L1 repeat is a basic unit of heterochromatin satellites in cetaceans

FIG. 1.—Alignment of dolphin satellite monomer and L1 consensus sequence. The top sequence is the satellite monomer (GenBank sequence M36451; opposite orientation) and the bottom sequence is the L1MA9 consensus sequence (Smit et al. 1995) (positions 81–685; RepBase Update: http://www.girinst.org). Numbering refers to satellite sequence positions. Identical nucleotides are indicated by asterisks, transversions by dots, transitions by colons, and alignment gaps by hyphens. Mammalian heterochromatin, pericentric, telomeric, or intercalary, is usually composed of long arrays of tandemly repeated DNA sequences called satellites, which contribute up to 15%–30% of the entire genome. Rapid divergence of heterochromatin satellites, sometimes between closely related species, raises the fundamental question of whether satellite monomers rapidly evolve from the preexisting pool of satellites or some of them are recruited from nonsatellite DNA. Previous analyses of sequence periodicities indicate that alphoid satellites might have evolved from smaller heptanucleotide units (Zaitsev and Rogaev 1986), which suggests continuous evolution of satellites from the preexisting satellites. The alternative model is that individual satellite units can be recruited from a wide spectrum of nonsatellite DNA and propagated into satellites by unequal crossing over (Smith 1976). This model is rooted in the fact that no stringent functional requirements seem to be imposed on individual satellite units, and many DNA fragments, particularly abundant interspersed repeats, should occasionally be expanded into satellites. However, to date, no evidence of such events has been recorded. In this correspondence, we report that the so-called ‘‘common satellite’’ in cetaceans originated by amplification of a DNA fragment which includes the L1 interspersed repetitive element (Boeke 1997). The common cetacean satellites, constituting about 15% of all cetacean genomes, of both odontocetes (toothed whales) and mysticetes (whalebone whales) were first used to prove a monophyletic origin of the Cetacea at the DNA level (Arnason, Hoglund, and Widegren 1984). These satellites are predominantly located in most interstitial and terminal heterochromatin Cbands. The monomeric repeat is ;1,760 bp long in all cetaceans except dolphins, where its length is ;1,580 bp (Arnason, Gretarsdottir, and Widegren 1992; Gretarsdottir and Arnason 1992). There is no reported satellite DNA similar to the common cetacean satellite in other mammalian species. We have found that a 540-bp DNA fragment of the common cetacean satellite monomer is 63% similar to the 39-terminal portion of the mammalian L1 (LINE-1) retrotransposon (figs. 1 and 2). This is also the first sequence record of the L1 retroelement in the genome of marine mammals. In addition to the nucleotide similarity, we have found protein sequence similarity to the