To make insight into the cytological basis of hybrid sterility, we examined chromosome synapsis and recombination in male and female sterile hybrids between two sibling species of the grey vole, Microtus arvalis (dams) and M. levis (sires). These species diverged from 0.5 to 4.3 MYA3233, differ by a series of chromosomal rearrangements3334, yet remain morphologically indistinguishable35. In nature, hybrids occur at the zone of sympatry in the Urals, and can easily be produced in laboratory settings3536373839. Reciprocal hybrids between M. arvalis and M. levis are completely sterile; however, males differ from females in the stage of reproductive collapse38. In males, testis mass is severely reduced and no sperm is found in the epididymis. Male meiosis arrests at prophase I but occasional nuclei reach diakinesis-metaphase I, where they mainly show univalents. More advanced stages of spermatogenesis have not been detected40. In female hybrids, oocyte growth and development was described as normal. However, follicular atresia was detected in hybrids even at the primordial follicle stage, and, at all stages, was more pronounced than in females of the parental species. Hybrid females display abnormal ovulation without follicle wall breakage. Mature oocytes move into Graafian follicles where they undergo the second meiotic division. No corpus luteum was detected in hybrid females41. According to Gileva et al.37 backcross progeny may be produced very rarely, but in our hybridisation experiments none occurred. Thus, these hybrids provide a promising model for studying cellular mechanisms of male and female hybrid sterility.