A reevaluation of the diffuse coevolution model in Trachymyrmex ants

Although fungus-gardening ants are often used as examples of 1:1 coevolution, this is generally true only at higher phylogenetic levels, i.e., when comparing clades of ants to clades of their symbiotic fungi.  However, within clade, horizontal exchange (symbiont sharing) is thought to be more common, hence it is thought that a diffuse (many to one) coevolution model is more appropriate. One possible explanation is that phylogenetically weak markers might obscure the evolutionary history of symbiont exchange, so that diffuse coevolution might be an artifact of poor phylogenetic resolution.  In the following study, we tested the diffuse coevolution model by conducting cross-fostering experiments where we forced newly mated queens to grow different clades of ant fungi and compared these results with phylogenetic analysis of specificity using single-nucleotide polymorphisms. The experiments indicated significant synergisms among specific clades of fungi (some fungal clades were better at producing ant offspring than other ant fungi), whereas the phylogenetic overview supported the coevolution model. Between now and November we expect to have an explanation for why the experiments and phylogenetics produce different results.