This is the peer reviewed version of the following article: Lancaster, L. T., Dudaniec, R. Y., Chauhan, P., Wellenreuther, M., Svensson, E. I. and Hansson, B. (2016), Gene expression under thermal stress varies across a geographical range expansion front. Mol Ecol, 25: 1141–1156., which has been published in final form at doi:10.1111/mec.13548. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.