Abstract
The relative role of genetic adaptation and phenotypic plasticity is of fundamental importance in evolutionary ecology [M. J. West-Eberhard, Proc. Natl. Acad. Sci. U.S.A. 102 (suppl. 1), 6543-6549 (2005)]. European eels have a complex life cycle, including transitions between life stages across ecological conditions in the Sargasso Sea, where spawning occurs, and those in brackish and freshwater bodies from northern Europe to northern Africa. Whether continental eel populations consist of locally adapted and genetically distinct populations or comprise a single panmictic population has received conflicting support. Here we use whole-genome sequencing and show that European eels belong to one panmictic population. A complete lack of geographical genetic differentiation is demonstrated. We postulate that this is possible because the most critical life stages-spawning and embryonic development-take place under near-identical conditions in the Sargasso Sea. We further show that within-generation selection, which has recently been proposed as a mechanism for genetic adaptation in eels, can only marginally change allele frequencies between cohorts of eels from different geographic regions. Our results strongly indicate plasticity as the predominant mechanism for how eels respond to diverse environmental conditions during postlarval stages, ultimately solving a long-standing question for a classically enigmatic species.
Highlights
How species adapt to the diverse environmental conditions that they experience through their life is fundamental to understanding evolutionary processes
Evaluating the hypothesis that the European eel consists of a single panmictic population is central to understanding eel ecology and evolution, and how eel populations may be affected by global change and other environmental threats [9, 10]
Our results, based on whole-genome sequencing, provide ultimate and conclusive evidence that it should be considered as one single panmictic population
Summary
Species that occur across extreme environmental gradients must respond to a diverse range of conditions This can be accomplished by individual-level phenotypic plasticity, meaning that individuals adjust their physiology to the prevailing environmental conditions [1], and by local genetic adaptation that may lead to reproductively isolated subpopulations. Which geographic region across Europe and North Africa eels inhabit as adults appears to be driven largely by a stochastic process These observations are consistent with a single panmictic population and would preclude genetic adaptation to local conditions in Europe and North Africa. A lack of genetic differentiation even at many loci does not exclude the possibility that the European eel is divided into partially reproductively isolated subpopulations, genetically adapted to the diverse ecological conditions that individuals are exposed to during postlarval stages. We report a complete lack of genomic regions with significant differentiation between geographically separated samples
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