Inter‐individual variation in parasite manipulation of host phenotype: A role for parasite microbiomes?

Alterations in host phenotype induced by metazoan parasites are widespread in nature, yet the underlying mechanisms and the sources of intraspecific variation in the extent of those alterations remain poorly understood. In light of the microbiome revolution sweeping through ecology and evolutionary biology, we hypothesise that the composition of symbiotic microbial communities living within individual parasites influences the nature and extent of their effect on host phenotype. The interests of both the parasite and its symbionts are aligned through the latter's vertical transmission, favouring joint contributions to the manipulation of host phenotype. Our hypothesis can explain the variation in the extent to which parasites alter host phenotype, as microbiome composition varies among individual parasites. We propose two non‐exclusive approaches to test the hypothesis, furthering the integration of microbiomes into studies of host–parasite interactions. Parasite microbiomes may influence host phenotypic manipulation by parasites. The presence or abundance of particular symbiotic microbes within a parasite affects the extent of host phenotypic change; for example, certain microbes may allow a parasite to induce more pronounced changes in host colour. As a result, parasites inducing similar phenotypic changes in their hosts may harbour similar microbial communities. The principal coordinate capturing most of the variation in the composition of microbial communities within parasites should correlate positively with the extent of phenotypic change induced in the host.