Authors
A Wohlleben1; S A Foster1; J A Baker1; 1 clark University, United States Discussion
Parasite burdens often vary substantially between host populations, both on spatial (globally and locally) and temporal scales. In this study, we took advantage of the Gasterosteus aculeatus (threespine stickleback) – Schistocephalus solidus host-parasite system to answer questions regarding the timing of infection and parasite burden in different host populations. Following the last glacial maximum, when oceanic stickleback independently colonized freshwater habitats, they first encountered S. solidus, a trophically-transmitted cestode with a long history of parasitizing freshwater stickleback. Adapting to this new freshwater environment and parasite pressure, some stickleback populations evolved to have persistently low parasite loads, while other populations evolved to have consistently high loads or extreme fluctuations in parasite loads across years. With the timing of infections being of great importance in host-parasite interactions and co-evolution, we investigated the timing of infection by S. solidus in four Alaskan stickleback population with different S. solidus infection rates. Our goal was to see if young-of-year stickleback, having hatched within the last few months, were already be infected with S. solidus and if so, to determine the development of the infection over the winter. We chose host populations with either high or low adult infection rates and were interested, if these differences would already be detectable in young-of-year fish. We found young-of-year stickleback harbored S. solidus parasites, with infection rates differing between the host populations: Depending on the year, the infection rate in young-of-year stickleback was between 10-20% for the low infection population and between 30-70% for the high infection populations. For the 2019 cohort, infection rate increased dramatically over the winter, while it did not change in the 2018 cohort. This study leads to two conclusions: First, S. solidus infecting young-of-year stickleback find themselves in hosts that are too small for the parasite. The parasite must allow it’s hosts to grow after infection, or the parasite won’t be able to reach its definitive size and with that sexual maturity. Second, infections seem to continue under the ice cover. 
