More than 90% of pachytene cells in female hybrids contained synaptic aberrations. Supplementary Fig. S2 shows examples of these aberrations. The most common were multivalents containing more than three lateral elements (from four to twenty, 6.5 ± 3.4 on average) (Figs 4 and 5). The number of multivalents varied from one to four per cell (1.7 ± 0.9 on average) and most multivalents were composed of large chromosomes. The probability for large metacentric chromosomes 1 to 4 of M. arvalis and the acrocentric pairs of M. levis to be involved in multivalent formation was 0.60 ± 0.06. This probability was also high for the X chromosomes (0.42 ± 0.11) and submetacentric chromosome 5 of M. arvalis with homologous acrocentric chromosome 1 of M. levis (0.32 ± 0.11). The probability for medium sized and small chromosomes, however, was very low (0.05 ± 0.01).