Chromosome pairing and recombination in male M. arvalis and M. levis have been described previously454647. Both species showed asynapsis of the X and Y chromosomes (Fig. 1a,b, Supplementary Fig. S1) which underwent meiotic sex chromosome inactivation and were labeled by γH2A.X antibodies (Fig. 1c, Fig. S1). This feature is characteristic of the entire arvalis lineage of the genus Microtus47. Asynapsis of autosomes at pachytene stage, when most chromosomes contained MLH1 foci, was very rare (less than 2% in both sexes) and always affected small chromosomes. M. arvalis and M. levis males did not differ from each other in total autosomal SC length (t212,1 = 2.6, P = 0.09) or MLH1 focus number per cell (t212,1 = 1.1, P = 0.29) (Table 1). The number of MLH1 foci was usually restricted to one per autosomal bivalent in spermatocytes, as was in oocytes (Fig. 1d, Table 1). Assuming at least one focus on the XX bivalent, no significant sex differences in recombination rates can therefore be seen in M. levis.