Molecular taxonomy and epidemiology of Blastomyces and Histoplasma species

Publisher Website
Google Scholar
Abstract
Summary. Cladistic analysis of partial 26S rRNA sequences was used to estimate evolutionary distances among species and varieties of the dimorphic onygenalean genera Blastomyces, Coccidioides, Emmonsia, Histoplasma and Paracoccidioides. With the exception of Coccidioides, all genera were closely related, with about 5% base differences and even less (1–2%) between Blastomyces and Emmonsia. These data were supported by a teleomorph in the same genus Ajcllomyces. In a phylogenic study of a wide range of ascomycete orders and families, Coccidioides immitis was found to be closest to Aphanoascus fulvescens and Chrysosporium keratinophilum, and to have relative distances to the remaining dimorphic genera (family Onygenaceae) similar to those of the dermatophytes (family Arthrodermataceae). The sequencing data were confirmed by genomic comparisons. All dimorphic genera had a nuclear DNA base composition in the same range of 46.6–47.3% G + C. The DNA melting curves of Blastomyces and Histoplasma strains showed irregularities that were ascribed to the presence of AT-rich stretches in satellite DNA rather than in mitochondrial DNA. Derivative profiles proved to be highly reproducible within regional populations and coincided with differences in clinical behaviour of each species. Blastomyces dermatitidis generated two kinds of curves, corresponding to the geographically distinct serotypes 1 and 2. The African type (serotype 2) was characterized by a classical sigmoidal melting curve similar to that for all strains of Coccidioides, Emmonsia and Paracoccidioides. In contrast, the American type (serotype 1) contained satellite DNA (27% G + C). A rRNA base difference of 1.5% was observed between geographical types, a value slightly higher than that noted between Histoblasma capsulatum and its variety farciminosum (0.9%). All three H. capsulatum varieties presented irregularities in their DNA melting curves. The molecular data support the recognition of two of them as agents of blastomycosis and the assignment of more than one species and two varieties to the genus Emmonsia. Zusammenfassung. Die Analyse partieller 26S rRNA-Sequenzen wurde zur Bestimmung evolutionärer Distanzen zwischen Arten und Varietäten der zu den Onygenales gehörenden dimorphen Gattungen Blastomyces, Coccidioides, Emmonsia, Histoplasma und Paracoccidioides eingesetzt. Mit Ausnahme von Coccidioides erwiesen sich alle Gattungen mit weniger also 5% Basendifferenzen eng verwandt. Bei Blastomyces und Emmonsia lag die Differenz sogar nur bei 1 bis 2%; diese Daten wurden auch durch das gemeinsame Teleomorph Ajellomyces gestützt. In einer phylogenetischen Studie über einen weiten Bereich von Ascomyceten-Ordnungen und -Familien erwies sich Coccidioides immitis als nächst verwandt mit Aphanoascus