Diploid cells of the yeast Schizosaccharomyces pombe carrying the mating-type allele h(90) are capable of sporulation and copulation. After copulation karyogamy does not always occur. In this case both nuclei will undergo meioses separately (twin meiosis). Asci with eight haploid spores derive from this event. Diploid cells homozygous for the mating-type alleles h(+) or h(-) do not sporulate. However, their nuclei can perform meiosis when they are in a common cytoplasm with a diploid nucleus of compatible mating type.