Genotypic and Phenotypic Analyses of Two "Isogenic" Strains of the Human Fungal Pathogen Cryptococcus neoformans var. neoformans.

The Cryptococcus neoformans species complex is a model organism for fungal studies. Many studies have used two strains, JEC20 and JEC21, and their derivatives. These two strains were obtained through 10 rounds of backcrosses and have been assumed near identical except at the mating-type locus. Here we obtained and compared the JEC20 genome sequence with the published "JEC21" genome. Our comparison revealed 5322 single nucleotide polymorphisms (SNPs) with the majority (N = 3816, 71.7%) located in three genomic regions, including the previously noted mating-type region. The remaining 1506 SNPs (28.3%) were distributed throughout all 14 chromosomes, predominantly at chromosomal ends. To study the potential effects of these three SNP-rich regions on phenotypes, 24 progenies from the JEC20 × JEC21 cross representing eight recombinant genotypes were analyzed for their mating ability, melanin production, capsule formation, and growths at 30 °C and 40 °C. Significant phenotypic variations were found among the progeny. However, the observed phenotypic variations could not be explained by the three SNP-rich regions. Further genome sequencing of our JEC21 and the 24 progenies revealed only six segregating SNPs outside of the three SNP-rich regions between JEC20 and JEC21, a result indicating that the 1500 SNPs identified in the published "JEC21" genome might be caused by sequencing errors and/or strain mixing. However, the six SNPs and the three SNP-rich regions could not explain the observed phenotypic variations. Our analyses suggest that spontaneous mutations accumulated under laboratory conditions could have significant effects on phenotypes and on our interpretations of experimental results.