidian carried to the granddaughters, with grandmaternal exposure influencing neonate size. A parame-terized mathematical model translated these impacts to the population level, finding that transgenerational virulence often reduces host density, infected host density, and infection prevalence, which could have implications for host conservation and spillover risk. Given widespread prior evidence for transgenerational effects of predators, resources, and other factors, we propose that transgenerational virulence may be common. ### Competing Interest Statement The authors have declared no competing interest.