Bacterial partners in ant-plant farming: the role of novel Sodalis-allied endosymbionts

Ants have evolved a diverse range of specialised lifestyles, with ant-plant mutualisms being an extreme example of this. Across the insects, maintenance of extreme lifestyles and feeding strategies have been assisted by nutrient supplementing bacterial partners. However, it is currently unclear whether bacterial endosymbionts have facilitated the evolution of ant-mutualisms. Here, I test this hypothesis using ant-plant mutualists Anonychomyrma and Philidris, who engage in complex plant-farming behaviour with various species of Squamellaria and Myrmecodia epiphytes, and have also evolved strong associations with Sodalis-allied bacteria. I assembled and annotated the genomes of the Sodalis found across three species of facultative plant-farmers Anonychomyrma, and obligate plant-farmer Philidris nagasau. I examine genome characteristics, such as GC content, size and coding density for evidence of an ancient acquisition and vertical transmission, including fluorescent in situ hybridisations in P. nagasau queens to determine bacterial localisation. I find strong evidence of an ancient association in both genera with support for vertical transmission and co-speciation within host lineages. Using gene presence and absence I also investigate the putative function of the symbionts, with genomic evidence suggesting nutrient-provisioning may occur for both the ant and plant host, indicating that these Sodalis-allied allied symbionts may have acted as evolutionary precursors to ant-plant farming relationships in these genera. More broadly, I discuss the repeated evolution of bacterial Sodalis-allied endosymbionts in ants, and the potential role they have played in the evolution of extreme lifestyles across the ants, and insects overall.