Population-wide differentiation of tolerance to microcystins and other cyanobacterial non- microcystin effects in Daphnia

Cyanobacteria cause major concern due to toxic secondary metabolites, especially microcystins. Adaptation of Daphnia populations to these toxic cyanobacteria has repeatedly been shown; however, it remains unclear if this adaptation indeed reflects increased tolerance to microcystins or to other cyanobacterial metabolites. To shed light on this question, we compared the response of Daphnia to a toxic cyanobacterium and to its microcystin-free mutant. We analysed clones originating from an experienced Daphnia magna population that coexists with toxic cyanobacteria as well as from a naive population. Diets containing 10 % and 20 % of either cyanobacterium resulted in stronger relative reduction of somatic growth in animals exposed to the toxic cyanobacterium. These effects did not differ among Daphnia populations. Contrastingly, tolerance to the mutant cyanobacterium was higher in the experienced population. This indicates that adaptation to cyanobacterial non-microcystin effects in Daphnia populations is possible and that the molecular mechanisms of tolerance are different for microcystins and other non-microcystin effects. Finally, a connection of high tolerance to microcystins and low somatic growth tentatively points at trade-offs of microcystin tolerance, which might constrain evolutionary adaptation of Daphnia to microcystins.