Taxonomy and phylogeny of the epiphytic sooty molds in family Metacapnodiaceae (class Eurotiomycetes, subclass Chaetothyriomycetidae)

Metacapnodiaceae is one of several sooty mold families in Ascomycota. Its species grow as dense, black, spongy mats on the surfaces of plant leaves and twigs, typically in association with insect honeydew or plant leachates. Studying Metacapnodiaceae has been challenging because multiple species intermingle in the same small patches of mycelium, and they are difficult to culture or to distinguish by their morphology.

Prior to this research, DNA sequences were available for only two species in the family. Here, with the goal of better characterizing species diversity, we determined complete or partial DNA sequence barcodes for ribosomal internal transcribed spacer (ITS) regions for 16 collections of Metacapnodium using a Metacapnodium - specific primer, followed by phylogenetic and morphological analyses. Tapering, moniliform hyphae, cells wider than long, and a distinctive phialidic asexual state were good predictors of membership in a well-supported Metacapnodium clade.

Sequences from the 16 collections represent 9 named species of Metacapnodium. Barcoded species include: M. stanhughesii sp. nov., M. vancouverensis, sp. nov. and M. australis comb. nov. Based on morphological characters, we propose M. atro-olivaceus comb. nov., M. novae-zelandiae comb. nov., and M. pacificus comb. nov.

We provide a key to identification of 15 species studied. To investigate the deeper phylogenetic relationships of Metacapnodiaceae, we sequenced partial nuclear ribosomal large subunit (LSU) gene regions from five specimens and elongation factor 1 - alpha (ef 1 - α) gene regions from two specimens. In our analysis of concatenated sequences from ribosomal DNA, ef 1 - α, and from rpb 2, the gene encoding the 2 nd largest subunit of RNA polymerase II protein, Metacapnodium appeared within the subclass Chaetothyriomycetidae, class Eurotiomycetes with strong support, and as the sister group to Pleostigmataceae but without strong statistical support.

Our study adds Metacapnodiaceae to the clades of enigmatic, slow growing fungi of harsh environments with lichenized, lichenicolous, resinicolous and rock-inhabiting niches. Resolving family relationships is relevant to age estimates of Ascomycota, as fossilized Metacapnodium specimens in amber potentially contribute to calibration of divergence times.