MOLECULAR-GENETIC EVIDENCE FOR PARALLEL LIFE-HISTORY EVOLUTION WITHINA PACIFIC SALMON (SOCKEYE-SALMON AND KOKANEE, ONCORHYNCHUS-NERKA)

The Pacific salmon Oncorhynchus nerka typically occurs as a sea-run fo rm (sockeye salmon) or may reside permanently in lakes (kokanee) thoug hout its native North Pacific. We tested whether such geographically e xtensive ecotypic variation resulted from parallel evolutionary diverg ence thoughout the North Pacific or whether the two forms are monophyl etic groups by examining allelic variation between sockeye salmon and kokanee at two minisatellite DNA repeat loci and in mitochondrial DNA (mtDNA) Bgl II restriction sites. Our examination of over 750 fish fro m 24 populations, ranging from Kamchatka to the Columbia River, identi fied two major genetic groups of North Pacific O. nerka: a ''northwest ern'' group consisting of fish from Kamchatka, western Alaska, and nor thwestern British Columbia, and a ''southern'' group consisting of soc keye salmon and kokanee populations from the Fraser and Columbia River systems. Maximum-likelihood analysis accompanied by bootstrapping pro vided strong support for these two genetic groups of O. nerka; the pop ulations did not cluster by migratory form, but genetic affinities wer e organized more strongly by geographic proximity. The two major genet ic groups resolved in our study probably stem from historical isolatio n and dispersal of O. nerka from two major Wisconsinan glacial refugia in the North Pacific. There were significant minisatellite DNA allele frequency differences between sockeye salmon and kokanee populations from different parts of the same watershed, between populations spawni ng in different tributaries of the same lake, and also between sympatr ic populations spawning in the same stream at the same time. MtDNA Bgl II restriction site variation was significant between sockeye salmon and kokanee spawning in different parts of the same major watershed bu t not between forms spawning in closer degrees of reproductive sympatr y. Patterns of genetic affinity and allele sharing suggested that koka nee have arisen from sea-run sockeye salmon several times independentl y in the North Pacific. We conclude that sockeye salmon and kokanee ar e para- and polyphyletic, respectively, and that the present geographi c distribution of the ecotypes results from parallel evolutionary orig ins of kokanee from sockeye (divergences between them) thoughout the N orth Pacific.