Fig. 2

SWS1-SWSAP1 is required for meiotic homolog synapsis and RAD51 and DMC1 focus assembly. a Histone H1t staining indicates that few spermatocytes in Sws1 and Swsap1 mutants progress to mid-pachynema and beyond. Total number of mid-pachytene, late-pachytene, and diplotene spermatocytes divided by the total number of spermatocytes analyzed from adult testes. Mice: control, n = 6; Sws1^−/−, Swsap1^−/−, n = 3. ****, P ≤ 0.0001; Fisher’s exact test, two-tailed. b, c Sws1 and Swsap1 mutants show altered meiotic progression and abnormal chromosome synapsis. Percentage of spermatocytes in each of the indicated meiosis prophase I stages in b with representative images in c. Sws1^−/− and Swsap1^−/− cells at leptonema are indistinguishable from controls. At later stages abnormal cells with synaptic defects are observed; because synapsis is abnormal these stages are appended with the word “-like” (orange bars). At early zygonema, chromosomes begin to synapse in the majority of mutant cells, although the number of synaptic stretches are usually reduced; however, delayed synapsis onset is apparent in a subset of mutant cells, as indicated by the lack of SYCP1 stretches. At late zygonema, abnormal cells with fully formed chromosome axes (SYCP3) but little or no synapsis (SYCP1) predominate. Fully-synapsed chromosomes with thicker and shorter SYCP3 axes characterize early pachynema, indicative of chromatin condensation, however, abnormal cells with unsynapsed chromosomes and chromosomes with non-homologous synapsis and/or partial asynapsis are frequent in the mutants. Mid-pachytene (H1t-positive) abnormal cells contain unsynapsed chromosomes, broken bivalents, or parts of chromosomes involved in non-homologous synapsis. Scale bars, 10 µm. Boxes in c in early pachytene-like cells highlight non-homologous synapsis and arrows indicate unsynapsed chromosomes. d–g RAD51 and DMC1 focus counts are reduced in Sws1 and Swsap1 mutant spermatocytes. Representative chromosome spreads from adult mice at early zygonema (-like) are shown in d, f with focus counts for all stages in e, g. n = 3. Error bars, mean ± s.d. Scale bars, 10 µm. Each circle in e, g indicates the total number of foci from a single nucleus. Solid circles in e, g normal cells. Open circles in e, g cells with abnormal synapsis. Abnormal cells at early zygonema in the Sws1 and Swsap1 mutants are not indicated because they are a fraction of cells and are not distinguishable without SYCP1 costaining. **P ≤ 0.01; ****P ≤ 0.0001; Mann–Whitney test, one-tailed