Multilocus Phylogenetic Study of the Scheffersomyces Yeast Clade and Characterization of the N-Terminal Region of Xylose Reductase Gene

Many of the known xylose-fermenting (X-F) yeasts are placed in the Scheffersomyces clade, a group of ascomycete yeasts that have been isolated from plant tissues and in association with lignicolous insects. We formally recognize fourteen species in this clade based on a maximum likelihood (ML) phylogenetic analysis using a multilocus dataset. This clade is divided into three subclades, each of which exhibits the biochemical ability to ferment cellobiose or xylose. New combinations are made for seven species of Candida in the clade, and three X-F taxa associated with rotted hardwood are described: Scheffersomyces illinoinensis (type strain NRRL Y-48827T  =  CBS 12624), Scheffersomyces quercinus (type strain NRRL Y-48825T  =  CBS 12625), and Scheffersomyces virginianus (type strain NRRL Y-48822T  =  CBS 12626). The new X-F species are distinctive based on their position in the multilocus phylogenetic analysis and biochemical and morphological characters. The molecular characterization of xylose reductase (XR) indicates that the regions surrounding the conserved domain contain mutations that may enhance the performance of the enzyme in X-F yeasts. The phylogenetic reconstruction using XYL1 or RPB1 was identical to the multilocus analysis, and these loci have potential for rapid identification of cryptic species in this clade

Electron Microscopy of Some Special Cell Contacts in Yeasts

Anastomosis in Endomycopsis javanensis and some other filamentous yeasts was brought about by contact of a denticle from one cell with the wall of another cell, resulting in the disappearance of the outer layer and the thickening of the inner layer of the cell wall of the contacted cell. Another form of contact between cells was the penetration of one cell by a denticle on another cell which had grown out to a stalk; this occurred between cells of E. javanensis and between cells of this species and other yeast species

Bipolar budding in yeasts - an electron microscope study

Bud formation in yeasts with bipolar budding was studied by electron microscopy of thin sections. Budding in yeasts of the species Saccharomycodes ludwigii, Hanseniaspora valbyensis and Wickerhamia fluorescens resulted in concentric rings of scar ridges on the wall of the mother cell. The wall between the ridges consisted of the scar plug left by the former budding and opened up in the formation of the next bud. The wall of the bud arose from under the wall of the mother cell. In the yeasts of the species Nadsonia elongata more than one bud might be formed from the same plug. In Schizoblastosporion starkeyi-henricii the scar ridges were close together and apparently not separated by the entire plug. In all species a cross wall was formed between mother cell and bud which consisted of an electron-light layer between two layers of more electron-dense material. The cells separated along the light layer.