Genome-wide comparisons reveal broad variations in intraspecific SNP frequencies among species in Agaricomycetes, Basidiomycota

Background: Genome sequence analyses can provide crucial information for understanding population history, speciation, and taxonomy. In Class Agaricomycetes where most mushroom-forming fungi belong, most species so far have been defined based on morphological, ecological, and/or molecular features. At present, there is little information on how species defined based on such features reflect their genome sequence diversity. In this study, we investigated genome-wide single nucleotide polymorphism (SNP) frequencies between strains within species to understand the patterns of variation. Methods: A total of 112 species in 72 genera of Agaricomycetes contained the nuclear and/or mitochondrial genome sequences from at least two strains each in public databases. Together, we obtained 398 and 106 available nuclear and mitochondrial genomes respectively from these taxa. Pairwise strain comparisons of the nuclear and mitochondrial genomes within individual species were conducted to obtain their SNP frequencies. Results: The SNP frequencies between nuclear genomes within individual species ranged 0–7.69% while for the mitochondrial genome, the pairwise strain SNP frequencies ranged 0–4.41%. The Spearman’s non-parametric rank correlation test showed a weak but statistically significant positive correlation between the paired nuclear and mitochondrial genome SNP frequencies. Overall, we observed a significantly higher SNP frequency in the nuclear genome than in the mitochondrial genomes between strains within most species. Interestingly, across the broad Basidiomycetes, the ratios of mitochondrial genome SNPs and nuclear genome SNPs between pairs of strains within each species were almost all lower than 1, with a mean of 0.24. Conclusions: Our analyses revealed broad variations among species in their intraspecific SNP frequencies in both the nuclear and mitochondrial genomes. However, there was broad consensus among the examined species in their mitochondrial to nuclear genome SNP ratios, suggesting that such a ratio could potentially serve as an indicator for genome sequence-based species identification.