Nucleotide sequence, genomic organization and evolution of a major repetitive DNA family in tilapia(Oreochromis mossambicuslhomorum)

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Abstract
A highly repetitive DNA sequence from tilapia (Oreochromis mossambicus/hornorum) has been cloned and sequenced. It is a tandemly arrayed sequence of 237 bp and constitutes 7% of the fish genome. The copy number of the repeat is approximately 3 .times. 105 per haploid genome. DNA sequence analysis of 7 cloned repeats revealed a high degree of conservation of the monomeric unit. Within the monomeric unit, a 9 bp AT rich motif is regularly spaced approximately 30 bp apart and may represent the progenitor of the amplified sequence. One cloned repeat, Ti-14, contained a 30 bp deletion at a position flanked by a 7 bp direct repeat. The Ti-14 sequence appears to have been amplified independently of the major 237 bp tandem array. A higher-order repeat unit, defined by longer-range periodicities revealed by restriction endonuclease digestion, is further imposed on the tandem array.