Diversity, host specificity, and evolution of trophic modes among endophytic, endolichenic, and saprotrophic fungi

Background/Question/Methods Fungal endophytes occur in the healthy, aerial tissues of every plant species surveyed to date, forming symbioses with bryophytes, ferns and allies, conifers, and angiosperms in a wide range of terrestrial communities. Fungi that resemble endophytes also have been discovered in association with algal photobionts within apparently healthy lichen thalli. These so-called “endolichenic fungi” appear more closely related to endophytes than to previously recognized lichen-associated fungi (i.e., lichen-forming mycobionts and lichenicolous fungi). Although frequently considered ‘incidental’ inhabitants of their hosts, growing evidence suggests that endophytes and endolichenic fungi play important, if cryptic and variable, ecological roles. Moreover, these little-known but apparently hyperdiverse symbionts provide an unprecedented opportunity to examine the evolution of major ecological modes in fungi. To examine their diversity, host specificity, and evolutionary relationships, we isolated endophytic fungi from the interior of living leaves (10 plant species, including bryophytes, ferns, conifers, and angiosperms), saprotrophic fungi from dead leaves in the canopy and host-associated leaf-litter (three focal plant species, each of which was also examined for endophytes), and endolichenic fungi from healthy lichen thalli (10 phylogenetically diverse lichen species) in the Chiricahua Mountains of southeastern Arizona. Using a culture-based method, we recovered 732 endophytic and endolichenic isolates, and 355 saprotrophic isolates. The internal transcribed spacer (ITSrDNA) was amplified and sequenced bidirectionally for all isolates and used to designate operational taxonomic units for estimating diversity and host-specificity.

Results/Conclusions We found remarkable diversity of fungi from all trophic modes: over 300 distinctive genotypes were recovered from only the first 700 isolates, and >70% of these were singletons. Genotypic assessments indicate the similarity of endolichenic and endophytic fungal assemblages, with mosses sharing more fungi with lichens than do other plant lineages. Phylogenetically distinct lichens share many fungi regardless of their saxicolous or corticolous growth forms. Communities of saprotrophs from dead leaves in the canopy were genotypically similar to endophytes, but leaf litter contained numerous fungi that were never found among endophytic or endolichenic assemblages. Preliminary phylogenetic analyses suggest some host-specific saprotrophic clades containing both dead-leaf and leaf litter fungi, and underscore the evolutionary distinctiveness of many symbiotic and saprotrophic taxa. We will examine the taxonomic distribution of fungi across these guilds and the clade-specific diversity they represent, and will present additional phylogenetic analyses that consider the evolutionary relationships – and evolution of trophic or symbiotic modes – of these novel fungi.