Host phylogeny and microbiome composition predict gut nematode community composition within a diverse assemblage of African herbivores.

Gut nematodes influence animal health and fitness, with effects shaped by their abundance and community composition. While closely related host species tend to harbour similar nematodes, the relative roles of host ecology and phylogeny in structuring nematode communities remain unclear. Here, we assess how host holobiont traits-body size, diet, space use, potentially pathogenic gut microbes and overall gut microbiome composition-predict gut nematode abundance and composition, while controlling for host phylogenetic relatedness. We jointly analyse DNA metabarcoding data on nematodes, microbiomes and diets from 17 free-ranging African herbivore species. Host phylogeny and microbiome composition were the strongest predictors of nematode community structure. Physical proximity and diet also contributed, although to a lesser extent, whereas body size did not. Nematode abundance correlated positively with the richness of putative pathogenic bacteria, which in turn increased with diet richness. Nematode presence/absence covaried with the microbiome and diet composition, and we identified pairwise associations between nematodes, putative pathogenic bacteria and diet plants. Our findings illustrate that host ecology and phylogeny jointly influence gut nematode communities. In particular, the gut microbiome is a key predictor of nematode communities, even after accounting for host phylogeny, emphasizing the ecological interconnectedness of these gut constituents.