Several features of meiotic disturbances in female hybrids between M. arvalis and M. levis also characterise male meiosis in many cases of male-only hybrid sterility. The zygotene-pachytene transition is mainly affected, asynapsis and heterosynapsis occur in both heteromorphic and homomorphic chromosomes, while cells vary widely in the degree of disturbances and the number of chromosomes affected1112515253.