Male meiosis in hybrids was generally arrested at leptotene. The majority of spermatocytes carried only fragments of the axial elements (Fig. 6a, Fig. S2), indicating incomplete assembly. The most advanced spermatocytes contained almost normal axial elements, but even in these cells no initiation of synapsis was seen (Fig. 6b). The halved total length of the lateral elements in the male hybrids at this stage was about 10% longer than the total SC length of the male M. levis at pachytene (t92,1 = 4.4, P < 0.001: Table 1). Hybrid spermatocytes showed heavy labeling with RAD51 antibodies indicating DSB presence.