Microbiomes mediate host-parasite interactions.

Parasitic and parasitoid organisms, including nematodes, trematodes, flies, ticks and wasps, rely on chemical cues to locate a host. In previous research, these cues were assumed to originate directly from the host, but in some cases it appears that the microbiome is responsible for emitting volatile organic compounds that can attract or repel parasites (Chavez et al., 2021; Takken & Verhulst, 2013). In a From the Cover paper in this issue of Molecular Ecology, Lutz et al. (2021) use multiscale analyses to demonstrate that members of the skin and oral, but not gut microbiome are correlated with ecto- and endoparasite occurrence in Afrotropical bats. That the microbiome may act as a beacon for searching parasites is an innovative mechanism for explaining how parasites find their host. This mechanism also changes our understanding of how evolutionary arms races occur between a host and parasite. Instead of reciprocal adaptations that allow the host to better defend and the parasite to better attack, each host and parasite may be attempting to manipulate or overcome manipulation of the microbiome that mediates the host-parasite interaction. While Lutz et al. (2021) establish that the skin and oral microbiomes are distinct between parasitized and nonparasitized individuals, they acknowledge that the directionality of this change cannot be determined from their data (i.e., does the microbiome differentially attract parasites or does the microbiome reflect the infection status of its host?). This leaves us with several interesting directions for future research. All of these future avenues of research have the potential to change our understanding of host-parasite coevolution, by necessitating that we extend our examination of this seemingly bipartite interaction to include a third actor-the microbiome.