The curious normality of the synaptic association between the sex chromosomes of two arvicoline rodents: Microtus oeconomus and Clethrionomys glareolus.

In all eight species of arvicoline (microtine) rodents previously described, the X and Y chromosomes have remained asynaptic throughout pachynema. Since synapsis is presumed to be a prerequisite for crossing over, it has been concluded that the sex chromosomes in these species are also achiasmatic, but the mechanism(s) of their disjunction remains an enigma. Their asynaptic, achiasmatic condition has been attributed to loss of the pseudoautosomal region (Borodin et al. 1991; Carnero et al. 1991; Jiménez et al. 1991). This loss has been postulated to include all arvicoline rodents. We describe here the sex chromosome behavior during meiotic prophase of two additional species in this group: Microtus oeconomus and Clethrionomys glareolus. In both species there is extensive synapsis between the X and Y, providing the usual opportunity for XY recombination. These findings challenge the concept of the pseudoautosomal region as an evolutionarily conserved region of homology, at least within the arvicoline rodents. The unexpected finding of synapsis in two very different species, one with a derived and one with a primitive karyotype is discussed within its phylogenetic context.