Is a jack-of-all-temperatures a master of none? An experimental test with Daphnia pulicaria (Crustacea: Cladocera)

A fundamental question in evolutionary ecology is the existence and magnitude of constraints. For example, the proverb ‘the jack-of-all-trades is a master of none’ refers to one possible constraint, a genetic trade-off of fitness between a generalist and a specialist. It remains an object of controversy. A new method was developed to test experimentally the hypothetical fitness trade-off between a generalist and a specialist that can be used to study populations of both unicellular and multicellular organisms. The method is based on properties of the tolerance curve. The tolerance curve can be defined as the relationship between fitness of a genotype and environmental conditions. Three analytical approaches are possible with respect to tolerance curves: (1) investigation of a genetic trade-off between height and breadth of the tolerance curve; (2) analysis of variance of the area under the tolerance curve among genotypes; and (3) estimation of genetic correlations of fitness on pairs of temperatures. Age of reproduction and clutch size of 29 Daphnia pulicaria genotypes from three closely related populations were investigated under seven temperature conditions. Based on these data, fitness – measured as an intrinsic rate of increase (r) – was estimated for each Daphnia genotype at seven temperatures. The estimated correlation between height and breadth of the tolerance curve was weakly negative. The analysis of variance of the area under the tolerance curve revealed statistically significant differences among the Daphnia clones. The majority of the estimated pairwise genetic correlation coefficients between temperatures were weak, either positive or negative. In general, we observed ‘winner’ and ‘loser’ genotypes of D. pulicaria in response to temperature, suggesting the need to re-examine the traditional paradigm based on the controversial proverb ‘the jack-of-all-trades is a master of none’.