Telomeres not bound by SUN1 tend to lack CCDC79 in Smc1b -/- spermatocytes. Chromosome axes are shorter, and homologue alignment and SC formation are impaired in Smc1b -/- spermatocytes as compared to wt spermatocytes (see Figure 4). Therefore, the stage that is equivalent to wt pachytene is called the zygotene-like pachytene stage in the Smc1b -/- mutant. SYCP3 (chromosome axes), TRF1 (telomeres), SUN1 (telomere attachment plate) and CCDC79 were detected by IF in nuclear surface spreads of wild type (SMC1b +/+, not shown, see Figure 4) and SMC1B-deficient (Smc1b -/-) spermatocytes obtained from adult testis. (A) Images of an Smc1b -/- spermatocyte at a zygotene-like pachytene stage are presented: upper panel shows single channel images of the four detected proteins, lower panel shows the overlay of SYCP3/axis with either CCDC79 and TRF1 signal (left) or CCDC79 and SUN1 signal (right). Enlarged views of the same individual chromosome (dashed line box) from both merged images are indicated. On the highlighted chromosome, CCDC79 is absent from one of the TRF1 marked telomeres (right, upper panel). The same chromosome end also lacks SUN1 (right, lower panel). Scale bars 10 μm. B) Quantification shows the fraction of telomeres, at which CCDC79 and/or SUN1 were detected in Smc1b +/+p achytene spermatocytes and Smc1b -/- zygotene-like pachytene spermatocytes. Telomeres were detected by TRF1 staining. CCDC79 and SUN1 co-localize on almost all telomeres (99.4%) in wt spermatocytes, and only on 83.7% of all telomeres in SMC1B-deficient spermatocytes. The mutant spermatocytes exhibit a much higher rate of CCDC79-free telomeres (12%) as compared to wt spermatocytes (0.2%). The large majority (76% = 9.5% out of 12%) of these CCDC79-free telomeres also fail to associate with SUN1.