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Asynaptic behavior of X and Y chromosomes in the Virginia oppossum and the southern pygmy mouse.

The meiotic behavior of silver-stained X and Y chromosomes of the Virginia opossum, Didelphis virginiana, and the southern pygmy mouse, Baiomys musculus, was studied by light microscopy. While the sex chromosomes of these two species differ in both size and morphology, their meiotic behavior is very similar. In both species a typical sex vesicle is formed during pachytene, but unlike most other mammalian species thus far studied, a synaptonemal complex does not form between the X and Y chromosomes. During pachytene, variable modes of association of the ends of the sex chromosomes (both heterologous and autologous) are sometimes observed. During diplotene, diakinesis, and metaphase I, the X and Y rarely appear to be associated. In some pachytene nuclei of B. musculus, the single axis of the biarmed X chromosome folds back upon itself, and the telomeres form a close association. While synaptic pairing segments of the sex chromosomes of the Virginia opossum may be displaced by telomeric heterochromatin on the X, true pairing regions between the X and Y of the pygmy mouse may be lacking.