Stress tolerance in a novel system: Genetic and environmental sources of (co)variation for cold tolerance in the butterfly Eurema smilax

The physiological ability to survive climatic extremes, such as low temperature, is a major determinant of species distribution. Research suggests that tropically restricted insect populations may possess low to zero variation in stress tolerance, thereby limiting any potential to adapt to colder climates. This paradigm derives largely from contrasts among Drosophila populations and species along the tropical-temperate cline of eastern Australia. Butterfly groups, such as the variously distributed representatives of the genus Eurema, offer opportunities to test the taxonomic breadth of this paradigm. We contribute here by investigating plasticity, repeatability and heritability (h(2)) for cold tolerance in Eurema smilax. This continentally widespread species (extending from the Torres Strait to the south coast of Victoria) offers an important comparative basis for evaluating stress tolerance in geographically restricted congenerics. We reared two generations of E.smilax under laboratory conditions and measured recovery from a chill-coma assay, which is one of the commonly used methods for characterizing adult cold stress tolerance. Trials on F2s conducted over three consecutive days revealed individual repeatability (r=0.405). However, recovery time decreased systematically across trials, which is characteristic of a phenotypically plastic hardening' response to prior cold exposure. Generalized linear modelling, wherein genetic variance was estimated via an animal model' approach, indicated no difference between sexes and no effect of body size, but a significant additive genetic term, corresponding to a heritability estimate of h(2)=0.414 +/- 0.100. These data suggest significant adaptive potential for cold tolerance in E.smilax but show that individuals may also respond directly to extremes of cold via phenotypic plasticity. This indicates the potential to adapt to varied thermal extremes, which would be expected for a broadly distributed species that is resilient to climate change.