Genetic differentiation in life history between Daphnia galeata populations: an adaptation to local predation regimes?

Abstract

Species of the water flea Daphnia exhibit constitutive as well as phenotypically inducible anti-predator defence strategies, involving life history, morphological and behavioural traits. We explored the hypothesis of genetic differentiation in anti-predator defence strategies using Daphnia clones originating from two different water bodies: Tjeukemeer (the Netherlands) and Fish Pond (Belgium). Both water bodies are inhabited by zooplanktivorous fish. In contrast to Tjeukemeer, Fish Pond is also inhabited by larvae of the phantom midge Chaoborus. The life history responses of the two sets of clones to kairomones from fish (Perca), to kairomones from Chaoborus, and to a mixture of both were compared. Clones from Tjeukemeer and Fish Pond showed strong responses to the presence of fish kairomone, with reductions in adult and neonate body length, in age at first reproduction and in the total number of neonates produced during the first three adult instars. Responses to Chaoborus kairomone were much less pronounced, although there was a tendency towards an increase in the number of neonates in the first brood. Significant inter-population genetic differences were found for all the investigated traits. However, there was no indication for genetic adaptation of the Fish Pond clones to negative size-selective predation by Chaoborus. Compared to Tjeukemeer clones, Fish Pond clones had a lower size at first reproduction, produced smaller neonates and produced a higher number of neonates in their first brood. This suggests adaptation to positive rather than to negative size selective predation. Genetic differences between populations were observed mainly for constitutive life history traits, rather than for phenotypic shifts in response to the presence of predator kairomones.