Acquisition of Uropygial Gland Microbiome by Hoopoe Nestlings

Mutualistic symbioses between animals and bacteria depend on acquisition of appropriate symbionts while avoiding exploitation by non-beneficial microbes. The mode of acquisition of symbionts would determine, not only the probability of encountering but also evolutionary outcomes of mutualistic counterparts. The microbiome inhabiting the uropygial gland of the European hoopoe (Upupa epops) includes a variety of bacterial strains, some of them providing antimicrobial benefits. Here, the mode of acquisition and stability of this microbiome is analyzed by means of Automated rRNA Intergenic Spacer Analysis and two different experiments. The first experiment impeded mothers' access to their glands, thus avoiding direct transmission of microorganisms from female to offspring secretions. The second experiment explored the stability of the microbiomes by inoculating glands with secretions from alien nests. The first experiment provoked a reduction in similarity of microbiomes of mother and nestlings. Interestingly, some bacterial strains were more often detected when females had not access to their glands, suggesting antagonistic effects among bacteria from different sources. The second experiment caused an increase in richness of the microbiome of receivers in terms of prevalence of Operational Taxonomic Units (OTUs) that reduced differences in microbiomes of donors and receivers. That occurred because OTUs that were present in donors but not in receivers incorporated to the microbiome of the latter, which provoked that cross-inoculated nestlings got similar final microbiomes that included the most prevalent OTUs. The results are therefore consistent with a central role of vertical transmission in bacterial acquisition by nestling hoopoes and support the idea that the typical composition of the hoopoe gland microbiome is reached by the incorporation of some bacteria during the nestling period. This scenario suggests the existence of a coevolved core microbiome composed by a mix of specialized vertically transmitted strains and facultative symbionts able to coexist with them. The implications of this mixed mode of transmission for the evolution of the mutualism are discussed.