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Obligate mutualistic symbioses rely on mechanisms that secure host-symbiont commitments to maximize host benefits and prevent symbiont cheating. Previous studies showed that somatic incompatibilities correlate with neutral-marker-based genetic distances between fungal symbionts of Panamanian Acromyrmex leaf-cutting ants, but the extent to which this relationship applies more generally remained unclear. Here we showed that genetic distances accurately predicted somatic incompatibility for Acromyrmex echinatior symbionts irrespective of whether neutral microsatellites or AFLP markers were used, but that such correlations were weaker or absent in sympatric Atta colombica colonies. Further analysis showed that the symbiont clades maintained by A. echinatior and A. colombica were likely to represent separate gene pools, so that neutral markers were unlikely to be similarly correlated with incompatibility loci that have experienced different selection regimes. We suggest that evolutionarily derived claustral colony founding by Atta queens may have removed selection for strong incompatibility in Atta fungi, as this condition makes the likelihood of symbiont swaps much lower than in Acromyrmex , where incipient nests stay open because queens have to forage until the first workers emerge. [ABSTRACT FROM AUTHOR]

Abstract: Though inconspicuous in healthy nests, Pseudoxylaria species are almost always present and overgrow deteriorating fungus-growing termite gardens. Whether these fungi are detrimental to the fungus-garden, benign, or even beneficial is unclear. We hypothesize that Pseudoxylaria is a stowaway that practices a sit-and-wait strategy to survive in the termite nest. Using isolates from three different termite genera to test our hypothesis, we compared Pseudoxylaria’s growth on 40 carbon sources with that of Termitomyces and tested its interaction with Termitomyces. The C-source use of both fungi largely overlapped, indicating potential for competition. One-to-one interactions between Pseudoxylaria, Termitomyces and free-living relatives showed that Pseudoxylaria and Termitomyces strains interacted differently with each other than with each other’s free-living relatives. Both fungi grew less together than when growing alone, confirming that they compete. Pseudoxylaria was more strongly inhibited by Termitomyces than free-living Xylariaceae were. The results suggest that the symbiotic lifestyle adopted by Pseudoxylaria goes together with reduced antagonism towards Termitomyces, consistent with Pseudoxylaria being a stowaway. [Copyright &y& Elsevier]