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Two reproductively isolated demes of brown trout coexist in a small Swedish mountain lake, Lake Bunnersjörna. We electrophoretically examined 102 specimens from that lake for 27 enzymes encoded by 54 loci. The two demes are fixed for different alleles at a lactate dehydrogenase locus (LDH-1); statistically significant allele frequency differences at five other loci further support the complete lack of gene flow between these demes. There are significant differences in growth rates between fish in the two demes, but no further morphological differentiation h-s been detected.--In light of these findings, the genetic distance between these populations is surprisingly small (Nei's I = 0.975). These demes represent one of the least genetically divergent, reproductively isolated sympatric pair of vertebrate populations that have been identified. The results are discussed from both an evolutionary and ecological perspective.

A previously described isozyme polymorphism at one of two skeletal muscle LdhA loci in brown trout is due to a null allele, Ldh1(n), producing no detectable catalytic activity. Homozygotes for this allele have approximately only 56% of the LDH activity in skeletal muscle relative to homozygotes for the active allele. The remaining activity results from enzyme subunits produced by other LDH loci. The Ldh1(n) allele is common and widespread throughout brown trout populations in Sweden and is also found in populations from Ireland. The persistence of duplicate gene expression for the LdhA loci in almost all salmonid species is best explained by natural selection against individuals containing null alleles. However, there is no indication of natural selection against brown trout with the Ldh1(n) allele: We suggest that the selection against individuals containing null alleles that is apparently responsible for the persistence of duplicate LdhA loci in salmonids occurs only under certain environmental conditions.