Ribosomal RNA Gene-Based and Multigene Phylogenies of Smittium (Harpellales) and Allies—Toward Unraveling Relationships Among Early-Diverging Fungi

Smittium is one of the oldest members of the Harpellales, a group commonly referred to as the “gut fungi”. Gut fungi are endosymbiotic microorganisms that live in the digestive tracts of various Arthropods, worldwide. During the 75 years since the first species, Smittium arvernense, was described, Smittium has increased in number and now includes 81 species. Research on this genus has also helped to advance our understanding of the gut fungi, by serving as a “model” for laboratory studies of the fungal trichomycetes. Many isolates of Smittium have been used for ultrastructural, physiological, host feeding, serological, as well as isozyme, and now ongoing molecular systematic studies. Previous and current molecular studies have shown that Smittium is polyphyletic but with consistent separation of Smittium culisetae, one of the most common and widespread species, from the remainder of Smittium species. Morphological (zygospore and trichospore shape), molecular (18S and 28S rRNA genes), immunological, and isozyme evidence are used to establish a new genus Zancudomyces, and to accommodate Smittium culisetae. A multigene dataset (consisting of 18S and 28S rRNA genes, with RPB1, RPB2, and MCM7 translated protein sequences) for Smittium and related Harpellales (Austrosmittium, Coleopteromyces, Furculomyces, Pseudoharpella, Stachylina and Trichozygospora) was used for phylogenetic analyses and provided strong support at multiple levels in the trees generated. The clades and branches of the consensus tree are assessed relative to morphological traits, including holdfast shape, thallus branching type, trichospore or zygospore characters as an aid to inform the current taxonomy and eventual systematic revisions and reclassification. Some patterned separation was found within the “Smittium” clade, including the separation of “True Smittium” clade and “Parasmittium” clade, which was supported also by thallus branching types and trichospore shapes, and perhaps lending support to an earlier narrower definition of the genus. Suggestions are offered for future morphological- and molecular-based studies, as ongoing efforts are unfolding