Data from: RAD-sequencing highlights polygenic discrimination of habitat ecotypes in the panmictic American eel (Anguilla rostrata)

The two primary ways that species respond to heterogeneous environments is
through local adaptation and phenotypic plasticity. The American eel
(Anguilla rostrata) presents a paradox; despite inhabiting drastically
different environments, the species is panmictic. Spawning takes place
only in the southern Sargasso Sea in the Atlantic Ocean. Then, the
planktonic larvae (leptocephali) disperse to rearing locations from Cuba
to Greenland, and juveniles colonize either freshwater or
brackish/saltwater habitats, where they spend 3–25 years before returning
to the Sargasso Sea to spawn as a panmictic species. Depending on rearing
habitat, individuals exhibit drastically different ecotypes. In
particular, individuals rearing in freshwater tend to grow slowly and
mature older and are more likely to be female in comparison to individuals
that rear in brackish/saltwater. The hypothesis that phenotypic plasticity
alone can account for all of the differences was not supported by three
independent controlled experiments. Here, we present a genome-wide
association study that demonstrates a polygenic basis that discriminates
these habitat-specific ecotypes belonging to the same panmictic
population. We found that 331 co-varying loci out of 42,424 initially
considered were associated with the divergent ecotypes, allowing a
reclassification of 89.6%. These 331 SNPs are associated with 101 genes
that represent vascular and morphological development, calcium ion
regulation, growth and transcription factors, and olfactory receptors. Our
results are consistent with divergent natural selection of phenotypes
and/or genotype-dependent habitat choice by individuals that results in
these genetic differences between habitats, occurring every generation
anew in this panmictic species.