12 Mating and Sporulation in Schizosaccharomyces pombe

Abstract

I. INTRODUCTION
Haploid cells of the fission yeast Schizosaccharomyces pombe display one of the two mating types denoted as h⁺ (or P) and h⁻ (or M). Homothallic strains, denoted as h⁹⁰, change their mating type during proliferation (for review, see Klar, these volumes), whereas heterothallic strains are fixed as either h⁺ or h⁻. S. pombe cells are most stable in the haploid state and are essentially asexual as long as they are well fed. They initiate sexual development only under starvation conditions. Thus, unlike Saccharomyces cerevisiae, homothallic S. pombe cells keep growing as haploids and do not undergo conjugation in rich medium. When starved for nutrients, especially for nitrogen, haploid cells mate to form zygotes, which then undergo meiosis and generate haploid spores (zygotic ascospores). Zygotes can grow as diploids if they are transferred to rich medium immediately after conjugation. However, these diploid cells undergo meiosis and form spores (azygotic ascospores) at the end of vegetative growth. The absence of glucose is not mandatory for mating and meiosis in S. pombe, but these events proceed more efficiently if the concentration of glucose in the medium is lowered.

Nitrogen starvation induces expression of genes essential for sexual development, which results in the enhanced production of mating pheromones and their receptors. Cells are then able to recognize the pheromone secreted by cells of the opposite mating type, and h⁺ and h⁻ cells elongate conjugation tubes toward each other. The cells agglutinate in pairs at the tips of these tubes, fuse, and then...