Phylogenetic heterogeneity of the genus Williopsis as revealed by 18S rRNA gene sequences

A phylogenetic investigation of the ascomycetous yeast genus Williopsis was performed by using 18S rRNA gene sequence analysis. Comparative sequence analysis revealed the genus to be phylogenetically heterogeneous. The five varieties of Williopsis saturnus [var. mrakii, var. sargentensis, var. saturnus (type), var. suaveolens and var. subsufficiens] were found to have identical 18S rRNA gene sequences and formed a distinct group, quite separate from all other Williopsis and non-Williopsis species examined. Williopsis mucosa was found to be the closet phylogenetic relative to the Williopsis saturnus group, however a sequence divergence of approximately 2.3% suggests this species may belong to a separate genus. The recently described species Williopsis salicorniae was found to exhibit a relatively close association with Ogataea minuta (identical to Pichia minuta), the type species of the genus Ogataea. The remaining two members of the genus, Williopsis californica and Williopsis pratensis, were found to form distinct lineages, displaying no specific association with any other Williopsis or non-Williopsis species. Based on comparative analysis of 18S rRNA genes it is apparent that the genus Williopsis as presently constituted is not monophyletic, and that the five currently recognized species form separate sublines each potentially worthy of separate generic status. The genus Williopsis should be restricted to the type species Williopsis saturnus and its five varieties. Despite the five varieties of Williopsis saturnus being genealogically indistinguishable at the 18S rRNA gene level, sequence analysis of the Internal transcribed spacer (ITS) region revealed that the five varieties could be differentiated on both their ITS1 and their ITS2 sequences, providing further evidence of the value of ITS sequences for discrimination of yeasts at the subspecies level.