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

Abstract

Cytogenetic evidence suggests that the caprids (sheep and goats) evolved from a common ancestor with a 2n=60 karyotype. Although goats (Capra) retained the primitive 2n=60 karyotype, sheep (Ovis) 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 (Ammotragus) and urial (O. vignei), 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 Ammotragus and Ovis (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 Ovis are discussed.

An Enhanced Linkage Map of the Sheep Genome Comprising More Than 1000 Loci

“…These include the translocation of a region equivalent to the centromeric end of OAR 9 and CHI 14 to the centromeric region of BTA 9 (Crawford et al 1995;Vaiman et al 1996) and the structure of the X chromosome (Galloway et al 1996;Ponce de Leon et al 1996). It has been proposed that caprids evolved from a common ancestor with a 2n = 60 karyotype and no metacentric chromosomes (Bunch et al 1976). However, evidence from the mapping of the trichohyalin (THH) gene in sheep, cattle, and goats suggests that the common ancestor may have had at least one metacentric chromosome equivalent to OAR1.…”

Cytogenetics and reproduction of sheep with multiple centric fusions (Robertsonian translocations)

“…Roosen-Runge (1973) (Mayr, 1963). The number of reports from several countries of the unimpaired mating which takes place between different species of sheep with different chromosome numbers (Nadler, Lay & Hassinger, 1971;Bunch, Foote & Spille«, 1976;Y. Sharipov, personal communication) further supports this conclusion.…”

Brief communication. Heteroplasmy of mitochondrial DNA in the iphiuroid Astrobrachion constrictum

“…Ammotragus (2n ϭ 58), commonly called the Barbary sheep or Aoudad, and Pseudois (2n ϭ 54 and 56), are regarded as aberrant sheep with goatlike affinities. True sheep of the genus Ovis have 2n ϭ 52, 54, 56, or 58 ( Bunch et al 1976;Bunch and Nadler 1980;Nadler and Bunch 1977;Nadler et al 1973).…”