Figure 5 and Fig. S2 show immunolocalisation of MLH1 at the SCs in pachytene oocytes of F1 hybrids between M. аrvalis and M. levis. The mean number of MLH1 foci was significantly lower in hybrid oocytes than in oocytes of M. levis (t96,1 = 9.6, P < 0.001: Table 1), while there was no difference in total SC length (t78,1 = 1.0, P = 0.32: Table 1). The decrease in hybrids was mainly due to a lack of foci in univalents. There was no substantial decrease in recombination between the homologous segments in homomorphic and heteromorphic bivalents, trivalents or multivalents. Most paired arms had at least one MLH1 focus. Even in the cells containing very complex multivalents (Fig. 5a), a series of multivalents (Fig. 5b), or long asynaptic regions (Fig. 5c) we did not observed a substantial decrease in the number of MLH1 foci.