“Sympatric parasites have similar host-associated, but asynchronous, patterns of diversification”

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Kayce C. Bell, John R. Demboski, and Joseph A. Cook (Sep 2018)

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Parasites living in the same host have different evolutionary histories

Least chipmunk (Tamias minimus), photo credit Philip Myers (CC-BY-NC-SA 3.0). Inset, chipmunk roundworm (Heteroxynema cucullatum) SEM, credit Stephen Greiman.

Because parasites are often restricted to just a few hosts with limited opportunities for dispersal, it has been thought that their evolutionary histories might mirror those of their hosts. Detailed investigations, however, are revealing more complex dynamics shaping parasite diversity. By comparing multiple parasites inhabiting the same hosts, researchers can shed light on the forces that drive parasite diversification.

In this study, Bell and colleagues find evidence of distinct evolutionary histories in two parasites that infect the same hosts. They explore the genetic relationships within each of two species of roundworm parasites found in a diverse group of hosts, western North American chipmunks. Although both pinworm species have lineages primarily associated with one chipmunk species or a closely related group of chipmunk species, each roundworm species has a distinct evolutionary history. The unique histories are manifested as different relationships among the host associated genetic lineages, as well as distinct timing of divergence events within each parasite species. Comparative investigations such as this suggest that even when parasites share the same host and experience similar environments, different timing and patterns of evolutionary diversification can emerge.


Parasitism is a common symbiotic interaction across diverse natural systems. Using a comparative evolutionary approach, we investigated the contributions of both host phylogeny and abiotic factors towards diversification of phylogenetically independent endoparasites that inhabit essentially the same physical space. We tested for host-parasite and parasite-parasite phylogenetic concordance in western North American chipmunks (Rodentia: Sciuridae) and two distantly related species of pinworms (Nematoda: Oxyurida). Deep structure in molecular phylogenies revealed signals of host-associated divergence in both parasite species, while shallower phylogeographic structure varied between the two parasites. This suggests that although these parasites experienced similar landscapes and cyclic climate processes, temporally distinctive diversification events were associated with differences in the initiation of their association with host lineages. When climate cycles initiate diversification, partially congruent, but asynchronous host-associated parasite phylogenies may emerge.