Data from: Intraspecific genetic admixture and the morphological diversification of an estuarine fish population complex

The North-east American Rainbow smelt (Osmerus mordax) is composed of two
glacial races first identified through the spatial distribution of two
distinct mtDNA lineages. Contemporary breeding populations of smelt in the
St. Lawrence estuary comprise contrasting mixtures of both lineages,
suggesting that the two races came into secondary contact in this estuary.
The overall objective of this study was to assess the role of
intraspecific genetic admixture in the morphological diversification of
the estuarine rainbow smelt population complex. The morphology of
mixed-ancestry populations varied as a function of the relative
contribution of the two races to estuarine populations, supporting the
hypothesis of genetic admixture. Populations comprising both ancestral
mtDNA races did not exhibit intermediate morphologies relative to pure
populations but rather exhibited many traits that exceeded the parental
trait values, consistent with the hypothesis of transgressive segregation.
Evidence for genetic admixture at the level of the nuclear gene pool,
however, provided only partial support for this hypothesis. Variation at
nuclear AFLP markers revealed clear evidence of the two corresponding
mtDNA glacial races. The admixture of the two races at the nuclear level
is only pronounced in mixed-ancestry populations dominated by one of the
mtDNA lineages, the same populations showing the greatest degree of
morphological diversification and population structure. In contrast,
mixed-ancestry populations dominated by the alternate mtDNA lineage showed
little evidence of introgression of the nuclear genome, little
morphological diversification and little contemporary population genetic
structure. These results only partially support the hypothesis of
transgressive segregation and may be the result of the differential
effects of natural selection acting on admixed genomes from different
sources.