Seasonal dynamics of sestonic protease inhibition: impact on Daphnia populations

Daphnia populations often show rapid microevolutionary adaptation to environmental changes. Here, we investigated the possibility that microevolution of Daphnia populations could be driven by natural sestonic Protease Inhibition (PI). We hypothesized that PI changes seasonally, which might lead to concomitant changes in tolerance to PI in a co-occurring Daphnia magna population. In order to test this, seston from a eutrophic pond was sampled regularly over two successive years. Extracts of these freeze-dried samples were used to determine their Inhibitory Potential (IP) on D. magna gut proteases. In the summer seston the IP against chymotrypsins exceeded that of spring seston 200-fold. In order to test for possible impacts on the co-existing D. magna population, we isolated clones before (spring) and after (fall) the peak of the IP. Microsatellite analyses revealed that the two subpopulations were genetically distinct. Individual exposure of three clones from each population to varying concentrations of a cyanobacterium that contains chymotrypsin inhibitors revealed a decrease in population and somatic growth rate for each clone, but no seasonal effects on Daphnia’s tolerance. In order to include maternal effects, we conducted a multi-clonal competition experiment on various cyanobacterial concentrations. However, no evidence for seasonally increased tolerance of D. magna to dietary protease inhibitors could be found.