Genetic differentiation among sympatric migratory and resident life history forms of rainbow trout in British Columbia

Rainbow trout Oncorhynchus mykiss in coastal areas of North America occur as two divergent migratory types: steelhead that migrate to salt water and return to their natal river to spawn and resident rainbow trout that either do not migrate or migrate locally within the freshwater system. We extracted DNA from sympatrically occurring steelhead and rainbow trout collected from five major drainages in British Columbia. We used three types of genetic markers to test for differentiation within the sympatric population: microsatellite DNA markers, gene intron polymorphisms (restriction fragment length polymorphisms [RFLPs] and fragment size polymorphisms), and major histocompatibility complex (MHC) exon polymorphisms (RFLPs). The migratory and resident forms were genetically differentiated in only one sympatric population based on microsatellite data, but no significant differentiation was found using the combined gene locus marker data. Overall, most of the observed allele variation at all marker types was attributable to among-drainage variance, while migratory life history contributed a nonsignificant component based on hierarchical analysis of molecular variance. Interestingly, the migratory and resident forms were genetically differentiated in two different populations based on the gonadotropin and p53 intron polymorphisms, while no significant genetic differentiation was found between the two migratory types at any of the MHC exon or growth hormone intron-length polymorphisms. Furthermore, the neighbor-joining cluster dendrogram based on the microsatellite markers reflected geographic patterns, while the neighbor-joining dendrogram using gene intron and MHC markers did not. Our data indicate that the migratory and resident life histories have probably evolved independently in different drainages in British Columbia. The disagreement between the results of the analyses using the microsatellite and gene locus markers probably reflects differences in selection, mutation, or both, acting at these two types of marker loci.