Host traits measurements after evolution with or without parasites during range expansions

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.

种群范围扩张过程中的快速进化改变，可导致种群核心区与前沿种群产生分化，并催生扩散综合征（dispersal syndromes）。除种内效应外，种群范围扩张还可能受到种间相互作用的影响，例如寄生作用。尽管寄生虫可对宿主施加额外的选择压力，潜在影响巨大，但目前学界对其在种群范围扩张中所扮演的角色仍未得到充分探索。
本研究旨在探究寄生虫是否会影响空间扩散过程中宿主扩散综合征的进化。我们以尾草履虫（Paramecium caudatum）及其细菌性寄生虫波纹全孢菌（Holospora undulata）在微宇宙景观中扩散的种群为研究对象，分别设置存在寄生虫与不存在寄生虫两种实验处理组，开展实验范围扩张相关研究。
我们发现，种群范围扩张与寄生虫处理对宿主扩散综合征的进化产生了独立作用。前沿宿主种群在游泳行为与种群增长参数间呈现出与核心种群截然不同的性状关联，表明二者发生了趋异进化。
寄生虫的存在重塑了这些性状关联模式以及前沿与核心种群间的分化格局：暴露于寄生虫的宿主整体上表现出更低的扩散能力，但抗性有所提升。
本研究表明，种群范围扩张过程中的宿主-寄生虫相互作用可推动种群走向全新的进化轨迹，同时可能影响种群范围扩张本身以及传播性流行病的扩散速度。