Meiotic Recombination in Schizosaccharomyces pombe: A Paradigm for Genetic and Molecular Analysis

Abstract

The fission yeast Schizosaccharomyces pombe is especially well-suited for both genetic and biochemical analysis of meiotic recombination. Recent studies have revealed ~50 gene products and two DNA intermediates central to recombination, which we place into a pathway from parental to recombinant DNA. We divide recombination into three stages - chromosome alignment accompanying nuclear "horsetail" movement, formation of DNA breaks, and repair of those breaks - and we discuss the roles of the identified gene products and DNA intermediates in these stages. Although some aspects of recombination are similar to those in the distantly related budding yeast Saccharomyces cerevisiae, other aspects are distinctly different. In particular, many proteins required for recombination in one species have no clear ortholog in the other, and the roles of identified orthologs in regulating recombination often differ. Furthermore, in S. pombe the dominant joint DNA molecule intermediates contain single Holliday junctions, and intersister joint molecules are more frequent than interhomolog types, whereas in S. cerevisiae interhomolog double Holliday junctions predominate. We speculate that meiotic recombination in other organisms shares features of each of these yeasts.

Figure 1

A pathway for meiotic recombination in S. pombe. The upper panels portray the fusion of cells and nuclei and the formation of the “horsetail” nucleus. The middle and lower panels portray the chromosome and DNA events that occur during the horsetail stage, which ceases shortly before meiosis I. Identified gene products required at each stage are indicated above the arrow leading to that stage. Additional proteins required for meiotic recombination, but whose points of action are not clear, include rec13, rec17 – 21, mug1, mug5, pds5, rqh1, mcp7, and meu13 (see sections 2, 3, and 8). MRN, Rad32 (Mre11)-Rad50-Nbs1 complex. Modified from Ellermeier and Smith (2005).

Figure 2

Linear elements containing Rec10 protein. Spreads of meiotic nuclei were stained using an antibody to Rec10 protein. Dots (a), filaments (b), and bundles (c) appear to occur in that order during meiosis. The filaments and bundles appear to reflect the linear elements seen by electron microscopy (see text). Bar indicates 5 μm. Figure supplied by J. Loidl.

Figure 3

A scheme that produces either crossover or non-crossover recombinants from single or double Holliday junctions. Resection of a DSB produces two 3′ single strand overhangs, one of which invades a homologous duplex, producing a D-loop. A single HJ results if this D-loop is cut before second end capture (left). A double HJ results if the D-loop remains uncut before second end capture (right). In both cases resolution of the HJ(s) results in crossover or non-crossover products, depending on the strands cleaved.

Figure 4

Single Holliday junction intermediates in S. pombe meiotic recombination. DNA from meiotic mus81 mutant cells was separated by two-dimensional gel electrophoresis, and DNA in the joint molecule region was visualized with an electron microscope. The junction of the four DNA duplex segments is splayed out in a “traffic circle” due to formamide in the spreading mixture. Bar indicates 0.2 μm. From Cromie et al. (2006).