Phylogenetic relationships among taxa in the Jahnulales inferred from 18S and 28S nuclear ribosomal DNA sequences

Jahnulales is an order of freshwater, lignicolous, bitunicate ascomycetes characterized by wide (10-40 mu m), brown, septate hyphae, stalked and (or) sessile ascomata, ascomal walls of 2-6 layers of large cells, and 1-septate ascospores. A variety of ascospore modifications are represented among the species in the order, including wall roughening, gelatinous sheaths, appendages and (or) pads, and apical caps or spines. To clarify generic boundaries and phylogenetic relationships within the Jahnulales and to assess the taxonomic significance of various morphological characters, a molecular study was carried Out using 18S and 28S rDNA sequence data from 15 species representing the four genera in the order. In addition, Brachiosphaera tropicalis Nawawi and Xylomyces chlamydosporus Goos, R.D. Brooks & Lamore, two mitosporic species that co-occur with Jahnula Kirschst., species and have wide (>10 mu m), brown, septate hyphae were included in the study to determine whether these species are members of this order. Maximum likelihood analyses confirmed the monophyly of the Jahnulales and resolved four clades. Two robustly supported clades comprise the genera Aliquandostipite Inderb. and Megalohypha A. Ferrer & Shearer. A third well-supported clade encompassed species of Brachiosphaera, Jahnula, and Xylomyces. The fourth clade contained isolates of the type species of the genus Jahnula, Jahnula aquatica (Plottn. & Kirschst.) Kirschst., and two other members of this genus, but this clade was weakly supported. Our data suggest that the presence of very wide, brown, septate hyphae is an important character defining the Jahnulales. Based on molecular and morphological data, we propose the transfer of Jahnula siamensiae Sivichai & E.B.G. Jones and Patescospora separans Abdel-Wahab & El-Shar. to Aliquandostipite and emend the description of the Jahnulales.