High-resolution cytological localization of the XhoI and EcoRI repeat sequences in the pachytene ZW bivalent of the chicken.

The pachytene ZW pair of the chicken has been studied with a novel combination of fluorescent in situ hybridization and silver staining of the synaptonemal complexes, and with electron microscopic in situ hybridization. Probes for the EcoRI and the XhoI repeat sequences were used for light microscopy and probes for XhoI for electron microscopy. XhoI repeats are pericentromeric and correspond to 27.3% of the W axis length. EcoRI repeats form three distinct domains: domain I covers the distal 19.4% of the late-synapsing arm, and domains II and III cover respectively 21.7% and 8.5% of the W axis. The early-synapsing end containing the recombination nodule is free from any of both signals. This non-hybridizing region accounts for 25.9% of the W axis. It is suggested that this region is composed of a proximal region containing other repeat types and a terminal region which is the recombining or pseudoautosomal region. The successful combination of silver staining and fluorescent in situ hybridization will be generally useful for high-resolution localization of DNA sequences in meiotic chromosomes.