Multiclonal study of Daphnia magna with respect to adaptation to toxic cyanobacteria

Mass developments of toxic cyanobacteria have increased in frequency due to global warming and eutrophication. Such cyanobacterial blooms impact whole freshwater ecosystems, especially reducing the abundance of herbivory species of the genus Daphnia. These negative effects on Daphnia have frequently been attributed to cyanobacterial secondary metabolites, among them hepatotoxic microcystins and protease inhibitors. Protease inhibitors inhibit major digestive proteases in the gut of Daphnia which results in reduced fitness, that is, population growth. To date evidence for local adaptation of Daphnia to cyanobacteria is confined to microcystin-producing cyanobacteria and based on comparison of individual clones from different populations but lacks evidence from multiclone microcosm experiments. In the present study, D. magna clones from a Swedish lake where they coexist with the microcystin-free Microcystis sp. strain BM25 were compared to clones from a Polish population without cyanobacteria, first in single-clone experiments and subsequently in a multiclonal experimental population. The Swedish clones were assumed to be locally adapted to this protease inhibitor-producing cyanobacterium and indeed showed higher population growth rates, a proxy for fitness, and dominated the population in the presence of dietary Microcystis sp. BM25, but not in the absence of this cyanobacterium. The results indicate an adaptive tolerance of the Swedish population and point at local adaptation to locally co-occurring protease inhibitor-producing cyanobacteria.