The Population Biology of Coccidioides

Abstract

Studies of field- and patient-derived isolates conducted over the past 75 years have provided a general picture of the population structure of Coccidioides, the cause of coccidioidomycosis. Premolecular studies provided a general outline of the geographical range, epidemiology and distribution of the fungus. Recent studies based on molecular markers have demonstrated that the genus is comprised of two genetically diverse, and genetically isolated, species: Coccidioides immitis and C. posadasii. Both species are composed of biogeographically distinct populations. Structure for two of these populations (C. immitis from central California, and C. posadasii from southern Arizona) indicates that frequent genetic recombination occurs within the entire geographic range of each population, even though sex has never been observed in the genus. Outbreaks of coccidioidomycosis are not the result of the spread of a single clonal isolate, but are caused by a diversity of genotypes. Although it is now possible to match patient isolates to populations, the lack of apparent structure within each population and the current paucity of environmental isolates limit map-based epidemiological approaches to understanding outbreaks. Therefore, a comprehensive database comprised of soil-derived isolates from across the biogeographic range of Coccidioides will improve the utility of this approach. Appropriate collection of environmental isolates will assist the investigation of remaining questions regarding the population biology of Coccidioides. The comparative genomics of representative genotypes from both species and all populations of Coccidioides will provide a thorough set of genetic markers in order to resolve the population genetics of this pathogenic fungus.

Molecular and Morphological Data Support the Existence of a Sexual Cycle in Species of the Genus Paracoccidioides

“…In addition, we have identified all orthologs of the mating machinery in both C. immitis and C. posadasii (see Table S4 in the supplemental material), although no evidence of sexual structures has yet been uncovered. It is well known that the Coccidioides genus represents a cryptic speciation (65), and there is also evidence of a recombination process in this genus (66), supporting the existence of a sexual cycle. Furthermore, the ratio of MAT1-1/MAT1-2 idiomorphs is 1:1, providing molecular support for the presence of a sexual stage in the life cycle (40).…”

The Evolution of Sex: a Perspective from the Fungal Kingdom

“…P. brasiliensis, the causative agent of paracoccidioidomycosis, is widespread in Latin America (262). C. immitis and C. posadasii are causative agents of coccidioidomycosis, or valley fever, which is endemic to the Southwestern United States and Central and South America (13). B. dermatitidis is the causative agent of blastomycosis and is endemic to the Americas (26).…”

Use of Population Genetics to Assess the Ecology, Evolution, and Population Structure ofCoccidioides

“…Because coccidioidomycosis is increasing and disease severity is highly variable, defining genotypic distribution is important for monitoring outbreaks and determining whether increased pathogenicity is an emerging trait ( 24 ). Previous analysis showed that a single clone did not cause the rise in infection rates in Arizona; rather, each isolate recovered from a patient was unique ( 25 ).…”