Pachytene oocytes were very variable in their appearance, containing asynaptic and heterosynaptic configurations. However, about 10% of oocytes had 23 “correctly” paired configurations, with four large trivalents, twelve heteromorphic and seven homomorphic bivalents (six acrocentric and one metacentric) (Fig. 3a). The heteromorphic bivalents comprised the X pair with long arms of different length and misaligned centromeres, and one large and ten smaller bivalents with the two centromere signals shifted. This is in a good agreement with pairing expected from comparative metaphase analysis of the parents (Fig. 3b). Parental karyotypes differ by at least four tandem fusions (producing trivalents in F1 hybrids), one paracentric and six pericentric inversions (which may produce inversion loops or straight bivalents with misaligned centromeres), four to seven putative centric transpositions (producing bivalents with misaligned centromeres), and an insertion of a large heterochromatic block in the X of M. levis3334.