The Clonal Ecology of Daphnia magna (Crustacea: Cladocera): II. Thermal Differentiation among Seasonal Clones

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Abstract
(1) In natural populations of Daphnia magna, clonal frequencies may change markedly between seasons. The range of seasonal variation in thermal response was investigated by comparing the effect of temperature (5-30 C at 5.degree.C intervals) on electrophoretically distinct clones that dominated a shallow lake in spring, summer, autumn and winter. (2) Four females sharing the same electrophoretic phenotype and representing the abundant clones of each season (seasonal clones) were obtained. Genetically identical lineages of each female were acclimated to a constant environment (15 .+-. 5.degree.C, L:D 24:0) for several parthenogenetic generations. (3) There were significant differences in the thermal response (viability and fecundity) of seasonal clones. The preferred temperature ranges generally corresponded to those of the season at which clones were most abundant, demonstrating genetic adaptation to ambient temperature. Differences between clone temperature preferenda of 5-10.degree.C were greatest between summer and winter clones. Spring and autumn clones, though thermally distinct from those of summer and winter, were less clearly differentiated from each other. (4) The four females comprising a seasonal clone were thermally distinct, when taken collectively over the entire temperature range, though variation in fitness was significantly greater between, than within seasonal clones. (5) These results demonstrate marked thermal differentiation among D. magna clones from a single population. Major clonal frequency changes between seasons were directly related to differences in the thermal tolerance of specific genotypes. Daphnia must therefore be examined from several seasons to obtain realistic limits to environmental stress.