Ribosomal DNA sequence polymorphism and the delineation of two ascosporic yeast species: Metschnikowia agaves and Starmerella bombicola

Abstract

The relationship between mating success and sequence divergence in the internal transcribed spacer (ITS)/5.8S-D1/D2 rDNA region was examined in isolates tentatively assigned to Metschnikowia agaves and Starmerella bombicola. Both species are haplontic and heterothallic, such that the formation of mature asci can be used as a measure of genetic compatibility. Parsimony haplotype network analysis and mating success confirmed that all known isolates of M. agaves are conspecific. The previously reported D1/D2 polymorphism of five substitutions was not corroborated; the maximum divergence observed between any two strains was three substitutions, four with ITS. Of 39 putative S. bombicola strains, 36 formed an ITS-D1/D2 haplotype network using the 95% criterion. Thirty-five strains could mate with one or more compatible partner. The excluded strains did not mate. Mature asci arose from crosses between individuals differing by as many as five, but not six or seven substitutions in the D1/D2 domain. All strains capable of mating formed mature asci with at least one partner and all network members could be linked to another member by three or fewer substitutions. These results support the use of sequence divergence as a criterion for species delineation, but caution against describing poorly sampled species solely on the basis of that criterion.

Starmerella orientalis f.a., sp. nov., an ascomycetous yeast species isolated from flowers

“…Starmerella) floricola but a higher bootstrap value was obtained in the case of the tree reconstructed by combined ITS-D1/D2 sequences. These results confirmed that D1/D2 and ITS sequences should be considered equally important in identifying yeast species as discussed previously by Lachance et al (2011b).…”

Ogataea saltuana sp. nov., a novel methanol-assimilating yeast species

“…2). Despite the considerable sequence divergences (7-9 substitutions), the analysis of the D1/D2 sequences alone failed to exclude O zsoltii from the network formed by the strains of the proposed new species at the same connection limit (results not shown) supporting the proposal that combined sets of D1/D2 and ITS sequences should be analysed (Lachance et al 2010(Lachance et al , 2011. Hart and Sunday (2007) reviewed hundreds of network analyses dealing with barcoding sequences of well-defined animal and plant species.…”

Zygosaccharomyces favi sp. nov., an obligate osmophilic yeast species from bee bread and honey

“…2). Unlike suggested by Lachance et al (2010Lachance et al ( , 2011b in this case gapped positions were not excluded from the analysis, because the aim was not to separate different species, but to demonstrate the intraspecies and intragenomic ITS heterogeneity (mainly indels) and the distribution of different ITS copies among the strains of the new species. In parsimony network the copy variants formed two groups, the first group consisted of copies belonging to strains NCAIM Y.02134 and NCAIM Y.02135 and one copy of NCAIM Y.02137, while the second of copies belonging to strains NCAIM Y.02136 and NCAIM Y.02137.…”