Comparative landmark loci for the X chromosomes in domestic cattle, river buffalo and sheep

Different from autosomes, the X chromosomes in Bovidae differentiate by more complex chromosome rearrangements. In particular, the Bos taurus X chromosome (BTAX) is submetacentric, whereas the X chromosomes of Bubalus bubalis (BBUX) and Ovis aries (OARX) are both acrocentric. Unlike BBUX, OARX possesses a very short p-arm. The different centromere positions originate from centromere transposition or repositioning events. Furthermore, BBUX is larger than BTAX and OARX due to the presence of an extra block of constitutive heterochromatin existing exclusively in BBUX next to the centromere. However, cytogenetic loci of BTAX and BBUX suggest synteny and evolutionary conservation of gene and marker order between both chromosomes. On the other hand, in sheep OARX, only the p-arm, harboring the pseudoautosomal region (PAR), and the proximal half of the OARX q-arm, display a similar gene order as found in BTAX and BBUX. In contrast, the distal half of the OARX q-arm consists of rearranged syntenic segments compared to BTAX and BBUX. We carried out comparative gene mapping by BAC FISH on RBPI-banded chromosomes to investigate the complex sex chromosome rearrangements that occurred during karyotype evolution in Bovidae. The new cytogenetic mappings evaluate and refine previously identified synteny between OARX, BTAX, and BBUX. The comparison of data with human evaluated conserved syntenic segments between bovid X chromosomes and the human X chromosome. The data present the most advanced cytogenetic maps of the X chromosomes in bovids, improve the knowledge about sex chromosome evolution, and are useful for clinical cytogenetics. The new assignments represent useful cytogenetic landmarks, which support the bovid X chromosome sequence annotation process and will finally identify the corresponding X chromosome bands and genome sequence contigs. Our data show that some of the loci in OARX and BTAX differ from loci assembled by genome sequencing. We suggest the relocation of affected sequence contigs according to the cytogenetic assignments.