Interspecific coexistence selects for variants enhancing mutualism and productivity. Xanthomonas retroflexus strain variants.. Interspecific coexistence selects for variants enhancing mutualism and productivity. Xanthomonas retroflexus strain variants.

Coexistence is an interplay between inter- and intraspecific interactions. The aim of this study was to experimentally investigate this interplay in a facultative mutualistic co-culture. Our results show that interactions between Paenibacillus amylolyticus and Xanthomonas retroflexus could select for a new, wrinkled variant of X. retroflexus and allow for it to establish in the co-culture. However, the initial emergence of the new wrinkled variant was not dependent on the presence of P. amylolyticus. In co-cultures with P. amylolyticus, the wrinkled X. retroflexus significantly enhanced the mutualistic interaction benefitting both species, which provided a positive selection for the wrinkled phenotype in the co-cultures. We examined how competition between the two genotypes of the X. retroflexus affected the stability of the facultative mutualistic dual-species interaction and found at stabilising effect on the two species’ coexistence. Our results showed that interspecies interactions can shape the diversification of the other member in the co-culture by selection and by this selection enhance the interaction. This illustrates the importance of considering internal species variation when evaluating co-culture dynamics.