From the Tunnels into the Treetops: New Lineages of Black Yeasts from Biofilm in the Stockholm Metro System and Their Relatives among Ant-Associated Fungi in the Chaetothyriales.

Martina Réblová,Mats Wedin,Olle Thureborn,Johannes Lundberg,Magnus Ivarsson,Vit Hubka,Therese Sallstedt,Jason E Stajich

doi:10.1371/journal.pone.0163396

Abstract

Rock-inhabiting fungi harbour species-rich, poorly differentiated, extremophilic taxa of polyphyletic origin. Their closest relatives are often well-known species from various biotopes with significant pathogenic potential. Speleothems represent a unique rock-dwelling habitat, whose mycobiota are largely unexplored. Isolation of fungi from speleothem biofilm covering bare granite walls in the Kungsträdgården metro station in Stockholm yielded axenic cultures of two distinct black yeast morphotypes. Phylogenetic analyses of DNA sequences from six nuclear loci, ITS, nuc18S and nuc28S rDNA, rpb1, rpb2 and β-tubulin, support their placement in the Chaetothyriales (Ascomycota). They are described as a new genus Bacillicladium with the type species B. lobatum, and a new species Bradymyces graniticola. Bacillicladium is distantly related to the known five chaetothyrialean families and is unique in the Chaetothyriales by variable morphology showing hyphal, meristematic and yeast-like growth in vitro. The nearest relatives of Bacillicladium are recruited among fungi isolated from cardboard-like construction material produced by arboricolous non-attine ants. Their sister relationship is weakly supported by the Maximum likelihood analysis, but strongly supported by Bayesian inference. The genus Bradymyces is placed amidst members of the Trichomeriaceae and is ecologically undefined; it includes an opportunistic animal pathogen while two other species inhabit rock surfaces. ITS rDNA sequences of three species accepted in Bradymyces and other undescribed species and environmental samples were subjected to phylogenetic analysis and in-depth comparative analysis of ITS1 and ITS2 secondary structures in order to study their intraspecific variability. Compensatory base change criterion in the ITS2 secondary structure supported delimitation of species in Bradymyces, which manifest a limited number of phenotypic features useful for species recognition. The role of fungi in the speleothem biofilm and relationships of Bacillicladium and Bradymyces with other members of the Chaetothyriales are discussed.

Highlights

Fungi from rock-dwelling habitats can endure extreme conditions; they are of polyphyletic origin, apparently lack sexual reproductive structures, are largely undescribed and usually figure under strain or sample numbers in large-scale multilocus phylogenetic analyses
ITS rDNA sequences of three species accepted in Bradymyces and other undescribed species and environmental samples were subjected to phylogenetic analysis and in-depth comparative analysis of ITS1 and ITS2 secondary structures in order to study their intraspecific variability
During our research of the Stockholm metro system we primarily focused on fungi classified in the Chaetothyriales living in the biofilm covering bare granite walls of the artificial cave of the Kungsträdgården metro station

Summary

Introduction

Fungi from rock-dwelling habitats can endure extreme conditions; they are of polyphyletic origin, apparently lack sexual reproductive structures, are largely undescribed and usually figure under strain or sample numbers in large-scale multilocus phylogenetic analyses Their closest relatives are often well-described taxa from various biotopes, and other unnamed strains or whole lineages of such organisms thriving in the most unlikely places [1,2,3,4]. Members of the Chaetothyriales, often referred to as black yeasts, play a diverse role in nature They act as saprobes, biotrophs and weak plant pathogens, and opportunistic pathogens, some of which can cause fatal infections in humans and animals. They include lichenicolous, bryophilous, epilithic and endolithic taxa. Numerous undescribed and morphologically poorly differentiated chaetothyrialean fungi isolated from nests or tunnelshaped runway galleries of tropical ants were discovered recently and placed in the Chaetothyriales with the aid of molecular data [14,15,16,17]

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