Forward and reverse evolution of multivariate responses to cyanobacteria in experimental waterflea populations

Studies of rapid evolution under changing environments are important for understanding the evolutionary fate of populations and the future of biodiversity. We used experimental evolution to test whether the presence of toxic cyanobacteria and its subsequent removal could lead to rapid forward and reverse genetic changes in multiple traits of the zooplankter Ceriodaphnia cornuta. In the forward selection experiment, the covariant pattern in multivariate trait space provided strong evidence of adaptive evolution in multiple traits at two selection levels, in the presence of toxic cyanobacteria. The C. cornuta population under high selection pressure showed higher tolerance than those under low selection pressure. The total trait change generated through evolutionary responses (constitutive evolution and evolution of plasticity) ranged from 0.2% to 32.2%, depending on the trait and selection level. After the relaxation of selection, almost all evolved traits were restored to their original state when the C. cornuta population were fed pure green algae. That is, the toxin-tolerant phenotypes were culled off from the population in the absence of the selection pressure, suggesting a heavy cost of maintaining tolerance. The genotype dynamics indicated that asymmetric intraspecific competition may be the main force driving the rapid reverse evolution. These results suggest that the change in cyanobacteria biomass, from proliferation to decline, resulted in rapid forward and reverse evolution of multivariate phenotypic responses in the freshwater zooplankton grazer C. cornuta.