Individual cyanobacterial species can occupy a diversity of symbiotic lifestyles yet how this lifestyle is determined is unknown. In this study we focus on the symbiotic lifestyles of the cyanobacterial genus Nostoc, and define their lifestyle based on their symbiotic state: free-living or symbiotic with lichen-forming fungi. Within an evolutionary context, members of one symbiotic lifestyle can be found more closely related to members of another symbiotic lifestyle and do not form monophyletic groups. This pattern can be explained by either deterministic or variable factors, by whether or not switches between lifestyles occur infrequently or frequently, and by limitations on symbiont dispersal. We began this study by focusing within a single habitat in which both free-living and symbiotic Nostoc sp. individuals occur in order to test amongst hypotheses. Using the rbcLX genetic region we placed individuals within a larger phylogenetic context using a worldwide dataset of free-living and symbiotic Nostoc sp. Free-living Nostoc sp. growing at this site were distinct from co-occurring lichenized strains, but closely related to symbiotic Nostoc sp. from other continents. We expanded our sampling to a larger geographic area, including free-living Nostoc sp., as well as two additional sites in which free-living and lichenized Nostoc sp., co-occurred. We found no evidence of spatial structure on a regional scale. Based on these results, we conclude that symbiotic lifestyles within distinct Nostoc sp. lineages are determined by variable factors, and their distributions are constrained and exclude physical proximity. Finally, in the course of identifying the fungal symbionts of Nostoc we identified two species of Peltigera using ITS rDNA sequences; These species are either under-reported in Wisconsin (P. ponojensis ) or new to the U.S.A. (P. islandica).
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Vol. 120 • No. 4