On Homo- and Heterothallism in Schizosaccharomyces Pombe

SUMMARY

The mating-type system of Schizosaccharomyces pombe has previously been studied by Leupold. In his culture, Leupold found heterothallic (h⁺, h^-) and homothallic (h⁹⁰) strains; the various strains showed specific patterns of spontaneous mating-type mutations. We have analyzed three additional cultures of S. pombe of different geographical origins, which were obtained from the CBS in Delft. From these cultures we isolated single-cell cultures called EF4, EF5, and EF6.

When cells of EF4 and EFS are plated, all resulting colonies show sporulating and nonsporulating sectors. Even after repeated streaking, neither pure sporulating nor pure nonsporulating clones were obtained. Therefore, EF4 and EF5 may be called homothallic, but, in contrast to Leupold's h⁹⁰ strains, they do not form homogeneously sporulating colonies. Furthermore, when EF4 is plated together with h⁺ or h^- cells, part of the resulting EF4 colonies form, if they are contiguous with heterothallic colonies, distinct sporulating lines in the zones of contact. Thus, colonies of EF4 appear to have basically a heterothallic mating type. We assume that the homothallism of EF4 and EF5 is caused by frequent mutations from h⁺ to h^- and vice versa.

From the EF6 culture, we isolated two heterothallic strains of opposite mating types (h⁺ and h^-); homothallic mutants were not found. In the EF6 strains, mutations from h⁺ and h^- and from h^- to h⁺ occur rarely. The cells of EF6 show a strong vegetative agglutination when grown or suspended in liquid medium.