Parasitism can be a powerful selective force, yet host populations routinely show substantial variation in susceptibility to parasites. The freshwater zooplankton Daphnia dentifera can evolve increased resistance to the virulent fungal parasite Metschnikowia bicuspidata over a single epidemic. Notably, the parasite doesn’t co-evolve due to surprisingly limited genetic variation. Despite the directional selection imposed by repeated epidemics, Daphnia populations routinely harbor substantial variation in disease resistance. This may be explained, in part, by a resistance-fecundity trade-off under which more susceptible phenotypes are fittest between outbreaks and during mild epidemics. Another possible explanation is genetic admixture from the diapausing egg bank of Daphnia. To determine the factors maintaining resistance diversity we monitored disease outbreaks in a set of lakes in Indiana, quantifying infection prevalence from September 14th through December 1st, 2015. We collected animals from two lakes, one with high and one with low disease prevalence. Our study uses infection assays on D. dentifera mothers and their sexually produced daughters to quantify the heredity of resistance to the fungal parasite. We also assay animals hatched from the diapausing egg bank the following spring to assess its role in maintaining polymorphism.
Results/Conclusions
The size of the disease outbreak differed between our two focal populations. In 2015 (the year during which we collected mothers and their sexually produced offspring), Hackberry Lake had a maximum prevalence of 0.05% for Metschnikowia infection, while Midland Lake had a maximum prevalence of 17%. Based on assays of a subset of Daphnia collected from Hackberry Lake in 2015, the narrow-sense heritability of susceptibility is 0.532. The average fraction of infected hosts from the mother group ranged from 0.10 to 0.95 (mean = 0.515, SD = 0.214). Thus, this lake retained substantial variation in susceptibility even at the end of the 2015 epidemic season. The average fraction of infected hosts from the daughter group ranged from 0.075 to 0.725 (mean = 0.338, SD = 0.186). Thus, even if the only animals that hatch in the following year are those produced in the previous year, there would be substantial diversity in susceptibility to disease in this lake. Analyses of the resistance of the remaining clones are ongoing.