Studies on the pollen-trapping fungal genus Retiarius: Taxonomy, phylogenetics, and metabolism with a special focus on western Canada

Abstract

Fungi can obtain nutrients from a variety of sources, including some which capture and degrade pollen grains. Retiarius Olivier (Orbiliomycetes, Ascomycota) is an anamorphic hyphomycete that traps airborne pollen grains with aerial hyphae. Only four species, described from South Africa and Hungary, are known within the genus. Retiarius has been widely reported in Europe, with several dispersed records world-wide including a single record from North America. In recent years, Retiarius has been detected in metagenome sequencing projects of lichens. In addition, microscopic observations of pollen grain deposits on the undersurfaces of lichens revealed hyphae infecting pollen grains. With these observations, I suspected that Retiarius was more prevalent in Western Canada than the previous sampling would suggest. In my thesis, I developed genus-specific PCR primers to screen for the presence of Retiarius on lichens and adjacent substrate from forests in Canada, the western US, and northern Mexico. I isolated strains in axenic culture of Retiarius from the lower surface of rock-dwelling lichens. Two of the strains were used in phenotype profiling experiments to characterize carbon and nitrogen utilization. The environmental screening suggested that Retiarius is ubiquitous in lichens and adjacent substrates of all forested ecosystems included in my sampling. In the Canadian Arctic, Retiarius was detected at low frequency rates. Phylogenetic analyses using a multilocus tree revealed two undescribed species: Retiarius canadensis nom. prov., and Retiarius crescentus nom. prov. Both were isolated from the lower surface of lichens, with morphological characteristics different from the known species of Retiarius. Using sequences derived from environmental sequencing, I identified an additional 17 putative species-level lineages of Retiarius. The phenotype profiling data were inconsistent in characterizing Retiarius carbon and nitrogen utilization. As a resource for future studies, the genome of Retiarius crescentus nom. prov. was sequenced and annotated, representing the first genome of a non-nematode trapping species in Orbiliomycetes. This research will serve as a benchmark for studying the genus Retiarius, expanding the current knowledge of its phylogeny and species richness. The evidence indicates a great diversity of species yet to be described, present in a wide variety of ecosystems across North America, and it might play a key role in nutrient cycling by tapping into the nutrient-rich pollen grains.