Résumé
1. Populations that expand their range can undergo rapid evolutionary
adaptation of life-history traits, dispersal behaviour, and adaptation to
the local environment. Such adaptation may be aided or hindered by sexual
reproduction, depending on the context. 2. However, few empirical and
experimental studies have investigated the genetic basis of adaptive
evolution during range expansions. Even less attention has been given to
the question how sexual reproduction may modulate such adaptive evolution
during range expansions. 3. We here studied genomic adaptation during
experimental range expansions of the protist Tetrahymena thermophilain
landscapes with a uniform environment or a pH-gradient. Specifically, we
investigated two aspects of genomic adaptation during range expansion.
Firstly, we investigated adaptive genetic change in terms of the
underlying numbers of allele frequency changes from standing genetic
variation and de novo variants. We focused on how sexual reproduction may
alter this adaptive genetic change. Secondly, we identified genes subject
to selection caused by the expanding range itself, and directional
selection due to the presence or absence of the pH-gradient. We focused
this analysis on alleles with large frequency changes that occurred in
parallel in more than one population to identify the most likely candidate
targets of selection. 4. We found that sexual reproduction altered
adaptive genetic change both in terms of de novo variants and standing
genetic variation. However, sexual reproduction affected allele frequency
changes in standing genetic variation only in the absence of long-distance
gene flow. 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. 5. In the absence of gene flow, sexual reproduction
may have aided genetic adaptation. Gene flow may have swamped expanding
populations with maladapted alleles, thus reducing the extent of
evolutionary adaptation during range expansion. Sexual reproduction also
altered the genetic basis of adaptation in our evolving populations via de
novo variants, possibly by purging deleterious mutations or by
revealing fitness benefits of rare genetic variants.