Light Microscopic Observations on the Behavior of Silver-stained Trivalents in Pachytene Cells of Sigmodon Fulviventer (Rodentia, Muridae) Heterozygous for Centric Fusion

Silver-stained pachytene cells of male cotton rats, Sigmodon fulviventer, heterozygous for a naturally occurring Robertsonian fusion, were examined by light microscopy. Silver staining clearly revealed synaptonemal complexes of all autosomal bivalents and of the translocation trivalent, as well as X-Y conformations within the sex vesicle. The synaptonemal complex of the trivalent conforms closely in morphology to that observed by electron microscopy in hybrids of other species. In all cases, the proximal telomeric knobs of the two acrocentric elements were paired and projected in the same direction from the metacentric element. The geometry of trivalent pairing in pachytene may predispose to normal disjunction and, therefore, account for the high fertility observed in S. fulviventer heterozygotes.