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.