Pattern of X-Y chromosome pairing in the Taiwan vole, Microtus kikuchii

Pairing of X and Y chromosomes at meiotic prophase and the G- and C-banding patterns and nucleolar organizer region (NOR) distribution were analyzed i n Microtus kikuchii. M. kikuchii is closely related to M. oeconomus and M. montebelli, karyologically and systematically. The formation of a synaptone mal complex between the X and Y chromosomes at pachytene and end-to-end ass ociation at diakinesis - metaphase I are only observed in three species in the genus Microtus; M. kikuchii, M. oeconomus, and M. montebelli. All the o ther species that have been studied so far have had asynaptic X-Y chromosom es. These data confirm that M. kikuchii, M. oeconomus, and M. montebelli ar e very closely related, and support the separation of asynaptic and synapti c groups on the phylogenetic tree.