Repetitive DNA sequences and the divergence offish populations: some hopeful beginnings

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<jats:p>Repetitive sequences are predominant components of eukaryotic genomes, including those of fishes, but they represent largely unchartered territory for the student of fish evolution and population differentiation. Three examples of the use of repetitive sequences are presented. In one, a Hind 111 monomeric unit of an adjacent repeat (satellite) sequence was characterized, by cloning and sequencing, from the genome of the sheepshead minnow, <jats:italic>Cyprinodon variegates</jats:italic>, a species which ranges from Cape Cod to Mexico. The characteristic monomeric sequence of each population sample (termed the ‘canonical sequence’) is divergent. The data demonstrate that molecular drive, usually invoked only for interspecific divergence, can apparently produce characteristic suites of repetitive sequences in conspecific local populations. The two other examples deal with dispersed simple‐sequences and minisatellites which provide individual‐specific DNA fingerprint phenotypes. In both cases, previous allozyme studies failed to resolve potentially important variation. In <jats:italic>Rivulus marmoratus</jats:italic>, a selling hermaphroditic species with homozygous, clonal population structure, fingerprinting demonstrates unprecedented levels of intrapopulation genetic heterogeneity, and strongly suggests that there are significant mutational distances between even sympatric clones. The data warrant serious re‐evaluation of the genetic variation of this species, its adaptive significance and its origins. In the striped bass, <jats:italic>Morone saxatilis</jats:italic>, a species of enormous economic and recreational importance, fingerprinting demonstrates that even hatchery stocks have very high levels of genetic variation. Though the species essentially lacks allozyme variation, it is apparently not approaching genome‐wide homozygosity. Taken together, the data argue that the analyses of various kinds of repetitive sequences will play an increasingly greater role in future attempts to understand the structure and differentiation offish populations.</jats:p>

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