Recent allopolyploid origin of Zygosaccharomyces rouxii strain ATCC 42981.

Zygosaccharomyces rouxii strain ATCC 42981 has been reported to have two copies of several genes including HOG1 and SOD2, whereas the type strain of Z. rouxii (CBS 732) has only one. To investigate the structure of the ATCC 42981 genome we sequenced random fragments from this genome and compared the data to the type strain. We found that ATCC 42981 contains two versions of the ribosomal RNA array, one of which is identical in the ITS1-ITS2 and 26S D1/D2 regions to Z. rouxii CBS 732, while the other is almost identical to a species provisionally named Z. pseudorouxii. We found that most genomic regions from Z. rouxii CBS 732 map in a one-to-two fashion to pairs of regions in ATCC 42981, with one of the ATCC 42981 regions having 97-100% DNA sequence identity to CBS 732 and the other having about 80-90% identity. Complete sequencing of regions containing 30 pairs of genes from ATCC 42981 and their orthologues in CBS 732 showed no evidence of the gene deletions or pseudogene formation that might be expected if ATCC 42981 had undergone whole-genome duplication several million years ago and was in the early stages of gene loss. Instead, we conclude that ATCC 42981 is a Z. rouxii-Z. pseudorouxii interspecies hybrid that was formed so recently that its genome has not had time to decay.