A synaptonemal complex-derived mechanism for meiotic segregation precedes the evolutionary loss of homology between sex chromosomes in arvicolid mammals.

Synapsis and reciprocal recombination between sex chromosomes are restricted to the pseudoautosomal region. In some animal species, sex chromosomes do not present this region, although they utilize alternative mechanisms that ensure meiotic pairing and segregation. The subfamily Arvicolinae (Rodentia, Cricetidae) includes numerous species with achiasmate sex chromosomes. In order to know whether the mechanism involved in achiasmate segregation is an ancient feature in arvicolid species, we have compared the sex chromosomes of both the Mediterranean vole (Microtus duodecimcostatus) and the water vole (Arvicola terrestris). By means of immunofluorescence, we have found that sex chromosomes in M. duodecimcostatus are asynaptic and develop a synaptonemal complex-derived structure that mediates pairing and facilitates segregation. In A. terrestris, sex chromosomes are synaptic and chiasmate but also exhibit a synaptonemal complex-derived filament during anaphase I. Since phylogenetic relationships indicate that the synaptic condition is ancestral in arvicolids, this finding indicates that the mechanism for achiasmate sex chromosome segregation precedes the switching to the asynaptic condition. We discuss the origin of this synaptonemal complex-derived mechanism that, in turn, could counterbalance the disruption of homology in the sex chromosomes of those species.