Novel Synaptonemal Complex Protein SYCE3 Figure 5. SYCE3 chromosome loading requires SYCP1 but is independent of SYCE1, SYCE2, and SYCP3. Confocal images showing the immunolocalization of SYCE3 (green) on spread preparations of (A) Sycp12/2 pachytene-like spermatocytes co-labeled with SYCP3 (red), of (B) Sycp32/2 spermatocytes co-stained for SYCP1 (red) and of (C) Syce12/2 and (D) Syce22/2 pachytene-like spermatocytes co-labeled with SYCP3 (red). In cells lacking SYCP1, SYCE3 loading is defective. In the absence of SYCP3 both SYCE3 and SYCP1 localize to synapsed regions. In cells deficient for SYCE1, SYCE3 localizes in a weak discontinuous pattern to AE independent of whether they are in close apposition or not (see inset). Here, asterisks mark SYCE3 staining on AEs that are not closely aligned. In Syce22/2 cells SYCE3 localizes to small foci corresponding to sites at which AEs are closely associated (see inset). Arrowheads point to sites of SYCE3 localization in Syce22/2 cells. Bars, 5 mm. doi:10.1371/journal.pgen.1002088.g005 advanced stages of prophase I suggests that DSBs are formed in Syce3-deficient spermatocytes, but that they are not efficiently repaired. To gain additional insights into further processing of DSBs in mutant cells, we performed immunostaining for proteins that are specific for different recombination nodules. In wild-type mice, one distinguishes between early nodules (ENs) which appear prior to synapsis and assemble at sites of DSBs [39,40], transitional nodules (TNs) during zygotene [41] and late recombination nodules (RNs) which mark sites of future crossover events [42]. ENs assemble at the leptotene stage and are made up of the two RecA homologs RAD51 and DMC1. They form numerous foci along chromosome cores and catalyze strand exchanges between homologous DNA molecules as a first step during processing of DSBs to crossover events [41,43]. During zygotene of wild-type mice, RAD51 and DMC1 are gradually replaced by RPA. Thereby ENs are transformed into TNs which appear isochronously to synapsis. It is likely that TNs are involved in stabilization or resolution of early PLoS Genetics | www.plosgenetics.org 6 May 2011 | Volume 7 | Issue 5 | e1002088