Based on recent phylogenetic studies of the basidiolichen subtribe Dictyonematinae, 149 species are now accepted in five genera, with hundreds of species remaining to be described. The identified photobionts of these species are members of a remarkable clade of cyanobacteria called Rhizonema, found predominantly associated with lichens, both basidiolichens and ascolichens (e.g., Coccocarpia, among others). To investigate the levels of photobiont diversity and specificity in this group, we generated 16S rDNA and rbcLX sequences from over 600 specimens representing all major clades within Dictyonematinae and an additional 21 ascolichens suspected to contain Rhizonema. This dataset contains samples collected from 18 countries, mainly from the Neotropics and Paleotropics. All lichens were found to harbor one of three putative species of Rhizonema: R. andinum, R. interruptum or R. neotropicum. There was no indication of photobiont-mycobiont co-evolution at the species level in any of the clades in Dictyonematinae; however, one of the three lineages of Rhizonema, R. andinum, appears to partner primarily with one of the two larger Cora clades, specifically the clade mostly composed of northern Andean taxa. Rhizonema interruptum was represented by a much larger number of haplotypes and associates with mycobionts from many clades, and an ancestral photobiont association reconstruction for Dictyonematinae indicated an early mycobiont association with this cyanobiont lineage. Based on the reconstruction of both fungal and cyanobacterial molecular clocks, we found that Rhizonema is much older than the Basidiomycota they associate with and was likely present in much older ascolichen clades (e.g., Coccocarpiaceae) before associating with basidiolichens. Our results for Dictyonematinae confirm other findings that the diversity of photobionts is far lower than that of the mycobionts, a view that lends further support to the “fungal farmers” hypothesis, in which photobiont lineages are under strong selection driven by lichenization.
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