Host taxonomy determines the composition, structure, and diversity of the earthworm cast microbiome under homogenous feeding conditions

Host evolutionary history is a key factor shaping the earthworm cast microbiome, although its effect can be shadowed by the earthworm's diet. To untangle dietary from taxon effects, we raised nine earthworm species on a uniform diet of cow manure and compared cast microbiome across species while controlling for diet. Our results showed that, under controlled laboratory conditions, earthworm microbiomes are species-specific, more diverse than that of the controlled diet, and mainly comprised of native bacteria (i.e. not acquired from the diet). Furthermore, diet has a medium to large convergence effect on microbiome composition since earthworms shared 16%-74% of their bacterial amplicon sequence variants (ASV). The interspecies core microbiome included 10 ASVs, while their intraspecies core microbiomes were larger and varied in ASV richness (24%-48%) and sequence abundance across earthworm species. This specificity in core microbiomes and variable degree of similarity in bacterial composition suggest that phylosymbiosis could determine earthworm microbiome assembly. However, lack of congruence between the earthworm phylogeny and the microbiome dendrogram suggests that a consistent diet fed over several generations may have weakened potential phylosymbiotic effects. Thus, cast microbiome assembly in earthworms seem to be the result of an interplay among host phylogeny and diet. Diet has a minor impact on composition, structure, and diversity of the earthworm microbiome under homogenous feeding conditions

Automated summary

Host evolutionary history and diet are important factors shaping the earthworm cast microbiome. Experimental results showed that earthworm microbiomes are species-specific and influenced by host evolutionary history and diet. Diet has a significant impact on microbiome composition, while under homogenous feeding conditions, it has a minor effect on microbiome composition, structure, and diversity.