Abstract
European (Anguilla anguilla) and American eel (A.
rostrata) represent a remarkable case of interspecific hybridization.
They are both panmictic and spawn in partial sympatry in the Sargasso
Sea, occasionally producing viable, fertile hybrids, primarily found in
Iceland. We studied introgressive hybridization from American into
European eel based on whole-genome sequences of 78 individuals,
encompassing European, American and 21 putative hybrid eels. Previous
studies using few genetic markers could not resolve whether
hybridization involved simple unidirectional backcrossing or a more
complex hybrid swarm scenario. However, local ancestry inference along
individual chromosomes revealed Icelandic hybrids were primarily F1
hybrids or first-generation backcrosses toward European eel, with a few
showing more complex backcrossing histories. All European eels outside
Iceland contained short chromosomal blocks from American eel, indicating
a porous genome. We found no evidence for previously stated hypotheses
about geographical gradients of introgression in European eel outside
Iceland. Several chromosomal regions showed high divergence between the
species, but haplotype blocks introgressed from American eel were
identified both within and outside these regions. There was little
correspondence between regions of high relative and high absolute
divergence (dXY), and they presumably reflect selective
sweeps within species or regions of reduced recombination rather than
barrier loci. We identified a single genomic region with evidence of
introgression from American into European eel at multiple occasions,
under positive selection in both species. Thus, although the two species
maintain genetic integrity, their gene pools are not independent and
represent a common pool of standing variation for future adaptive
responses.