Long-term cold adaptation leads to increased thermal plasticity of male body size and thermal canalization of fecundity in populations of Drosophila melanogaster: an experimental evolution study

From laboratory populations of Drosophila melanogaster maintained at 25{degrees}C for many decades, new populations were derived and maintained at 17{degrees}C for over 60 generations. Fitness traits such as body size and fecundity of the cold and warm selected populations were quantified at both 17{degrees}C and 25{degrees}C. Flies from both selection regimes had similar body weight when grown at 25{degrees}C, but 17{degrees}C selected males had greater dry weight and less relative water content than 25{degrees}C selected males, when grown at 17{degrees}C. Cold selected males thus evolved greater thermal plasticity of body weight and density compared to controls. Fecundity data showed, 25{degrees}C selected females laid less eggs at 17{degrees}C compared to 25{degrees}C treatment, but 17{degrees}C selected females did not show significant difference in fecundity between 25{degrees}C and 17{degrees}C. Cold evolved flies thus showed thermal canalization of fecundity, possibly buffering the repressing effect of cold. Whether or not (i) greater body mass and density of cold evolved males, and (b) unrepressed fecundity of cold evolved females, both observed when reared at cold, are causally related, remain to be investigated. The study thus shows thermal selection can lead to evolution of greater thermal plasticity in a trait, while result in thermal canalization of another.