Selection on growth rate and local adaption drive genomic adaption during experimental range expansions in the protist Tetrahymena thermophila

Populations that expand their range can undergo rapid evolutionary adaptation, which can be aided or hindered by sexual reproduction and gene flow. Little is known about the genomic causes and consequences of such adaptation. We studied genomic adaptation during experimental range expansions of the protist Tetrahymena thermophila in landscapes with a uniform environment or a pH-gradient, both in the presence and absence of gene flow and sexual reproduction. We used pooled genome sequencing to identify genes subject to selection caused by the expanding range and by the pH-gradient. Adaptation to the range expansion affected genes involved in cell divisions and DNA repair, whereas adaptation to the pH gradient additionally affected genes involved in ion balance, and oxidoreductase reactions. These genetic changes may result from selection on growth and adaptation to low pH. Sexual reproduction affected both de novo mutation and standing genetic variation, whereas gene flow and the presence of a pH-gradient affected only standing variation. Sexual reproduction may have aided genetic adaptation during range expansion, but only in the absence of gene flow, which may have swamped expanding populations with maladapted alleles.