• Microtus sikimensis;
  • karyotype;
  • synaptic sex chromosomes;
  • Microtus phylogeny


Pairing of the X and Y chromosomes during meiotic prophase and the G- and C-banding patterns were analysed in Microtus sikimensis. The karyotype of M. sikimensis from central Nepal, has 2n= 48 and FNa = 56. It has been derived from the all single-armed karyotype of M. carruthersi through a unique course of evolution. There is a synapsis between the X and Y chromosomes at pachytene and end-to-end association from diakinesis to metaphase I. Microtus sikimensis has no close karyological or morphological relationship to the other synaptic species studied so far. These data confirm that several species or lineages with synaptic condition exist in the genus Microtus, and that the ancestor of Microtus had synaptic sex chromosomes.