Cetacean molecular hybridization using balenopterid satellite DNA cRNAs as probes

Sequence organization and evolution, in all extant whalebone whales, of a DNA satellite with terminal chromosome localization

A heavy (GC rich) DNA satellite with terminal chromosomal localization is characteristic for all mysticete (whalebone whale) genomes. Sequences of 58 repeats of the satellite were compared in all ten extant mysticete species. In three families comprising eight species, the typical repeat length was 422 (421) bp. In two species, the northern right whale and the bowhead, of family Balaenidae (right whales) the repeats were much longer, typically ca. 900 and ca. 1200 bp. In all species the repeats were composed of a unique portion of constant length (212/211 bp), and a subrepeat portion, the length of which was variable. The evolutionary rigidity of the unique portion of the repeat is contrasted by the pronounced length variability of the subrepeat portion. The subrepeat portion consists essentially of 6 bp motifs, such that length differences are usually in multiples of 6 bp. The motif TTAGGG constituted 35%-50% of the subrepeats. Comparison between the unique portion of the 58 sequenced repeats revealed that the repeats divided into two primary groups, one comprising the two balaenids, the other including the eight remaining species. The mean difference between the two groups averaged 8.4%. In this sequence comparison the repeats of the pygmy right whale constituted a group that was separated from repeats of the other species. In all other cases repeats were intermingled to some extent between species. Comparison of individual repeats suggests that the unique portion evolves in concert, at a slow rate. A neighbor-joining comparison between the consensuses of all species suggests that the unique portion of the repeats evolves at a somewhat different rate in different lineages.

Composition and chromosomal localization of cetacean highly repetitive DNA with special reference to the blue whale, Balaenoptera musculus

Three highly repetitive DNA components--the common cetacean component, the heavy (GC-rich) satellite and the light (AT-rich) satellite--were were studied in the blue whale. Consensus sequences of the common component and the heavy satellite were determined on the basis of three repeats of the common component and eight repeats of the heavy satellite. The tandemly organized common cetacean component, which comprises a large portion of all cetacean--both odontocete (toothed whale) and mysticete (whalebone whale)--genomes has a repeat length of 1,760 bp and the three clones analysed showed a high degree of conformity. The repeat contains a 72 bp sequence with dyad symmetry and striking intrastrand complementarity. The rest of the repeat comprises a unique sequence. The repeat unit of the heavy satellite of the blue whale is 422 bp. Also this component is tandemly organized. About half the length of the repeat constitutes a unique sequence and the other half is made up of subrepeats with TTAGGG as a frequent motif. The light satellite has not been sequenced and its basic repeat unit has not yet been identified. The chromosomal localization of the three components was determined by in situ hybridization using 3H-labelled cloned fragments as probes. The common cetacean component was located in most interstitial and terminal C-bands. The heavy satellite occurred primarily in terminal C-bands. When the two components hybridized to the same terminal C-bands, the localization of the heavy satellite was distal to that of the common cetacean component. Neither component shared localization with the light satellite which is located in centromeric C-bands in just a few chromosome pairs.

Conservation of highly repetitive DNA in cetaceans

It is controversial whether odontocetes (toothed whales) and mysticetes (whalebone whales) have a common ancestry. Cetacean karyological uniformity, which is unique among mammalian orders, suggests a monophyletic origin; however, several anatomical authorities have maintained that odontocetes and mysticetes are diphyletic. We investigated the issue using Southern blot hybridization. Two labelled restriction fragment probes from the DNA of the sei whale (a mysticete) were hybridized to restricted DNA of cetacean species representing all extant families except the Eschrichtiidae, the gray whales. The probes hybridized to specific restriction fragments in all odontocete and mysticete materials. Hybridization showed preservation of hybridization homologies and a striking conservation of the length of highly repeated DNA sequences. The results are compatible with a common ancestry of odontocetes and mysticetes.