Chromosomes of the Siberian Snow Sheep, Ovis Nivicola, and Implications Concerning the Origin of Amphiberingian wild Sheep (subgenus Pachyceros)

Abstract

The chromosomes of Ovis nivicola, described for the first time, exhibit 2n = 52, the lowest diploid number to be reported for wild sheep and goats. The new chromosomal data, together with a review of the fossil history of the genus, lead us to conclude that the bighorned wild sheep (subgenus Pachyceros) evolved their distinctive characteristics while isolated in the ice-free Beringian refugium, and then migrated southward into western North America when the glacial barriers melted, as first suggested by Cowan (1940).

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

“…As yet, we have not examined 2n=52 karyotypes and can only speculate as to which acrocentrics probably were involved in the formation of the M4, or fourth pair of metacentrics. Korobitsyna et al (1974) compared the arm ratios of the M4 to the "Massey IF' translocation, which occurs in a select breed of Romney sheep, and suggested that the same acrocentric equivalents may have given rise to the M4. Such acrocentrics have been identified by Bruere et al (1974) as the 8 and 11 acrocentric pairs of the 2n=54 karyotype (Bruere and McLaren, 1967).…”

“…This form of chromosome rearrangement is clearly observed within Ovis (N adler et al, 1973). As a result, Ovis can be divided into four cytogenetic groupings, based upon chromosome number: 2n = 52 (O. nivicola): 2n = 54 (O. aries, O. canadensis, O. dalli, O. musimon, and O. orientalis); 2n=56 (O. ammon) : and 2n=58 (O. vignei) (Schmitt and U lbrich, 1968;W urster and B enirschke, 1968;N adler, 1971;N adi.er et al, 1971; Korobitsyna et al, 1974). All members of the genus Capra have 2n = 60 chromosomes (W urster and B enirschke, 1968).…”

G-band patterns of the Siberian snow sheep <i>(Ovis nivicol</i><i>a</i>) and their relationship to chromosomal evolution in sheep

“…Korobitsyna et al (1974) examined conventional karyotypes of O. nivicola and suggested that the morphology of M4 most closely resembled the Massey II (t3) translocation of 2n = 52 domestic sheep because of similarities in arm-length ratios. The availabilitty of G-band preparations from O. nivicola, which were compared with photographic illustrations of Massey tj, t2, and t:j translocations (Bruere et al, 1974), now confirms the earlier supposition of Korobitsyna et al (1974) that M4 of O. nivicola was not homologous with the Massey ti or t.i metacentrics. G-band patterns also differentiate M4 of O. nivicola from the Massey t> translocation by differences in banding of the short arms and the fact that the centromere of the former element occupies a more sub median position.…”

“…Wild sheep with 2n = 54 may have evolved monophyletically from an ancestral 2n = 58-56-54 population or polyphyletically by a series of independent, nonrandom fusions. In contrast, the fourth pair of biarmed autosomes in O. nivicola and in 2n = 52 domestic sheep variants may have resulted from random fusions of different chro mosomes.Wild sheep of the Holarctic Region belong to four groups based upon chromosome numbers, morphology, and other characters (N adi.i r et al" 1973a, b;Korobitsyna et al, 1974). These groups include mouflon sheep inhabiting the Mediterranean region and Near East (Ovis musimon and O. orientalis, 2n = 54).…”

“…Siberian snow sheep constitute the only known wild sheep taxon with 2n = 52, and they presumably evolved from a 2n = 54 pachycerinc an cestral stock isolated within the ice-free Beringian glacial refugium during the Riss-Illinoin period (Korobitsyna et al, 1974). Possible chromosomal homologies between O. nivicola and other wild sheep have not been eval uated nor have the relationships between the fourth pair of metacentric autosomes of O. nivicola and the three translocations reported in 2n 52 domestic sheep variants been examined (Bruiri et al" 1974(Bruiri et al" , 1976.…”