Does geographic variation in thermal tolerance in Daphnia represent trade-offs or conditional neutrality?

• Published in: Journal of thermal biology Vol. 98; p. 102934
• Main Authors: Coggins, B.L., Pearson, A.C., Yampolsky, L.Y.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01-05-2021; Elsevier BV
• Subjects: Algae; Cloning; Daphnia; Fluorescence polarization; Genetic load; Geographical variations; Heat tolerance; High temperature; Lipid peroxidation; Local adaptation; Range extension; Reproductive fitness; Temperature tolerance; Trade-offs; Daphnia; Heat tolerance; Genetic load; Local adaptation; Trade-offs
• ISSN: 0306-4565; 1879-0992
• DOI: 10.1016/j.jtherbio.2021.102934

Geographic variation in thermal tolerance in Daphnia seems to represent genetic load at the loci specifically responsible for heat tolerance resulting from conditional neutrality. We see no evidence of trade-offs between fitness-related traits at 25 °C vs. 10 °C or between two algal diets across Daphnia magna clones from a variety of locations representing the opposite ends of the distribution of long-term heat tolerance. Likewise, we found no evidence of within-environment trade-offs between heat tolerance and fitness-related traits in any of the environments. Neither short-term and long-term heat tolerance shows any consistent relationship with lipid fluorescence polarization and lipid peroxidation across clones or environments. Pervasive positive correlations between fitness-related traits indicate differences in genetic load rather than trade-off based local adaptation or thermal specialization. For heat tolerance such differences may be caused by either relaxation of stabilizing selection due to lower exposure to high temperature extremes, i.e., conditional neutrality, or by small effective population size followed by the recent range expansion.
•Populations of Daphnia magna show no thermal specialization or trade-offs between heat tolerance and fitness.•Likewise, no specialization is detected in lipid properties that correlate with heat tolerance.•Rather, differentiation evolved neutrally by relaxation of stabilizing selection in recently expanding Northern populations.