POSTGLACIAL GENETIC DIFFERENTIATION OF REPRODUCTIVE ECOTYPES OF KOKANEE ONCORHYNCHUS-NERKA IN OKANAGAN LAKE, BRITISH-COLUMBIA

Okanagan Lake, south-central interior of BC, contains two reproductive ecotypes of kokanee Oncorhynchus nerka; individuals spawn in tributar y streams ('stream-spawners') as well as on shoreline gravel areas ('b each-spawners'). We tested the hypothesis that these sympatric ecotype s comprise a single panmictic population by assaying variation in morp hological traits and at allozyme, mitochondrial and minisatellite DNA loci in fish collected from three stream-spawning and two beach-spawni ng sites. No morphological traits consistently distinguished the repro ductive ecotypes with the exception of the number of anal fin rays whi ch was greater in stream-spawning kokanee. Four of 18 allozyme loci sc reened were polymorphic, but no significant allele frequency differenc es were detected among populations within ecotypes or between ecotypes . Similarly, allele frequencies at two minisatellite DNA loci were not significantly different among populations or between ecotypes. By con trast, significant differences in the frequencies of mitochondrial DNA restriction fragment length polymorphism (mtDNA RFLP) haplotypes were detected between stream- and beach-spawners, but not among population s within ecotypes. Further, two RFLPs that distinguished stream- and b each-spawning adults were found in juvenile kokanee sampled from the l imnetic zone of Okanagan Lake. The two mtDNA RFLPs and a d-loop sequen ce variant appear to be unique to Okanagan Lake kokanee because we did not observe these haplotypes in sockeye salmon and kokanee sampled ou tside of Okanagan Lake. Our data suggest that: (i) there is restricted female-mediated gene flow between stream- and beach-spawning kokanee in Okanagan Lake, (ii) the forms have diverged within the lake basin s ince the retreat of the Wisconsinian glaciers (< approximate to 11 000 years ago), and (iii) distinct reproductive niches may promote diverg ence in north temperate freshwater fish faunas.