Phylogenetics of Heterocystous Cyanobacteria and the Evolution of Specificity and Selectivity in Cyanolichen Symbioses
I begin by presenting a multi-locus phylogenetic analysis of the order Nostocales. I show that reference cultures on which cyanobacterial taxonomy is based do not reflect the diversity of cyanobacteria encountered in nature. Nostoc comprises two distinct lineages. One contains the best-characterized Nostoc strains from culture collections and the other the type species of the genus and most of the strains that participate in symbiotic associations (Nostoc sensu stricto). I next show that within Nostoc sensu stricto, closely related strains are able to form associations with a broad range of plant and fungal hosts as well as growing free-living. Finally, I investigate fine-scale patterns of specialization by focusing on one particular symbiosis: the association between the lichen fungus Peltigera and its photobiont Nostoc. I use phylogenetic and population genetic approaches to characterize the population structures of both partners at nested spatial scales. I show that the fungi comprise a diverse community of reproductively isolated species that have recombining population structures while the population structure of Nostoc is predominantly clonal. There is tremendous variation in photobiont specificity with Peltigera, from species that associate exclusively with a single Nostoc lineage that is not found in any other association, to species that associate with all available Nostoc genotypes. By associating with a partner with a broad ecological amplitude that is ubiquitously distributed such as Nostoc, spores of Peltigera has been able to become one of the most diverse and ecologically dominant groups in many habitats.
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