Origins and relationships of native populations of Salmo trutta (brown trout) in Spain

Allele frequency variation at 25 protein-coding loci was compared among collections from 24 naturally spawning brown trout populations in Spain. Evidence of introgression was apparent but alleles of indigenous fish predominated in each of the nine populations sampled in areas where exogenous hatchery fish had been released. Native Spanish gene pools were characterized by strong individuality based on distinct allele frequencies of both adjacent and distant rivers (G(ST) = 0.64), the locally high frequency of rare and previously unreported alleles, and low heterozygosities (H-S = 0.031). Two major Spanish lineages are proposed within a previously postulated 'ancestral' grouping of populations existing in southern Europe, based on predominant alleles at the CK-A1* locus. An 'Atlantic' group (comprising populations in Atlantic flowing streams) extends to unglaciated areas of the southern British Isles and Britanny, and is characterized by high frequencies of the *115 allele. Mediterranean flowing streams are populated by a 'Mediterranean' group extending eastward through the Caspian basin and characterized by high frequencies of the *100 allele.