The aim of the present study was to clarify the mechanisms underlying the expression of phenotypic plasticity in Daphnia life history traits in response to fish kairomone and to explore how these mechanisms interact with genetic variation for life history traits. I studied the effect of fish kairomone on life history traits in 16 Daphnia galeata clones. Maturation occurred more often at the earlier instars in response to the fish treatment, resulting in earlier age and smaller size at maturity. The changes in age and size at maturity which occurred in the response to the fish treatment could be attributed to a large extent to this shift. In addition, fish kairomone resulted in smaller instar increments after the maturation process was initiated, but not during the juvenile period. Within groups of animals maturing at the same instar, larger size at birth resulted in larger size at maturity, but had no effect on age at maturity. However, larger size at birth brought about earlier maturity because animals which were larger at birth matured more often at the earlier instar. Fish treatment resulted in more, but smaller, offspring in the first clutch relative to the size of the mother. Genetic variation was found in all measured life history traits. Genetic variation in plasticities of life history traits was not detectable by standard methods, and was only shown when the above mechanisms of expression of life history traits were taken into account. This study illustrates the importance of a thorough understanding of the mechanisms underlying the expression of life history traits, in this case, sources and consequences of maturation in the multiple instars. Not only is this necessary to predict the long-term effect of the environment, but also to understand the response of life history traits and their plasticities to natural selection. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00442-002-0901-0.
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