Novel Synaptonemal Complex Protein SYCE3 Figure 2. SYCE3 selectively localizes to the synapsed areas of homologous chromosomes. (A) Mouse spermatocytes in zygotene, pachytene, diplotene and diakinesis stages stained for SYCE3 (green) and SYCP3 (red). Bar, 10 mm. (B) Pachytene spermatocytes marked with SYCE3 (green) and SYCP1 (red). (C) Mouse pachytene oocyte labeled with SYCE3 (green) and SYCP3 (red). Images were acquired using a fluorescence microscope. Bar, 7.5 mm. doi:10.1371/journal.pgen.1002088.g002 SYCE2, SYCE3 localized to small foci at closely aligned chromosome axes (Figure 5D, and insets therein). As expected, SYCE3 also localized to small foci in mice lacking Tex12 (data not shown). Taken together, the results described in the first part of this study lead to the conclusion that SYCE3 is a novel, meiosis- Figure 3. SYCE3 localization is restricted to the CE of the SC. (A) Immunoelectron microscopy of frozen rat testis sections marked with affinity purified rabbit anti-SYCE3 antibody (left panel) and a specific antibody directed against the coiled-coil region of SYCP1 (right panel). Arrowheads indicate gold-particles. Bar, 0.1 mm. (B) Quantification of gold-particle distribution in EM images of SYCE3 labeled (left, n = 304) and SYCP1 labeled samples (right, n = 135). The distance between the center of the SC and the LE was divided into seven sections and gold- particles were counted in each section. The CE of the SC is marked with a horizontal grey bar whereas the horizontal black bars correspond to LEs. doi:10.1371/journal.pgen.1002088.g003 specific component of the CE of mammalian SCs. Chromosome loading of SYCE3 appears to require SYCP1, but no other currently known, CE-specific proteins. SYCE3 Is Required for Male and Female Fertility To gain deeper insights into the function of SYCE3, we generated a mouse strain lacking the SYCE3 protein. To this end, we replaced the two exons coding for the full-length protein with a neomycin cassette by electroporating a modified pKSloxPNT vector for gene-replacement into R1/E ES cells (Figure S2A; [37]). Using PCR and Southern blot we identified one positive ES cell clone (Figure S2B, S2C), which was injected into blastocysts of a C57BL/6 mouse to generate chimeric animals. Mating of chimeras resulted in heterozygote animals, which produced PLoS Genetics | www.plosgenetics.org 4 May 2011 | Volume 7 | Issue 5 | e1002088