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Habitat-specific genetic and phenotypic differentiation in juvenile coho salmon Dobson, Diana

Abstract

Juvenile coho (Oncorhynchus kisutch) will spawn and have their offspring reared in very different habitat types in coastal British Columbian streams. For instance, some utilize main channel river areas whereas others take advantage of off-channel habitats. However, during winter, juveniles produced in the main channel areas are believed to move into off-channel habitats for reasons of protection. These main channel juveniles tend to be much larger at the start of winter than the off-channel resident juveniles. It is possible that these two size morphs reflect genetic differentiation that would indicate either the fine-scale population structure or heritable growth-related differences between individuals using different habitats. On the other hand, the variation may simply reflect phenotypic plasticity. There is little understanding of the correspondence between neutral molecular genetic variation and adaptive variation, yet patterns of molecular variation are most often used to develop management plans directed towards conserving genetic diversity in salmon populations. This study describes variation for both microsatellites (neutral genetic markers) and growth, presumably an adaptive trait, in closely located populations of coho salmon. The genetic basis of the observed size variation was explored by studying juvenile coho from the Mamquam River. After 6 months rearing in a common environment, no size differences were observed between juveniles originating from different habitats. More variation for size was observed between individuals sampled from within habitat types than was observed among habitats. This result suggested that the size variation is a result of phenotypic plasticity associated with environmental differences in rearing habitat. It does not exclude the possibility that variation for growth among individuals is associated with adaptive differences on a microhabitat scale. Microsatellite analysis revealed fine-scale population structure within the Mamquam River resulting from either founder effects associated with channel colonisation, the existence of separate races, or sampling bias. Heritability for growth over the six-month rearing period and size were estimated by a novel technique that uses molecular marker similarity to infer relatedness. Positive co-variances were observed between relatedness and phenotypic similarity for growth rate and initial size indicating heritability for these traits.

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