Relationship between large Y-chromosome and side-by-side pairing of the XY-bivalent observed in the chinese hamster, Cricetus griseus

Summary and Conclusion

• 1. In the Chinese hamster, a species of Eurasian Cricetinae, the Y-chromosome is almost as large as the X: both have long arms approximately equal in length.

• 2. During male meiosis when the major portion of the X and Y are heavily condensed, the distal two-thirds of the long arms of both the X and Y remain isopycnotic and serve as pairing segments which permit side-by-side association. Coincident with condensation of the sex chromosomes at first meiotic prophase, the smallest (tenth) autosomal bivalent also manifests positive heteropycnosis.

• 3. Matthey believes that the large Y seen in many species of Eurasian Cricetinae results from a translocation involving a small Y, a large X, and a homologous pair of autosomes. This hypothesis explains our present observations on the Chinese hamster.

• 4. Side-by-side pairing of the X and Y during male meiosis is limited to those very few mammalian species in which the unusually large Y diminishes the size difference between the sex chromosomes. Available evidence indicates that this rare situation results from a translocation with the autosomes.