Ascospore formation in the yeast Saccharomyces cerevisiae

Abstract

Sporulation of the baker's yeast Saccharomyces cerevisiae is a response to nutrient depletion that allows a single diploid cell to give rise to four stress-resistant haploid spores. The formation of these spores requires a coordinated reorganization of cellular architecture. The construction of the spores can be broadly divided into two phases. The first is the generation of new membrane compartments within the cell cytoplasm that ultimately give rise to the spore plasma membranes. Proper assembly and growth of these membranes require modification of aspects of the constitutive secretory pathway and cytoskeleton by sporulation-specific functions. In the second phase, each immature spore becomes surrounded by a multilaminar spore wall that provides resistance to environmental stresses. This review focuses on our current understanding of the cellular rearrangements and the genes required in each of these phases to give rise to a wild-type spore.

FIG. 1.

(A to G) Overview of the stages of spore and ascus formation. In the presence of a nonfermentable carbon source, diploid cells starved for nitrogen will undergo meiosis. During the second meiotic division, the SPBs (indicated as ⟂︁), which are embedded in the nuclear envelope (shown in red), become sites for formation of prospore membranes (shown in green). As meiosis II proceeds, the prospore membranes expand and engulf the forming haploid nuclei. After nuclear division, each prospore membrane closes on itself to capture a haploid nucleus within two distinct membranes. Spore wall synthesis then begins in the lumen between the two prospore membrane-derived membranes. After spore wall synthesis is complete, the mother cell collapses to form the ascus.

FIG. 2.

Stages of prospore membrane growth. (A) As cells enter meiosis II, the meiosis II outer plaque is formed on the cytoplasmic face of the SPB (black bar). (B and C) The meiosis II outer plaque becomes a site for the recruitment and subsequent fusion of secretory vesicles to form a prospore membrane (green). (D) As the prospore membrane expands to engulf a daughter nucleus, its growth is controlled by two membrane-associated complexes: the septins (orange), which form sheets thought to run down the nuclear-proximal side of the prospore membrane, and the leading-edge complex (purple), which forms a ring structure at the membrane lip. (E) Closure of the prospore membrane completes cytokinesis. All three of the prospore membrane-associated complexes, i.e., the septins, leading-edge complex, and meiosis II outer plaque, disassemble at about the time of prospore membrane closure.

FIG. 3.

Organization of proteins within the meiosis II outer plaque. The changes in organization and composition between a mitotic/meiosis I outer plaque and a meiosis II outer plaque are shown in the cartoon. The Spc72p and γ-tubulin complex proteins are removed and replaced with Ady4p, Mpc54p, Spo21p, and Spo74p, leading to a conversion from microtubule to membrane nucleating activity. In the upper right is shown an electron micrograph of a meiosis II outer plaque with associated prospore membrane. The proposed correspondence between the arrangement of proteins in the model and the structure as seen in the electron micrograph is indicated.

FIG. 4.

Nonsister dyad formation. (A to C) During the first meiotic division, homologous chromosomes segregate to opposite poles, and at the second meiotic division, sister chromatids are separated. (C) When sporulated under acetate-depleted conditions, only two of the fours SPBs, one from each spindle, form meiosis II outer plaques. (D) As a result, only two prospore membranes are formed and only two nuclei are packaged into spores. (E) The unpackaged nuclei degenerate during ascal maturation, resulting in a two-spored ascus. If a heterozygous centromere-linked marker (indicated by + and −) is followed, an NSD will contain one spore with each allele. The gray ⟂︁ indicates the daughter SPB formed at meiosis II (see text).

FIG. 5.

Pathways of acetate metabolism in sporulating cells. Carbon sources mentioned in the text are shown in italic, and whether cells form tetrads or NSDs when sporulated on those carbon sources is indicated. Positions at which mutations in the glyoxylate cycle or gluconeogenesis lead to NSD or tetrad formation in the presence of acetate are also indicated. Gray arrows denote metabolic reactions unique to the glyoxylate cycle. Dashed lines indicate multiple reactions. DHAP, dihydroxyacetone phosphate. G3P, Glyceraldehyde-3-phosphate. PEP, phosphoenolpyruvate.

FIG. 6.

Specific SNARE complexes mediate fusion with the plasma membrane and prospore membrane. Secretory vesicle fusion with the plasma membrane is mediated by a heterotrimer consisting of Sso1p or Sso2p, Sec9p, and Snc1p or Snc2p (left panel). At the prospore membrane the fusion of secretory vesicles requires Sso1p, Spo20p (although Sec9p can substitute to a limited extent), and probably Snc1p or Snc2p. The roles of Snc1p and Snc2p in this process are inferred from the protein localization but have not been directly demonstrated (107).

FIG. 7.

Comparison of the spore wall and the vegetative cell wall. An electron micrograph of a germinating ascospore is shown. On the left side, the cell is surrounded by spore wall with its four layers, mannan, beta-glucan, chitosan, and dityrosine (indicated by M, B, C, and D, respectively, in the close-up and in the cartoon). On the right side, the tip of the germinating cell is bounded by vegetative cell wall with it predominant beta-glucan and mannan layers (indicated by B and M). The mannan layer of the cell wall appears to be continuous with that of the spore wall. PM, plasma membrane.

FIG. 8.

A pathway of spore wall assembly. The steps in assembly of the spore wall are shown. Genes shown to be required for specific steps are indicated (13, 24, 28, 34, 42, 55, 94, 121, 145-147, 163, 168). Adapted from reference .