Abstract
Cells of the fission yeast Schizosaccharomyces pombe normally reproduce by mitotic division in the haploid state. When subjected to nutrient starvation, two haploid cells fuse and undergo karyogamy, forming a diploid cell that initiates meiosis to form four haploid spores. Here, we show that deletion of the mal3 gene, which encodes a homolog of microtubule regulator EB1, produces aberrant asci carrying more than four spores. The mal3 deletion mutant cells have a disordered cytoplasmic microtubule structure during karyogamy and initiate meiosis before completion of karyogamy, resulting in twin haploid meiosis in the zygote. Treatment with anti-microtubule drugs mimics this phenotype. Mutants defective in karyogamy or mutants prone to initiate haploid meiosis exaggerate the phenotype of the mal3 deletion mutant. Our results indicate that proper microtubule structure is required for ordered progression through the meiotic cycle. Furthermore, the results of our study suggest that fission yeast do not monitor ploidy during meiosis.
Highlights
Karyogamy, a process in which two haploid nuclei fuse to produce a diploid nucleus, must occur prior to initiation of meiosis in haploid organisms such as the fission yeast Schizosaccharomyces pombe
Mal3 is Essential for Proper Spore Production The sexual differentiation pathway, which involves mating, meiosis, and sporulation, was induced in mal3D cells in order to elucidate the role of Mal3 in meiosis
We determined the viability of spores in the aberrant asci produced by mal3D cells, and found that spore viability declined as the number of spores per ascus increased (45%, 15% and 2% in four, six- and eight-spore asci, respectively, Fig. 1C). These results suggest that Mal3 plays a significant role in meiosis
Summary
A process in which two haploid nuclei fuse to produce a diploid nucleus, must occur prior to initiation of meiosis in haploid organisms such as the fission yeast Schizosaccharomyces pombe. Haploid fission yeast cells display one of two mating types, designated as h+ and h2. Homothallic strains, designated as h90, undergo frequent switching of mating type between h+ and h2, whereas heterothallic strains are fixed as either h+ or h2. When starved for nutrients, nitrogen, haploid cells conjugate with cells of the opposite mating type, after which they undergo karyogamy and form diploid zygotes. Expression of the mei gene is induced [1,2]. The mei gene encodes an inhibitor of Pat kinase [3]. Pat kinase negatively regulates the initiation of meiosis [4,5]. Once Pat is inactivated by Mei, dephosphorylated Mei, which is a critical target of Pat, accumulates and triggers the initiation of meiosis [6,7,8]
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