Localization of NORs in cetacean karyotypes

Abstract

A silver staining technique was used for locating the nucleolar organizing regions (NORs) in seven cetacean species, four odontocetes and three mysticetes. The odontocetes were, the whitebeak dolphin, the killer, Antillean beaked and sperm whales, the mysticetes were the gray, minke and fin whales. The karyotypes of the dolphin and the three mysticete species conformed to the general 2n=44 cetacean karyotype. In these species, only one Ag-positive pair was present. This was the smallest pair of the complement. The 2n=44 karyotype of the killer whale, differing in certain respects from the general 2n=44 karyotype, has NORs in two chromosome pairs. The smaller of these pairs was similar to the NOR pair in the general karyotype. The 2n=42 Antillean beaked whale karyotype had two NOR pairs, one large and one small. The smaller pair was similar to the NOR pair in the general karyotypes, the larger pair was the second largest of the complement. The NOR sites, so far discussed, all had terminal localizations. The NOR site of the sperm whale, 2n=42, was located in the middle of the short arm of one of the largest chromosomes. Prior to the positive demonstration of NORs in the cetacean karyotypes, occasional associations have been described between telocentric chromosomes. These observations are discussed in the light of the present findings.