This study supports breeding records and histological assessments showing the sterility of hybrids between M. arvalis and M. levis353637383941. Hybrids of both sexes are sterile; however, males, in accordance with Haldane’s rule, are “more sterile” than females. Male meiosis is uniformly arrested at leptotene, while female meiosis is affected at pachytene with wide variation between oocytes in the degree of disturbances of chromosome pairing. This might be due to the sex difference in degree of methylation on the meiotic onset. Studies in mice demonstrated that female germ cells enter meiosis in a demethylated state, while the genome of male germ cells is heavily remethylated after mitotic arrest at the beginning of meiosis484950. A faster evolution of male hybrid sterility in the grey voles might also be accelerated by constitutive asynapsis and total lack of recombination between XY in this phylogenetic lineage47.