EXTENSIVE DIRECT-TANDEM ORGANIZATION OF A LONG REPEAT DNA-SEQUENCE ONTHE Y-CHROMOSOME OF CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA)

A long repetitive DNA sequence (OtY8) has been cloned from male chinoo k salmon and its genomic organization has been characterized. The repe at has a unit length of 8 kb and is present approximately 300 times pe r diploid male nucleus. All internal fragments within the 8-kb repeat segregate from father to son, suggesting that the entire repeat unit i s located on the Y chromosome. The organization of this sequence into an 8-kb repeat unit is restricted to the Y chromosome, as are several male-specific repeat subtypes identified on the basis of restriction-s ite variation. The repeat possesses only weak internal sequence simila rities, suggesting that OtY8 has not arisen by duplication of a smalle r repeat unit, as is the case for other long tandem arrays found in eu karyotes. Based on a laddered pattern arising from partial digestion o f genomic DNA with a restriction enzyme which cuts only once per repea t unit, this sequence is not dispersed on the Y chromosome but is orga nized as a head-to-tail tandem array. Pulse-gel electrophoresis reveal s that the direct-tandem repeats are organized into at least six separ ate clusters containing approximately 12 to 250 copies, comprising som e 2.4 Mb of Y-chromosomal DNA in total. Related sequences with nucleot ide substitutions and DNA insertions relative to the Y-chromosomal fra gment are found elsewhere in the genome but at much lower copy number and, although similar sequences are also found in other salmonid speci es, the amplification of the repeat into a Y-chromosome-linked tandem array is only observed in chinook salmon. The OtY8 repetitive sequence is genetically tightly associated with the sex-determination locus an d provides an opportunity to examine the evolution of the Y chromosome and sex determination process in a lower vertebrate.