Nuclear division phenotypes in Sporidiobolales and Trichosporonales

During nuclear division in basidiomycetous yeasts, the mother cell nucleus migrates to the daughter cell and divides inside that cell, followed by nuclear segregation in both cells. This mode contrasts sharply with that of Saccharomyces cerevisiae, where nuclear division occurs within the mother cell without prior nuclear migration. However, the universality of this division mode across all basidiomycetous yeasts remains unclear. Accordingly, we aimed to identify basidiomycetous yeasts that undergo nuclear division inside their mother cells by observing nuclear division in 51 Sporidiobolales and 33 Trichosporonales yeasts. The phenotype of a budded cell with nuclei that exists only in the daughter cell is a key indicator of nuclear migration to the daughter cell prior to nuclear division. Using 4',6-diamidino-2-phenylindole staining, we identified four candidate Trichosporonales species (Apiotrichum domesticum, Cutaneotrichosporon arboriforme, Trichosporon asahii, and Vanrija humicola) that exhibited a noticeable decrease in this phenotype. Conversely, all tested yeasts in Sporidiobolales showed the phenotype. Live cell imaging of T. asahii expressing an mCherry-tagged histone H2A protein confirmed nuclear division occurring inside the mother cell or at the bud neck, without prior nuclear migration to the daughter cell. Furthermore, we found that a nucleus is divided near the center of a cell in T. asahii. The specific location-mother or daughter cells-where nuclear division occurs depends on the length of the daughter cell in T. asahii. These findings provide insight into nuclear division mode diversity and may support a better understanding of fungal pathogenesis.

The dogma of nuclear division in basidiomycetous yeasts is that the nucleus present in the mother cell moves to the daughter cell and divides inside that cell. The mode of nuclear division is completely different from that in ascomycetous yeasts. Our results indicate that nuclear division follows the dogma in Sporidiobolales but not in all Trichosporonales yeasts. In the study, we identified four candidate species that do not follow basidiomycetous nuclear division (basidio-nuclear division), then we focused on Trichosporon asahii for deeper analysis. T. asahii surprisingly undergoes nuclear division inside the mother cell. Furthermore, we determined that the exact position of the nuclear division depends on the daughter cell length. This is the first report to identify T. asahii as a basidiomycetous yeast that does not follow basidio-nuclear division.