Achiasmatic sex chromosomes in Pitymys duodecimcostatus: mechanisms of association and segregation.

The meiotic behavior of the sex chromosomes of Pitymys duodecimcostatus was studied by electron microscopy of whole-mount synaptonemal complex preparations. The results established that the sex chromosomes of this species are achiasmatic and remain unassociated throughout meiotic prophase I in most spermatocytes. In other cells, nonspecific association of the X and Y occurred by means of filamentous bridges. Pitymys duodecimcostatus represents an additional example of a mammalian species lacking a homologous pairing segment in its sex chromosomes and extends current knowledge about this controversial subject. In this regard, we suggest that sex-chromosome association is a characteristic that probably followed different evolutionary paths in different mammals, leading to loss of the homologous segment in some species and its conservation in others. It is also suggested that in P. duodecimcostatus, and probably in many other species as well, three mechanisms may act in concert to permit joining of the X and Y chromosomes during meiotic prophase, and, consequently, to ensure proper segregation during anaphase I: (1) joining of the sex-chromosome axes at their ends to the nuclear membrane, (2) formation of fibrillar structures to hold the sex chromosomes together, and (3) cohesiveness due to sex-vesicle formation.