High-resolution cytological localization of theXhol andEcoRI repeat sequences in the pachytene ZW bivalent of the chicken

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Abstract
The pachytene ZW pair of the chicken has been studied with a novel combination of fluorescentin situ hybridization and silver staining of the synaptonemal complexes, and with electron microscopicin situ hybridization. Probes for theEcoRI and theXhol repeat sequences were used for light microscopy and probes forXhol for electron microscopy.Xhol 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 fluorescentin situ hybridization will be generally useful for high-resolution localization of DNA sequences in meiotic chromosomes.