Immigration shapes evolutionary tolerance to toxic cyanobacteria in two cladoceran grazers

Harmful algal blooms have triggered evolutionary tolerance in many grazers, but whether immigration can alter evolutionary dynamics is unknown. Using experimental evolution, the impacts of immigration on evolutionary tolerance to toxic cyanobacteria were investigated in two cladoceran grazers, the large-bodied Daphnia pulex and medium-bodied Simocephalus vetulus. Animals were randomly split into control, selection, and immigration lines; they were incubated with Microcystis aeruginosa (selection experiment), and then fed pure, nutritious green algae (relaxation experiment). Individuals in the control line were transferred to the immigration line, but not the selection line. To infer grazer tolerance, relative change in intrinsic rates of population increase when feeding on a mixed diet with M. aeruginosa and pure green algae was measured at the end of experiments. After a 3-month exposure, both grazers in the selection and immigration lines evolved higher tolerance. Immigration had a positive effect on the evolutionary tolerance in D. pulex, but little influence on S. vetulus. Following a 2-month relaxation of selection, immigration facilitated the loss of evolved tolerance in S. vetulus, but had no significant impact on D. pulex. These results underscore the importance of considering immigration when predicting the evolutionary responses of grazers to increasing harmful algae.