Effect of Protein Supplementation on the Gut Microbiome of Omnivorous and Herbivorous Goliath Beetles

Scarab beetles depend on gut microbes for digestive enzymes. Some species’ microbiomes show taxonomic conservation regardless of diet, while others have conserved functional profiles. We compared the hindgut microbiomes of two Cetoniinae beetle larvae: the obligately saproxylophagous Mecynorrhinella poggei and the omnivorous/predatory Goliathus goliatus, which requires protein supplementation when reared artificially. Two diets with and without supplementation were used. If diet drives the microbiome, then gut microbes in protein-supplemented hosts should produce fewer lignocellulolytic enzymes and more proteinases regardless of species. If microbiome composition is conserved within a species, then Mecynorrhinella is expected to have more lignocellulolytic microbes while Goliathus should have more proteinolytic microbes regardless of diet. In this study, low-protein diets reduced G. goliatus growth, but gut microbiome composition and predicted function remained largely stable, dominated by Bacteroidales including Dysgonomonas, Proteiniphilum, and Alistipes. Mecynorrhinella’s gut microbiome showed some reduced Proteiniphilum and increased Dysgonomonas relative abundance, but otherwise the microbiome composition was statistically stable with no effect of diet on growth or predicted microbiome function. These results highlight that microbiomes of closely related insects can markedly differ even if diet does not and that microbiome functional conservation tied to host physiology may occur even if greater plasticity could theoretically reduce malnutrition.