2.1 Samples
Lakes Bunnersjöarna are closely connected oligotrophic twin lakes (total area of 0.67 km2) located at an elevation of 955 m near the Norwegian border in the County of Jämtland, Sweden (Figure 1; Appendix S1, Figure S1). Both lakes are shallow, the southern lake only 0.5 m with a deeper middle (a few meters) and the northern lake c. 2 m deep. The brown trout in these lakes were sampled in 1975 as part of some of the first population genetic screenings of natural populations(Allendorf et al., 1976; Ryman et al., 1979). Material from that collection has been stored in a frozen tissue bank at the Department of Zoology, Stockholm University, Sweden. Here, we used 140 samples that were still available (out of 151 reported in Allendorf et al., 1976; Ryman et al., 1979); 68 and 72 from the northern and southern lake, respectively.
The allozyme studies showed contrasting homozygosity at one locus coding for lactate dehydrogenase (locus LDH-1 ). About half of the fish were homozygous for the 100 allele most common in brown trout in the study area, and the others homozygous for a rare null allele with no active enzyme product (Allendorf, Ståhl, & Ryman, 1984).
We classified the 140 fish into Deme I or II based on the LDH-1genotype, resulting in 68 individuals from Deme I (100/100 homozygous for LDH-1 ) and 72 individuals from Deme II (homozygous for the null allele). The 68 Deme I fish were from both lakes (northern:n =39, southern: n =29), as were the 72 Deme II fish (northern: n =23, southern: n =49).
We included data from eight additional brown trout populations that are part of other projects (Andersson et al., in prep; Kurland et al., in prep) to put the diversity and divergence patterns observed in Lakes Bunnersjöarna into perspective (Appendix S1, Figure S1).