status and characterization of Enteroramus dimorphus: a xylose-fermenting yeast attached to the gut of beetles

Enteroramus dimorphus from the gut of the passalid beetle Odontotaenius disjunctus was described originally as a yeast-like fungus of unknown taxonomic affiliation. The fungus can be observed in situ, attached by a specialized cell to the beetle hindgut wall. In a recent study of yeast endosymbionts from a variety of beetles, we discovered that E. dimorphus is a member of the Pichia stipitis (Saccharomycetes) clade, known for xylose fermentation and assimilation. The closest relative of E. dimorphus is the PASS1 isolate, repeatedly acquired from passalid beetles in eastern North America from Pennsylvania to Louisiana. In addition to xylose fermentation and assimilation, these yeasts are characterized by the production of hat-shaped ascospores in culture, assimilation of a wide range of sugars, and synthesis of several vitamins. Enteroramus dimorphus, however, can be distinguished from close relatives by several physiological characteristics and rDNA sequences, which vary slightly from the more widespread PASS1 genotype. We present an amended description of E. dimorphus and discuss its symbiotic phase in association with O. disjunctus, including a holdfast that parallels those of unrelated symbiotic yeasts associated with nematodes.