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Cytogenetic Characterization of a Small Evolutionary Rearrangement Involving Chromosomes BTA21 and OAR18

Both cattle (Bos taurus) and sheep (Ovis aries) belong to the Bovidae family but to different subfamilies, Bovinae and Caprinae, respectively. From a chromosomal point of view, apart from the already known centric fusions (that occurred during the evolutionary process in the Bovidae family) and the small differences in the chromosome classification, the 2 karyotypes are very similar in banding patterns. In this study, the combination of bioinformatics techniques and physical mapping of DNA markers enabled the identification of a micro-rearrangement, a small inversion involving bovine chromosome 21 (BTA21) and the corresponding sheep chromosome 18 (OAR18). The aim of this study was the cytogenetic characterization of this difference in genomic assemblies between cattle and sheep in this single chromosome region. To verify the inversion in FISH experiments, we used the BACs 442H08 and 222H03 from the INRA library and BACs 134H22 and 436P08 from the sheep-specific CHORI library. The results confirmed the presence of the inverted fragment in sheep compared to the cattle genome. Genomic rearrangements may have consequences depending on their influence on gene activity, but in this case no gene or transcribed DNA portion seemed to be involved. In conclusion, we showed for the first time, concerning autosomes, that besides the already known centric fusions also other differences exist between the bovine and sheep karyotypes. Furthermore, we demonstrated that the combination of a bioinformatics approach and physical mapping is a valid tool for the identification of currently unknown rearrangements between related species.