A MOLECULAR ANALYSIS OF RELATIONSHIPS AND BIOGEOGRAPHY WITHIN A SPECIES COMPLEX OF HOLARCTIC FISH (GENUS OSMERUS)

Episodes of trans-Arctic faunal exchange and isolation between the nor th Pacific and Atlantic ocean basins have been implicated as important historic geological events contributing to extant patterns of genetic diversity and structure in Holarctic faunas. We made a further test o f the significance of such biogeographic events by examining mitochond rial DNA (mtDNA) restriction fragment length and cytochrome b sequence polymorphism among north Pacific and Arctic, north-western Atlantic ( north-eastern North American), and north-eastern Atlantic (European) r egional forms of the boreal smelt, genus Osmerus. Our analyses also as sessed whether the regional forms within this 'species complex': (i) r epresent a single widely distributed and polytypic species, or is comp osed of three geographically distinct species, and (ii) resulted from a single split from north Pacific ancestral Osmerus or two independent Pacific-Atlantic divergences. MtDNA sequence divergence estimates amo ng forms ranged from 5.6-8.9% and from 6.1-8.5% based on restriction f ragment and 300 base pairs of cytochrome b sequencing, respectively. D ivergence within forms averaged less than 0.5% for fragment analysis a nd no differences were detected from sequence analysis. Provisional da ting of lineage separations in Osmerus based on our sequence divergenc e estimates suggested a mid-Pliocene to early Pleistocene time frame f or diversification among the forms. These estimated lineage separation dates support the idea that geological events in 'Beringia' and the s urrounding trans-Arctic area (e.g. opening of the Bering Seaway, Pleis tocene glacial advances), occurring over a similar time frame, have in fluenced radiation in Osmerus. Phenetic and parsimony analyses of the sequence divergence estimates and of sequence polymorphisms suggested that the north Pacific/Arctic form and the northwestern Atlantic form shared a common ancestor more recently than either has with the north- eastern Atlantic form, thus supporting the hypothesis that the species complex has arisen from two independent Pacific-Atlantic divergences probably beginning during the mid-Pliocene.