MOLECULAR TAXONOMY AND EPIDEMIOLOGY OF BLASTOMYCES AND HISTOPLASMA SPECIES

Cladistic analysis of partial 26S rRNA sequences was used to estimate evolutionary distances among species and varieties of the dimorphic on ygenalean 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%) b etween Blastomyces and Emmonsia. These data were supported by a teleom orph in the same genus Ajellomyces. In a phylogenic study of a wide ra nge of ascomycete orders and families, Coccidioides immitis was found to be closest to Aphanoascus fulvescens and Chrysosporium keratinophil um, and to have relative distances to the remaining dimorphic genera ( family Onygenaceae) similar to those of the dermatophytes (family Arth rodermataceae). The sequencing data were confirmed by genomic comparis ons. All dimorphic genera had a nuclear DNA base composition in the sa me range of 46.6-47.3% G+C. The DNA melting curves of Blastomyces and Histoplasma strains showed irregularities that were ascribed to the pr esence of AT-rich stretches in satellite DNA rather than in mitochondr ial DNA. Derivative profiles proved to be highly reproducible within r egional populations and coincided with differences in clinical behavio ur of each species. Blastomyces dermatitidis generated two kinds of cu rves, corresponding to the geographically distinct serotypes 1 and 2. The African type (serotype 2) was characterized by a classical sigmoid al melting curve similar to that for all strains of Coccidioides, Emmo nsia and Paracaccidioides. 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 tha t noted between Histoplasma capsulatum and its variety farciminosum (0 .9%). All three H. capsulatum varieties presented irregularities in th eir 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.