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Chromosome studies of native populations of Oncorhynchus mykiss (steelhead and rainbow trout) in western Washington and southern British Columbia revealed the presence of two evolutionarily distinct chromosome lineages. Populations between, and including, the Elwha River, Washington, and Chilliwack River, British Columbia, contained 2n = 60 chromosomes. Populations on the central Washington coast contained 2n = 58 chromosomes. The north Washington coast and western Strait of Juan de Fuca contained individuals with 58, 59, or 60 chromosomes, suggesting this is a transition zone between 58 and 60 chromosome groups. The differences in chromosomal structure between 2n = 58 and 2n = 60 groups are presumably a Robertsonian rearrangement and an inversion. Alelic variation at three microsatellite loci (One ,u6, One u11 and Omy 77) also was examined, and no significant variation was detected among the 58 and 60 chromosome races. A hypothesis is presented concerning the origin of the 60 chromosome lineage.

Most males showed a morphological difference between the X and Y chromosomes but a number of males with no apparent sex chromosome heteromorphism were observed. These fish were particularly common in some populations; these may represent areas in which the rearrangement resulting in a morphological difference between the X and Y has not become fixed in the population.

The Eagle Lake trout is a rainbow series trout inhabiting a Lahontan basin lake, where the Lahontan cutthroat trout would be expected to occur. The fish is thought to be descended from rainbow trout, or from a hybridization between rainbow and cutthroat trout, or rainbow and redband trout. Electrophoretic, karyotypic and meristic data argue against the cutthroat x rainbow hypothesis. The data are more equivocal concerning the other two hypotheses. Eagle Lake, rainbow and redband trout were found to be very similar electrophoretically. In addition, the Eagle Lake trout is similar to redband and some rainbow trout in having 58 chromosomes. Meristically, however, the Eagle Lake trout is similar to the rainbow and unlike the redband. Thus, the most plausible theory of origin of the Eagle Lake trout is the immigration or perhaps early introduction of a rainbow trout with 58 chromosomes.