ONE OF 2 GROWTH-HORMONE GENES IN COHO SALMON IS SEX-LINKED

Salmonid fishes have two growth hormone genes resulting from their pol yploid ancestry. We used the polymerase chain reaction to examine gene tic variation in the third intron (C) of both of these genes in coho s almon (Oncorhynchus kisutch). A polymorphism in the length of intron C in GH-1 is due to a variable number of copies of a 31-nt repeat that is absent from GH-1 of the closely related chinook salmon (Oncorhynchu s tshawytscha) and rainbow trout (Oncorhynchus mykiss). Thus, this tan dem repeat sequence has become established in the genome of coho salmo n since the separation of this species from its closest relatives. All male coho salmon examined have an allele at the second growth hormone gene, GH-2, that is not found in females. GH-2 is thus on the sex chr omosome and there is no recombination between GH-2 and the sex-determi ning locus (SEX). Sequences of intron C indicate much greater divergen ce between the X chromosome-specific allele and the Y chromosome-speci fic allele within coho salmon than between the X chromosome-specific a lleles of coho and the closely related chinook salmon. Thus, absence o f recombination between GH-2 and SEX apparently predates separation of these two species.