Achiasmatic giant sex chromosomes in the vole Microtus cabrerae (Rodentia, Microtidae).

Conventional and microspread preparations of Microtus cabrerae spermatocytes were made to investigate the chromosomes of this species. Three different types of Y chromosomes, varying in size of the heterochromatic block, were observed; they were alike, however, in regard to synapsis, which was consistently absent. Our results suggest that the heterochromatic blocks are not involved in the lack of synapsis and that asynapsis is a cytological feature common to all species of the family Microtidae. In addition, the co-aligned configuration of the ends of the sex-chromosome axes of this species and the lack of silver-stainable threads or filaments connecting them suggest the existence of two mechanisms for association of the sex chromosomes during prophase I and metaphase I: attachment of the ends of both sex chromosome axes to the nuclear envelope and heterochromatin "stickiness."