Résumé
Rapid evolutionary changes during range expansions can lead to the
divergence between range core and front populations, with the emergence of
dispersal syndromes. Besides intraspecific effects, range expansions may
be impacted by interspecific interactions such as parasitism. Yet, despite
the potentially large impact of parasites imposing additional selective
pressures on the host, their role on range expansions remains largely
unexplored. Here, we investigated whether parasites affect the evolution
of host dispersal syndromes during spatial spread. Using populations of
the ciliate Paramecium caudatum and its bacterial parasite Holospora
undulata spreading in microcosm landscapes, we studied experimental range
expansions under the presence or absence of the parasite. We found that
range expansion and parasite treatments acted independently on the
evolution of host dispersal syndromes. Front host populations showed
different trait associations between swimming behaviour and population
growth parameters than core populations, indicating divergent evolution.
The presence of parasites reshaped these trait associations and divergence
between front and core, with hosts exposed to the parasite exhibiting
overall reduced dispersal, but increased resistance. Our work shows that
host-parasite interactions during range expansions can shift the
contenders to novel evolutionary trajectories and may influence the speed
of the range expansion itself and of spreading epidemics.