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.

Travelling with a parasite dataset. Complete matrix of the traits measured for the 4 evolutionary treatments.