Extensive photobiont sharing in a rapidly-radiating cyanolichen clade

Recent studies have uncovered remarkable diversity Dictyonema s.lat.
basidiolichens, here recognized as subtribe Dictyonemateae. This group
includes five genera and 148 species, but hundreds more await description.
The photobionts of these lichens belong to Rhizonema, a recently
resurrected cyanobacterial genus known by a single species. To further
investigate photobiont diversity within Dictyonemateae, we generated 765
new cyanobacterial sequences from 635 specimens collected from 18
countries. The ITS barcoding locus supported the recognition of 200
mycobiont (fungal) species among these samples, but the photobiont
diversity was comparatively low. Our analyses revealed three main
divisions of Rhizonema, with two repeatedly recovered as monophyletic
(proposed as new species), and the third mostly paraphyletic. The
paraphyletic lineage corresponds to R. interruptum and partnered with
mycobionts from all five genera in Dictyonemateae. There was no evidence
of photobiont-mycobiont co-speciation, but one of the monophyletic
lineages of Rhizonema appears to partner predominantly with one of the two
major clades of Cora (mycobiont) with samples collected largely from the
northern Andes. Molecular clock estimations indicate the Rhizonema species
are much older than the fungal species in the Dictyonemateae, suggesting
that these basidiolichens obtained their photobionts from older ascolichen
lineages and the photobiont variation in extant lineages of Dictyonemateae
is the result of multiple photobiont switches. These results support the
hypothesis of lichens representing "fungal farmers", in which
diverse mycobiont lineages associate with a substantially lower diversity
of photobionts by sharing those photobionts best suited for the lichen
symbiosis among multiple and often unrelated mycobiont lineages.