Elemental stoichiometry and insect chill tolerance: evolved and plastic changes in organismal Na+ and K+ content in <i>Drosophila</i>

Acclimation and evolutionary adaptation can produce phenotypic changes that allow organisms to cope with challenges. Determining the relative contributions and the underlying mechanisms driving phenotypic shifts from acclimation and adaptation is of central importance to understanding animal responses to change. Rates of evolution have traditionally been considered slow relative to ecological processes that shape biodiversity. Many organisms nonetheless show patterns of genetic variation that suggest that adaptation may act sufficiently fast to allow continuous change in phenotypes in response to environmental change (called 'adaptive tracking'). In Drosophila, both plastic and evolved differences in chill tolerance are associated with ionoregulation. Here, we combine an acclimation experiment, field collections along a well-characterized latitudinal cline, and a replicated field experiment to assess the concordance in the direction, magnitude, and potential mechanisms of acclimation and adaptation on chill coma recovery and elemental (Na and K) stoichiometry in both sexes of Drosophila melanogaster. Acclimation strongly shaped chill coma recovery, spatial adaptation produced comparatively modest effects, and temporal adaptation had no significant effect. Leveraging knowledge on the mechanisms underlying variation in chill tolerance traits, we find that relationships between elemental stoichiometry and chill coma recovery in the context of acclimation may differ from those that are associated with spatial adaptive change.