Molecular evolution of the fungi: human pathogens
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B H Bowman1, J W Taylor, T J White
1Roche Molecular Systems, Alameda, California 94501.
Abstract:
The morphological, ecological, and clinical diversity among ascomycete fungi that are pathogenic to humans suggest that the potential for pathogenicity may have arisen multiple times within these higher fungi. We have obtained 18S ribosomal DNA sequences from a diverse group of human pathogenic fungi in order to determine their evolutionary origins. The fungi studied include a skin pathogen that is confined to humans (Trichophyton rubrum) and three systemic, facultative parasites that cause histoplasmosis (Histoplasma capsulatum), blastomycosis (Blastomyces dermatitidis) and coccidioidomycosis (Coccidioides immitis) in humans and other higher animals. Also included in our analysis are representatives of non-pathogenic fungi, as well as two opportunistic pathogens, Pneumocystis carinii and Candida albicans, that cause severe disease in immunocompromised individuals, especially those with AIDS. Two of the fungi we sequenced, T. rubrum and C. immitis, are limited to asexual modes of reproduction and therefore lack the sexual structures that are most useful for evolutionary comparison as well as being essential for classification among the higher fungi. Coccidioides immitis is particularly problematic owing to its contradictory and confusing asexual morphologies, which have caused it to be placed in three fungal classes and the protista. Our analysis shows that the specialized, superficial parasite and the systemic, facultative parasites, including C. immitis, are closely related ascomycetes, which clearly demonstrates the power of molecular characters to compensate for missing or confusing reproductive morphology. Analysis also shows that the opportunistic pathogens are more distantly related, with the likely explanation that pathogenicity has arisen more than once within the Ascomycetes.
Insights
Molecular analysis reveals human pathogenic fungi, including Trichophyton rubrum and Coccidioides immitis, are closely related ascomycetes. This suggests pathogenicity evolved multiple times within these fungi, highlighting molecular data
Area of Science:
- Mycology
- Evolutionary Biology
- Molecular Biology
Background:
- Human pathogenic fungi exhibit diverse morphologies and ecological niches.
- The evolutionary origins of fungal pathogenicity are not fully understood.
- Some pathogenic fungi, like Coccidioides immitis, present classification challenges due to asexual reproduction.
Purpose of the Study:
- To determine the evolutionary origins of human pathogenic fungi using molecular data.
- To investigate the phylogenetic relationships among various ascomycete fungi with pathogenic potential.
- To assess whether pathogenicity arose multiple times within the Ascomycetes.
Main Methods:
- Sequencing of 18S ribosomal DNA from a diverse group of human pathogenic and non-pathogenic fungi.
- Phylogenetic analysis of obtained DNA sequences.
- Comparison of molecular data with existing morphological and ecological classifications.
Main Results:
- Superficial and systemic pathogenic fungi, including Coccidioides immitis, are closely related ascomycetes.
- Molecular data effectively compensates for missing or ambiguous reproductive morphology in classification.
- Opportunistic pathogens like Pneumocystis carinii and Candida albicans are more distantly related, suggesting independent origins of pathogenicity.
Conclusions:
- Pathogenicity in ascomycete fungi has likely arisen independently on multiple occasions.
- Molecular phylogenetics provides robust insights into fungal evolution, even with limited morphological data.
- Understanding the evolutionary history of fungal pathogens is crucial for medical mycology.