Intra‐specific variations in reaction norms of predator‐induced polyphenism in the water flea Daphnia pulex

Abstract

AbstractPhenotypic plasticity is considered an important factor leading to phenotypic evolution through adaptations to changing environments. Interactions between environmental inputs and expressed phenotypes can be evaluated as “reaction norms”. Comparisons of these reaction norms among geographically distinct populations may provide evidence for genetic diversity and, moreover, important clues into understanding how the interactions affect evolutionary processes. The water flea Daphnia pulex exhibits predator‐induced polyphenism in response to a kairomone released by predatory phantom midge larvae (Chaoborus spp.). The polyphenism, a defensive structure formed on the head that enables juvenile larvae to avoid predation, is referred to as neckteeth. Differences in shape and/or plasticity of defensive structures have also been reported among related Daphnia species, suggesting that the species‐specific variation in defensive morphology is derived from intra‐specific differences in reaction norms. By determining reaction norms based on the incidence and number of neckteeth, we clarified differences in patterns of neckteeth formation in response to varying concentrations of Chaoborus kairomone. We then compared these differences among populations (strains) collected from six locations in Japan. Our findings showed that the reaction norms of both traits differed among strains. Furthermore, a molecular phylogeny using mitochondrial ND5 and COI sequences suggested that differences in the reaction norms of Japanese strains corresponded to their genetically different lineages, which likely originated from North American populations that were probably introduced to Japan independently.