Differential Thermotolerance Adaptation between Species of Coccidioides

Abstract

Coccidioidomycosis, or Valley fever, is caused by two species of dimorphic fungi. Based on molecular phylogenetic evidence, the genus Coccidioides contains two reciprocally monophyletic species: C. immitis and C. posadasii. However, phenotypic variation between species has not been deeply investigated. We therefore explored differences in growth rate under various conditions. A collection of 39 C. posadasii and 46 C. immitis isolates, representing the full geographical range of the two species, was screened for mycelial growth rate at 37 °C and 28 °C on solid media. The radial growth rate was measured for 16 days on yeast extract agar. A linear mixed effect model was used to compare the growth rate of C. posadasii and C. immitis at 37 °C and 28 °C, respectively. C. posadasii grew significantly faster at 37 °C, when compared to C. immitis; whereas both species had similar growth rates at 28 °C. These results indicate thermotolerance differs between these two species. As the ecological niche has not been well-described for Coccidioides spp., and disease variability between species has not been shown, the evolutionary pressure underlying the adaptation is unclear. However, this research reveals the first significant phenotypic difference between the two species that directly applies to ecological research.

A Review of Coccidioides Research, Outstanding Questions in the Field, and Contributions by Women Scientists

“…Continued disease surveillance and soil sampling will help to further resolve the ecological niche of Coccidioides and discrepancies between estimates, especially as we identify new endemic areas, such as Washington State [ 53 , 54 ]. By identifying differences between Coccidioides species, like thermotolerance [ 55 •], we can also determine if there is a niche unique to each species. This will help to delineate the geographical distribution of each species, which may be different than originally thought; for example, C. immitis , originally deemed the “California species,” has also been found in New Mexico, Utah, and Washington [ 53 , 56 , 57 ].…”

“…Later, analyses of relatively small populations (<10 individuals) documented differences in fungal growth rates on high salt media, further suggesting that C. immitis is more salt tolerant than C. posadasii [ 76 ]. Indeed, many scientists agree that salinity, temperature, and other environmental variables strongly shape Coccidioides physiology and distribution [ 40 , 55 •, 77 , 78 ]. The first study to interrogate phenotypic variation in a robust population of 39 C. posadasii and 46 C. immitis isolates [ 55 •] noted that while C. posadasii and C. immitis had similar growth rates at 28°C, C. posadasii grew significantly faster at 37°C.…”

“…Indeed, many scientists agree that salinity, temperature, and other environmental variables strongly shape Coccidioides physiology and distribution [ 40 , 55 •, 77 , 78 ]. The first study to interrogate phenotypic variation in a robust population of 39 C. posadasii and 46 C. immitis isolates [ 55 •] noted that while C. posadasii and C. immitis had similar growth rates at 28°C, C. posadasii grew significantly faster at 37°C. Earlier work from the same group suggested that C. immitis produces spherules synchronously during in vitro culture where C. posadasii does not [ 79 ].…”

Phylogenomic Placement of American Southwest-Associated Clinical and Veterinary Isolates Expands Evidence for Distinct <i>Cryptococcus Gattii</i> VGVI.

“…If indeed this population inhabits the American Southwest, several questions about adaptations of VGVI to warm, arid environments are raised; this includes whether VGVI has departed from traditional niches for C. gattii, usually found in moist and nutrient-rich microenvironments. Dry soils in temperature-extreme environments are generally unexpected places to find fungi [20]; however, microenvironments like the extensive cryptobiotic grounds in the southwest U.S. [21,22] and organism adaptations like those of Coccidioides spp.-both at the cellular [23,24] and life cycle levels (e.g. soil sterilization hypothesis [25,26] and small-mammal reservoir hypothesis [27,28])-illustrate the clear possibility of the presence of other cryptic environmental fungi in these ecosystems.…”

Real-time PCR assay for detection and differentiation of Coccidioides immitis and Coccidioides posadasii from culture and clinical specimens

“…Identifying Coccidioides to the species level is likely beneficial in the proper treatment of the patients and disease surveillance. Such a laboratory test could also become an important tool in ongoing studies aimed at defining similarities and differences in Valley fever caused by C. immitis and C. posadasii [7,8,12].…”