Co-evolutionary patterns and diversification of ant-fungus associations in the asexual fungus-farming ant Mycocepurus smithii in Panama.

Partner fidelity through vertical symbiont transmission is thought to be the primary mechanism stabilizing cooperation in the mutualism between fungus-farming (attine) ants and their cultivated fungal symbionts. An alternate or additional mechanism could be adaptive partner or symbiont choice mediating horizontal cultivar transmission or de novo domestication of free-living fungi. Using microsatellite genotyping for the attine ant Mycocepurus smithii and ITS rDNA sequencing for fungal cultivars, we provide the first detailed population genetic analysis of local ant-fungus associations to test for the relative importance of vertical vs. horizontal transmission in a single attine species. M. smithii is the only known asexual attine ant, and it is furthermore exceptional because it cultivates a far greater cultivar diversity than any other attine ant. Cultivar switching could permit the ants to re-acquire cultivars after garden loss, to purge inferior cultivars that are locally mal-adapted or that accumulated deleterious mutations under long-term asexuality. Compared to other attine ants, symbiont choice and local adaptation of ant-fungus combinations may play a more important role than partner-fidelity feedback in the co-evolutionary process of M. smithii and its fungal symbionts.

Multiple gains and losses of Wolbachia symbionts across a tribe of fungus-growing ants.

Although the intracellular bacterium Wolbachia is ubiquitous in insects, it has a unique relationship with New World ants on which particular bacterial strains have specialized. However, data are from distantly related hosts and detailed phylogenetic information which could reveal transmission dynamics are lacking. Here, we investigate host-Wolbachia relationships in the monophyletic fungus-growing ant tribe Attini, screening 23 species and using multilocus sequence typing to reliably identify Wolbachia strains. This technique reduces the significant problem of recombination seen using traditional single gene techniques. The relationship between Wolbachia and the fungus-growing ants appears complex and dynamic. There is evidence of co-cladogenesis, supporting vertical transmission; however, this is incomplete, demonstrating that horizontal transmission has also occurred. Importantly, the infection prevalence is frequently different between closely related taxa, with the Acromyrmex leaf-cutting ants appearing particularly prone to infection and there being no consistent relationship with any of the major life history transitions. We suggest that infection loss and horizontal transmission have driven epidemics or selective sweeps of Wolbachia, resulting in multiple gains and losses of infection across the fungus-growing ants.