Measures of cold tolerance in diploid and triploid Daphnia clones exposed to two temperatures

Despite decades of studies on the differential distribution of polyploid
organisms, the causes of this pattern have yet to be elucidated. This
study aimed to explore some of the possible physiological mechanisms
explaining the differential northern distribution of polyploid clones of
Daphnia pulex compared to the one of the diploid parental species. The
critical thermal minimum (CTmin) was measured in 17 D. pulex clones of
contrasted ploidy (diploid and triploid) and geographic origins (temperate
and subarctic climates) reared under low and high temperatures (16 and 24
°C). Triploid clones had better cold tolerance (lower CTmin) than both
sympatric and temperate diploid clones. No significant association was
found between CTmin and body size nor with cell size. We suggest that
triploids might express a cold shock resistant phenotype related to higher
gene expression and/or fatty acid profiles. Cold tolerance can be viewed
as one of the possible reasons for polyploid preponderance in subarctic
climates.