A multi-environment comparison of senescence between sister species of Daphnia.

Senescence is a general decline of physiological state that accompanies advancing age. It affects nearly all organisms, but patterns of senescence vary markedly, even among closely related taxa. Understanding the evolution of this diversity requires information about environmental effects on the expression of variation among taxa. I examined genetically-based variation of senescence within and between two species complexes of Daphnia in four environments. The environments were defined by large differences in food and temperature, two factors known to influence senescence. The species studied were chosen to represent sister species that likely experience divergent (D. pulex and D. pulicaria) or similar (D. mendotate and D. dentifera) selection pressures on senescence. Overall, D. pulex expressed the greatest demographic senescence, D. mendotae and D. dentifera were intermediate, and D. pulicaria expressed the least. In environments representative of typical natural conditions, D. pulex had greater senescence than D. pulicaria, regardless of how late-life performance was assessed. This shows that genetic-environment interactions do not confound the interpretation of senescence differences between these species as the result of selective differences between their habitats. Comparison of D. mendotae and D. dentifera primarily revealed similar life histories, although differences in reproductive declines occurred in some environments. The joint observation of similar mortality patterns but dissimilar fecundity declines suggests that the trade-off between survival and reproduction changes with age. This calls into question the utility of only studying mortality for understanding evolutionary change of senescence in nature.