Translocations of acrocentric chromosomes and their implications in the evolution of sheep <i>(Ovis)</i>

Cytogenetic evidence suggests that the caprids (sheep and goats) evolved from a common ancestor with a 2n=60 karyotype. Although goats <i>(Capra</i>) retained the primitive 2n=60 karyotype, sheep <i>(Ovis</i>) underwent a sequential reduction in the number of chromosomes by means of acrocentric translocation. The formation of the first metacentric autosome (Ml) occurred in the aoudad (<i>Ammotragus</i>) and urial <i>(O</i>. <i>vignei</i>), resulting in a 2n=58 karyotype. The G-bands are homologous, which implies both genotypes arose from a common ancestor, possibly a rupicaprid. Based on G-bands, acrocentric chromosomes 1 and 7 of the 2n=60 karyotype formed the Ml. The X chromosome, which is the second longest acrocentric in the 2n = 60 karyotype, became the longest acrocentric in <i>Ammotragus </i>and <i>Ovis </i>(2n = 58). The second pair of metacentrics to evolve, which is ranked in the M3 position of the 2n=54 karyotype, resulted from the translocations of acrocentric chromosomes 4 and 14 or 15 in the 2n = 60 karyotype. The M2 was the third pair of metacentrics to be formed and resulted from the translocations of acrocentric chromosomes 3 and 12 or 13 in the 2n = 60 karyotype. The G-bands of all 2n = 54 karyotypes are homologous, which indicates origin from a common ancestor. Evidence is presented that suggests a prezygotic selection is bringing about a reduction in diploid chromosome numbers. The possible roles of fission and fusion in the karyotypic evolution of <i>Ovis </i>are discussed.