Novel Synaptonemal Complex Protein SYCE3 Figure 8. Complete repair of DSBs and crossover formation is not observed in Syce32/2 spermatocytes. Immunostaining of spread preparations of wild-type pachytene and Syce32/2 pachytene-like spermatocytes for cH2AX (red) and SYCP3 (green), RAD51 (green) together with SYCP3 (red), RPA (green) co-labeled with SYCP3 (red) and MLH1 (green) co-stained with SYCP3 (red). Images shown in row two to four were acquired using a confocal laser scanning microscope. Bars, 10 mm. doi:10.1371/journal.pgen.1002088.g008 SYCP1 is a component of the CE and TFs, the latter three proteins only localize to the CE [23,24,28,29]. Here, we have identified SYCE3, a novel meiosis-specific protein that selectively localizes to the CE of the SC. SYCE3 Is Required for Initiation of Central Element Assembly and Chromosomal Loading of Central Element–Specific Proteins Detailed analyses of Syce2 and Tex12 knockout mice showed that loss of each of these two genes causes infertility in both sexes. In both mice, meiotic chromosomes align, and AEs form, but they do not synapse. Although synapsis between homologs is initiated at multiple positions along the axes, it fails to propagate along the entire chromosomes. SYCP1 and SYCE1 colocalize at these sites of synapsis initiation and, as revealed at the electron microscopical level, short CE-like structures become assembled. Furthermore, correct progression of meiotic recombination is altered in cells lacking SYCE2 or Tex12. Altogether, these data and the direct interaction of SYCE2 and Tex12 lead to the model that both SYCE2 and Tex12 are required for the longitudinal polymerization of SYCP1 filaments along the axial elements and thus for the propagation of synapsis along the homologs [31,32]. Elimination of SYCE1 - the other currently known CE protein - displays a different phenotype. The absence of SYCE1 leads to the alignment of homologous chromosomes with a disrupted synapsis, but, in contrast to SYCE2 and Tex12 knockout mice, SYCE1- deficient mice display no sites of synapsis initiation and no CE-like structures at all. In these mice, SYCP1 is located in a weak discontinuous pattern along AEs, whether they are closely aligned or not. This indicates that under physiological conditions SYCP1 alone is insufficient for formation of stable head-to-head polymers for which CE proteins are required. Despite this striking difference to SYCE2 and Tex12 mice, SYCE1-deficient mice also exhibit a disturbed progression of meiotic recombination [30]. Taken together, analysis of these central region knockout mice leads to three main conclusions: (1) The organization of the central region appears to be highly complex, (2) disruption of any currently known protein component causes defective synapsis leading to severe meiotic defects and infertility and (3) the correct assembly of the central region is also required for normal progression of meiotic recombination [27,30–32]. Here, we have demonstrated that in wild-type cells SYCE3 distribution closely resembles SYCE1 localization (Figure 4; [28,29]). Thus, we expected that SYCE3 is likely involved in a complex with SYCE1. Consistent with this assumption, no CE-like structures were detected in the EM analysis of Syce32/2 mouse testis sections (Figure 7B). The complete absence of any CE-like structure strongly suggests that, in contrast to SYCE2 and Tex12, PLoS Genetics | www.plosgenetics.org 9 May 2011 | Volume 7 | Issue 5 | e1002088