EVIDENCE FOR MULTIPLE ORIGINS AND SYMPATRIC DIVERGENCE OF TROPHIC ECOTYPES OF SMELT (OSMERUS) IN NORTHEASTERN NORTH AMERICA.

Abstract

The rainbow smelt, Osmerus mordax (Mitchill), is an osmerid fish that exhibits extensive life-history diversity throughout watersheds of northeastern North America. There are both ∗∗∗sea-run (anadromous) and lake-resident (lacustrine) populations and the latter have diversified further into "dwarf-" and "normal-sized" life-history types. Anadromous and lacustrine smelt may inhabit the same watershed and there are several instances where dwarf and normal populations reside within the same lake. We assayed variation among smelt for morphological traits linked to feeding performance in fishes to see if trophic ecology might promote life-history diversity in Osmerus. We also examined mitochondrial DNA (mtDNA) restriction site variation among forms to assess their evolutionary interrelationships. Dwarf smelt had significantly more gill rakers, larger eyes, but shorter upper jaws than normal lake and anadromous smelt. The populations clustered into two trophic "morphotypes"; an anadromous/normal lake group of populations and a group consisting only of dwarf smelt. The mtDNAs of 444 smelt from 16 populations were digested with 12 restriction enzymes revealing 93 composite mtDNA genotypes that clustered (UPGMA) into two major phylogenetic groups differing by 0.78% in sequence. Both genetic groups were present in dwarf and normal smelt as well as in anadromous fish. Further, geographic proximity, rather than trophic morphotype, appeared to be the major determinant of genetic affinities among populations. In two lakes, however, dwarf and normal smelt populations had significantly different mtDNA genotype frequency distributions indicating that the forms are reproductively isolated within both lakes. A clustering analysis of population affinities suggested that the divergence of sympatric dwarf and normal populations had occurred independently in the two lakes. We concluded (1) that trophic ecology is an important factor promoting differentiation in smelt life histories; (2) that smelt ecotypes are polyphyletic and there have been multiple, independent divergences of Osmerus life-history types throughout northeastern North America; and (3) that the biological and mtDNA differences between coexisting dwarf and normal lake smelt argue strongly that their genetic isolation may have developed sympatrically.