Reciprocal microbiome transplants differentially rescue fitness in two syntopic dung beetle sister species (Scarabaeidae: Onthophagus)

1. Microbial symbionts play a crucial role in the development, health, and homeostasis of their hosts. However, the eco-evolutionary conditions shaping these relationships and the evolutionary scale at which host–microbiome interactions may diverge warrant further investigation, especially in non-model systems. This study examines the impact of reciprocal gut microbiome transplants between two ecologically very similar, sympatric, and syntopic dung beetle sister species. 2. Onthophagus vacca and Onthophagus medius were specifically used to compare the growth, development, and fitness outcomes of individuals that were either (i) reared in the presence of a microbiome provided by a mother of the same species (“self-inoculated”), (ii) forced to develop with a microbiome derived from a heterospecific mother (“cross-inoculated”), or (iii) reared without a maternally transmitted microbiome. 3. This study found that individuals reared in the absence of a maternally derived gut microbiome incur detrimental changes in survival, as well as in several metrics signalling normative development. Furthermore, such negative effects are only partly rescued through inoculation with a heterologous microbiome. 4. Collectively, this study's results suggest that inoculation with a species-specific, maternally transmitted microbiome is critical for normative development, that the significance of maternally derived microbiota for host survival differs across species, and that the phenotypic outcomes resulting from host–microbiome interactions may diverge even between closely related, ecologically similar host species.