PARALLEL EVOLUTION OF LAKE-STREAM PAIRS OF THREESPINE STICKLEBACKS (GASTEROSTEUS) INFERRED FROM MITOCHONDRIAL-DNA VARIATION

Three drainage systems in British Columbia, Canada, contain divergent parapatric lake-stream pairs of threespine sticklebacks (Gasterosteus aculeatus): Drizzle and Mayer Lakes on Graham Island, Queen Charlotte Islands, and Misty Lake on northeastern Vancouver island. Ecological a nd morphological differences between members of all three lake-stream pairs are strikingly similar; lake fish are melanistic and slim bodied with smaller mouths and more gill rakers than the mottled-brown and r obust-bodied stream sticklebacks. We estimated the level of genetic di vergence between lake and stream fish in Misty Lake and tested hypothe ses of single versus multiple origins of the pairs by assaying mitocho ndrial DNA (mtDNA) restriction site variation in samples from the thre e lake systems. MtDNA analysis revealed the existence of two highly di vergent lineages differing by 2.7% in sequence. One lineage predominat ed in Misty stream fish (73%), whereas the ether lineage predominated in Misty Lake samples (96%). Comparable forms (lake or stream) in the different lakes did not cluster together in terms of mtDNA nucleotide divergence, suggesting that the pairs have had independent origins. We concluded that: (1) divergent mtDNA lineages in North Pacific stickle backs stem from historical isolation in the two major glacial refugia proposed for the North Pacific (Beringia and Cascadia); (2) the stream and lake pair in Misty Lake are distinct gene pools; (3) the divergen ce between parapatric lake and stream Gasterosteus represents parallel evolution having occurred at least twice in the North Pacific; and (4 ) different scales of evolutionary divergence exist in North Pacific G asterosteus, that is, a relatively ancient divergence of mtDNA clades as well as recent (i.e.: postglacial), divergence of ecotypes within m ajor clades.