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.

Keywords: coccidioidomycosis; fungal pathogen; growth rate; phenotypic variation; valley fever.

Figure 1

Temperature impacts growth ability of C. immitis isolates compared to C. posadasii on YE media. Seven mm diameter plugs were sub-cultured onto yeast extract plates and radial growth was documented over 16 days. (A) Radial growth measurements at 37 °C for 46 C. posadasii and 39 C. immitis isolates in triplicate. (B) Radial growth measurements at 28 °C for 46 C. posadasii and 39 C. immitis isolates in triplicate. (C) Representative of phenotypic variation observed for C. immitis on day 16. (D) Representative of phenotypic variation observed for C. posadasii on day 16.

Figure 2

Radial growth rate of 85 isolates of Coccidioides demonstrates species-specific response to temperature. Each line represents the mean diameter (y-axis) for each isolate in triplicate (46 C. immitis and 39 C. posadasii) at a given time point (x-axis). Dark lines represent mean growth rate of each species. Radial growth was measured at day 5, 7, 9, 12, 14 and 16. There is a significant difference in growth rate (slope) in response to higher temperature between species of Coccidioides. The radial growth rate of C. immitis is decreased at a higher temperature 37 °C (slope37 = 0.64 mm/day; 95% C.I. 0.51–0.78) compared to C. posadasii (slope37 = 1.82 mm/day; 95% C.I. 1.49–2.16). Both species appear to tolerate 28 °C and grow at a similar rate (C. immitis slope28 = 3.73 mm/day; 95% C.I. 3.53–3.92, C. posadasii, slope28 = 3.47 mm/day; 95% C.I. 2.98–3.90).