A cosmopolitan Serendipita forms mycothalli with sub-Antarctic leafy liverworts.

The occurrence of mycothalli, symbioses between liverworts and fungi, is poorly documented in sub-Antarctica, and biogeographical patterns in Serendipita, the main fungal genus forming the symbiosis, remain understudied. Here, 83 specimens of 16 leafy liverwort species were sampled from sub-Antarctic South Georgia and were examined for mycothalli. Microscopy was used to enumerate fungal structures in liverwort tissues, and sequencing of fungal ribosomal DNA was used to determine the taxonomic and biogeographical affinities of the fungi. Stained hyphal coils, a defining feature of the symbiosis, were found to be frequent (>40% of stem length colonised) in Barbilophozia hatcheri, Cephaloziella varians and Lophoziopsis excisa. A single species of Serendipita, based on a 3% cut-off for ITS2 region sequence divergence, was a frequent colonist of these liverworts. A further 18 basidiomycete and ascomycete taxa colonised other liverwort species. The presence of the Serendipita species was positively associated with the occurrence of stained hyphal coils in stem epidermal cells. Phylogenetic analyses, incorporating worldwide accessions from leafy liverwort-associated Serendipita, showed that the same species, which also occurs in Chile, mainland Europe and on Svalbard, is apparently the sole symbiont of sub- and maritime Antarctic leafy liverworts, and indicated much higher species richness of the genus outside Antarctica.