HIGH-RESOLUTION CYTOLOGICAL LOCALIZATION OF THE XHOL AND ECORI REPEATSEQUENCES IN THE PACHYTENE ZW BIVALENT OF THE CHICKEN

The pachytene ZW pair of the chicken has been studied with a novel com bination of fluorescent in situ hybridization and silver staining of t he synaptonemal complexes, and with electron microscopic in site hybri dization. Probes for the EcoRI and the Xhol repeat sequences were used for light microscopy and probes for Xhol for electron microscopy. Xho l repeats are pericentromeric and correspond to 27.3% of the W axis le ngth. EcoRI repeats form three distinct domains: domain I covers the d istal 19.4% of the late-synapsing arm, and domains II and III cover re spectively 21.7% and 8.5% of the W axis. The early-synapsing end conta ining the recombination nodule is free from any of both signals. This nan-hybridizing region accounts for 25.9% of the W axis. It is suggest ed that this region is composed of a proximal region containing other repeat types and a terminal region which is the recombining or pseudoa utosomal region. The successful combination of silver staining and flu orescent in situ hybridization will be generally useful for high-resol ution localization of DNA sequences in meiotic chromosomes.