By using three gene probes, one derived from the porcine major histocompatibility complex (MHC) and two from bovine cytokeratin genes, type I (KRTA) and type II (KRTB), the hypothesis of conservation of genome structure in two members of the family Bovidae was examined. Gene mapping data revealed the MHC to be in chromosome region 23q15→q23 in cattle (BOLA) and 20q15→q23 in sheep (OLA). KRTA was localized to chromosome region 19q25→q29 in cattle and 11q25→q29 in sheep and KRTB to 5q14→q22 in cattle and 3q14→q22 in sheep. The banding patterns of the chromosome arms to which the loci were assigned were identical in both species. Moreover, the resemblances of GTG- or QFQ-banding patterns between the cattle and sheep karyotypes illustrated further chromosome homologies. These studies, based on gene mapping comparisons and comparative cytogenetics, document that within bovid chromosomes, homology of banding patterns corresponds to a homologous genetic structure. Hence, we propose that gene assignments on identified chromosomal segments in one species of the Bovidae can be extrapolated, in general, to other bovid species based on the banding homologies presented here.
This appears to be the first reported case of a bull with a balanced autosomal reciprocal translocation associated with azoospermia. The analysis includes somatic chromosome banding, conventional meiotic analysis, and electron microscopy of synaptonemal complexes (SCs). The karyotype of the bull was found to be 60, XY, rcp(8;13)(ql l;q24). Electron microscopy of SCs in microspread pachytene spermatocytes revealed a high incidence of terminal asynapsis of the smallest arm of the quadrivalent. Most quadrivalents with such asynapsis and few with nonhomologous synapsis showed associations with the XY sex bivalent, leading to complete meiotic arrest at late pachynema. Except for one diakinetic cell, no diplotene and subsequent stages were encountered in air-dried meiotic preparations. The presence of degenerating primary spermatocytes in SC preparations, as well as in testicular sections, and the absence of spermatozoa in ejaculates confirm the chromosomally derived male sterility of the bull. X-chromosome reactivation, evidenced by the cytomorphological reversal of associated sex bivalents, appeared to be the initial step in the degeneration of spermatocytes. Consequently, the formation of a separate, fully developed XY body, which was previously demonstrated on the periphery of spermatocyte nuclei in fertile bulls, could not be attained in this case. Extensive end-to-end association of autosomal bivalents in meiotically arrested, as well as degenerating, spermatocytes was a consistent and unique observation of this study. Such associations may lead to enhanced reactivation of the X chromosome.
Summary. A genetic region, most likely the major histocompatibility complex, was assigned to bands q13–23 of cattle chromosome 23 by in situ hybridization using a cloned DNA sequence of a class I gene of the pig major histocompatibility complex.
The first gene assignment to a horse chromosome is reported for equine leucocyte antigen (ELA), the major histocompatibility complex of the horse. A cloned DNA sequence derived from a class I gene of the porcine major histocompatibility complex was used as a probe for an in situ hybridization experiment. We present the regional localization of ELA, using this sequence, to equine chromosome 20q14-q22.
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