Fine-scale local adaptation of cyanobacterial tolerance in Simocephalus vetulus (Crustacea: Cladocera)

The hypothesis that zooplankton evolve tolerance to cyanobacteria on a fine scale was tested by comparing the intrinsic rates of population increase (r) of a dominant cladoceran, Simocephalus vetulus, from seven small isolated lakes with the distances from 2.2 to 31.7 km. The toxic cyanobacterium Microcystis aeruginosa (poor food) significantly depressed the r values, compared with a nutritious green alga (good food). The tolerance index to cyanobacteria (r_poor/r_good) of 24 clones from six populations differed significantly among clones and populations. Twelve of 15 pairs of two populations had significant divergence with the shortest geographic distance of 2.2 km. Compared with these clones from six populations, 25 clones from another population had a comparable tolerance index. The amount of genetic variation available to selection pressure was sufficient within a population. The tolerance index increased and exhibited saturation with increasing microcystin concentrations in sediments, a proxy of the prevalence of cyanobacterial blooms. The divergence in tolerance between populations was more significantly correlated with the difference in microcystin concentrations than the geographic distance between two sites. These results suggest that local adaptation to toxic cyanobacteria in S. vetulus occurred on a microgeographic scale.