Global patterns in culturable soil yeast diversity.

Yeasts, broadly defined as unicellular fungi, fulfill essential roles in soil ecosystems as decomposers and nutrition sources for fellow soil-dwellers. Broad-scale investigations of soil yeasts pose a methodological challenge as metagenomics are of limited use for identifying this group of fungi. Here we characterize global soil yeast diversity using fungal DNA barcoding on 1473 yeasts cultured from 3826 soil samples obtained from nine countries in six continents. We identify mean annual precipitation and international air travel as two significant correlates with soil yeast community structure and composition worldwide. Evidence for anthropogenic influences on soil yeast communities, directly via travel and indirectly via altered rainfall patterns resulting from climate change, is concerning as we found common infectious yeasts frequently distributed in soil in several countries. Our discovery of 41 putative novel species highlights the continued need for culture-based studies to advance our knowledge of environmental yeast diversity.

Genetic Diversity and Dispersal of Aspergillus fumigatus in Arctic Soils.

Aspergillus fumigatus is a saprophytic mold and an opportunistic pathogen with a broad geographic and ecological distribution. A. fumigatus is the most common etiological agent of aspergillosis, affecting over 8,000,000 individuals worldwide. Due to the rising number of infections and increasing reports of resistance to antifungal therapy, there is an urgent need to understand A. fumigatus populations from local to global levels. However, many geographic locations and ecological niches remain understudied, including soil environments from arctic regions. In this study, we isolated 32 and 52 A. fumigatus strains from soils in Iceland and the Northwest Territories of Canada (NWT), respectively. These isolates were genotyped at nine microsatellite loci and the genotypes were compared with each other and with those in other parts of the world. Though significantly differentiated from each other, our analyses revealed that A. fumigatus populations from Iceland and NWT contained evidence for both clonal and sexual reproductions, and shared many alleles with each other and with those collected from across Europe, Asia, and the Americas. Interestingly, we found one triazole-resistant strain containing the TR34 /L98H mutation in the cyp51A gene from NWT. This strain is closely related to a triazole-resistant genotype broadly distributed in India. Together, our results suggest that the northern soil populations of A. fumigatus are significantly influenced by those from other geographic regions.

Fitness distribution and transgressive segregation across 40 environments in a hybrid progeny population of the human-pathogenic yeast Cryptococcus neoformans.

The opportunistic human fungal pathogen Cryptococcus neoformans includes two varieties, C. neoformans var. grubii and C. neoformans var. neoformans, which correspond to serotypes A and D, respectively. Recent population genetic studies revealed that multiple natural hybridizations have occurred recently between these two divergent lineages. However, the biological effects of such hybridizations are little understood. In this study, we used colony size as a proxy for vegetative fitness to examine the phenotypic effects of hybridization between these two lineages in a laboratory cross. Two genetically diverged parental strains that differed in their growth at different temperatures and on different media as well as in their susceptibility to the common antifungal drug fluconazole were chosen. A total of 269 progeny were obtained and their vegetative growth was determined in 40 environments that differed in nutrients, temperature, and fluconazole concentration. Our analyses indicated little evidence for outbreeding depression or heterosis in the average vegetative fitness of the hybrid progeny population. The progeny, each of the three environmental variables, and their two-way, three-way, and four-way interactions all contributed significantly to the overall vegetative fitness variation. Interestingly, a variable number of progeny displayed evidence of transgressive segregation in vegetative fitness among the tested environments. Our study suggests that hybridization could play a significant role in the phenotypic evolution of this important human-pathogenic fungus.