Vertical transmission of fungus-growing ant microbiota is species-specific and constrained by queens

Multipartite symbioses are inherently complex, involving dynamic ecological interactions between organisms with intertwined yet distinct evolutionary histories. The fungus-growing (attine) ants facilitate maintenance of a symbiotic species network through maternal vertical transmission of an obligate fungal symbiont. While the gut microbiomes of fungus-growing ant species are remarkably simple, their fungal gardens support diverse microbial communities. Here, we focus on an understudied transmission bottleneck: the fungal garden pellet that nest-founding queens transfer to inoculate a new fungal garden. We used 16S rRNA metagenomic sequencing to reconstruct the extent of vertical transmission of bacteria to new gardens via queen pellets in four sympatric fungus-growing ant species (Atta sexdens, Atta cephalotes, Acromyrmex echinatior, and Mycetomoellerius mikromelanos) from Central Panama. We also characterized the bacterial communities associated with queen eggs and somatic tissues (mesosomas, guts and ovaries) to assess whether queens are likely to transmit symbiotic bacteria of workers, such as cuticular Actinobacteria and endosymbionts (Wolbachia, Mesoplasma, and Spiroplasma). Our results suggest that garden-associated bacteria are mainly horizontally acquired as the bacterial communities of pellets shared few bacterial taxa with the mature gardens of the four ant species investigated. While the bacterial communities of garden pellets showed some species-specificity, a subset of prevalent bacterial taxa were shared across ant species. Further, our findings provide evidence for vertical transmission of species-specific endosymbiotic bacteria through a transovarial route and/or via fecal droplets. Overall, while we found mixed evidence for vertical transmission of garden bacteria, our results support maternal transmission as a primary route for gut-associated symbionts. While our results suggest that vertical transmission of fungus-growing ant bacterial associates is mediated by the ant hosts, the mechanism behind this host control is not yet understood.