Figure 1. Only a subset of DSBs become crossovers. Mouse spermatocyte spreads were stained for the chromosomal axial element SYCP3 (red) and either (A) RAD51 (green) or (B) MLH1 (green). DAPI staining is shown in blue. Each DSB gives rise to a chromosome-associated RAD51 complex, whereas MLH1 complexes localize only to sites that will become crossovers. There is an approximate 9-fold excess of DSB-associated foci relative to crossover-associated foci. Arrowheads point to an example of an autosome with two widely separated MLH1 foci, characteristic of crossover interference. (Images courtesy of Ignasi Roig, Molecular Biology Program, Memorial Sloan-Kettering Cancer Center). doi:10.1371/journal.pgen.1000576.g001 disjunction. Indeed, although pch2 mutation has little or no effect on spore viability on its own, introducing a spo11 mutation that reduces DSB activity by ,20% significantly reduced viability despite approximately wild-type crossover frequencies [13,14]. This reduction in spore viability was further exacerbated in spo11 hypomorphs that reduce DSB activity up to 80%. Although this is an indirect method of measuring crossover homeostasis, these findings pro- vide compelling evidence that Pch2 has a role in multiple aspects of crossover control during yeast meiosis. So what role could Pch2 play in this process? Pch2 is required for differential References 1. Allers T, Lichten M (2001) Differential timing and control of noncrossover and crossover recombination during meiosis. Cell 106: 47–57. 2. Borner GV, Kleckner N, Hunter N (2004) Crossover/noncrossover differentiation, synapto- nemal complex formation, and regulatory sur- veillance at the leptotene/zygotene transition of meiosis. Cell 117: 29–45. 3. Page SL, Hawley RS (2003) Chromosome chore- ography: The meiotic ballet. Science 301: 785–789. 4. Buhler C, Borde V, Lichten M (2007) Mapping meiotic single-strand DNA reveals a new landscape of DNA double-strand breaks in Saccharomyces cerevisiae. PLoS Biol 5(12): e324. doi:10.1371/ journal.pbio.0050324. 5. Bishop DK, Zickler D (2004) Early decision; Meiotic crossover interference prior to stable strand exchange and synapsis. Cell 117: 9–15. 6. Jones GH (1984) The control of chiasma distribution. Symp Soc Exp Biol 38: 293–320. 7. Martini E, Diaz RL, Hunter N, Keeney S (2006) Crossover homeostasis in yeast meiosis. Cell 126: 285–295. 8. Chen SY, Tsubouchi T, Rockmill B, Sandler JS, Richards DR, et al. (2008) Global analysis of the organization of chromosome structural proteins Hop1 and Red1 relative to the synaptonemal complex central element protein Zip1 [19]. In pch2D mutants, Hop1/Red1 and Zip1 exhibit a more uniform axial localization pattern than is observed in wild type. Joshi et al. now demonstrate that chromosome domains that are enriched for Hop1 and Red1 tend to colocalize with future sites of crossover formation, leading to the hypothesis that Pch2 functions to stabilize alternating domains enriched for either Hop1/Red1 or Zip1. Such domains are proposed to be modules that mediate crossover designation and interference. Interestingly, when PCH2 meiotic crossover landscape. Dev Cell 15: 401–415. 9. Kleckner N, Zickler D, Jones GH, Dekker J, Padmore R, et al. (2004) A mechanical basis for chromosome function. Proc Natl Acad Sci U S A 101: 12592–12597. 10. Hillers KJ (2004) Crossover interference. Curr Biol 14: R1036–R1037. 11. Muller HJ (1916) The mechanism of crossing over. Am Nat 50: 193–221. 12. Lynn A, Soucek R, Borner GV (2007) ZMM proteins during meiosis: Crossover artists at work. Chromosome Res 15: 591–605. 13. Zanders S, Alani E (2009) The pch2D mutation in baker’s yeast alters meiotic crossover levels and confers a defect in crossover interference. PLoS Genet 5(7): e1000571. doi:10.1371/journal.pgen. 1000571. 14. Joshi N, Barot A, Jamison C, Bo¨rner GV (2009) Pch2 links chromosome axis remodeling at future crossover sites and crossover distribution during yeast meiosis. PLoS Genet 5(7): e1000557. doi:10.1371/journal.pgen.1000557. 15. Wu HY, Burgess SM (2006) Two distinct surveillance mechanisms monitor meiotic chro- is deleted, not only is axial organization of Hop1/Red1 and Zip1 compromised, but appearance of both crossover and non- crossover products is delayed to similar extents [19]. It is not yet clear whether these different aspects of the pch2 mutant phenotype are consequences of the same molecular defect, nor is it yet clear precisely how Pch2 protein functions in wild-type cells. Nonetheless, the current findings provide new support for the idea that higher order chromosome structure plays a key role in crossover control [9], and furthermore implicate Pch2 as an impor- tant player in coordinating recombination with large-scale chromosome structures. mosome metabolism in budding yeast. Curr Biol 16: 2473–2479. 16. Bhalla N, Dernburg AF (2005) A conserved checkpoint monitors meiotic chromosome synap- sis in Caenorhabditis elegans. Science 310: 1683–1686. 17. Li X, Schimenti JC (2007) Mouse pachytene checkpoint 2 (Trip13) is required for completing meiotic recombination but not synapsis. PLoS Genet 3(8): e130. doi:10.1371/journal.pgen. 0030130. 18. Joyce EF, McKim KS (2009) Drosophila PCH2 is required for a pachytene checkpoint that moni- tors double-strand-break-independent events leading to meiotic crossover formation. Genetics 181: 39–51. 19. Borner GV, Barot A, Kleckner N (2008) Yeast Pch2 promotes domainal axis organization, timely recombination progression, and arrest of defective recombinosomes during meiosis. Proc Natl Acad Sci U S A 105: 3327–3332. 20. Diaz RL, Alcid AD, Berger JM, Keeney S (2002) Identification of residues in yeast Spo11p critical for meiotic DNA double-strand break formation. Mol Cell Biol 22: 1106–1115. PLoS Genetics | www.plosgenetics.org 2 July 2009 | Volume 5 | Issue 7 | e1000576