Loss of Mll2 leads to male sterility. (A) The fertility of 11 control (black) or mutant males (grey) was tested by weekly breedings to two wild-type females before and after tamoxifen administration. The mutants initially transmitted the unrecombined haplotype (light grey), and then the recombined haplotype (dark grey) and lost fertility by week 7. (B) The diagram illustrates a spermatogenic period of 40 days with mitosis (red), meiosis I (orange), meiosis II (yellow), spermiogenesis (green) and spermatozoa maturation (blue). Indicated are spermatogonia (SG), leptotene spermatocytes (LS), pachytene spematocytes (PS), rounded spermatids (RS) and elongated spermatids (ES). The transmission of the unrecombined haplotype (light grey) indicates that recombination did not occur in ES and only partially occurred in RS. Although recombination was complete in LS and PS, spermatogenesis proceeded (dark grey). Permanent sterility occurring in the sixth week indicated that spermatogenesis was interrupted before or at meiosis I (red line) (C, D) Wild-type, control and atrophic mutant testes 8 weeks after tamoxifen induction (C) or weighed during a time course (D). (E to J) Testis histological cross-sections reveal a block of germ cell differentiation and progressive loss of spermatogonia in Mll2FC/FC testis. Control testis (E) were normal after tamoxifen treatment. Mutant testis were sectioned 1 week (F), 2 weeks (G), 3 weeks (H), 4 weeks (I) and 14 weeks (J) after induction. (K, L) Increased levels of spermatogonial apoptosis in Mll2FC/FC testis, determined by TUNEL staining in control (K) and mutant (L) testis sections 3 weeks after tamoxifen induction. (M, N) Persistence of Tra98-positive cells 4 months after tamoxifen induction. Double staining with a Tra98 antibody and DAPI shows the expected distribution of Tra98 in control testis (M) and reduced staining in mutant testis (N). (O, P) Expression of Mll2 (O) and Mll (P) in normal testis detected by in situ hybridization with antisense probes.