Introgressive hybridization and latitudinal admixture clines in North Atlantic eels
- Equal contributors
1 Lehrstuhl für Zoologie und Evolutionsbiologie, Universität Konstanz, 78457 Konstanz, Germany
2 Institute of Integrative Biology, ETH Zürich, Universitätsstrasse 16, 8092, Zürich, Switzerland
3 Department of Ecosystem and Public Health, University of Calgary - Faculty of Veterinary Medicine, 3280 Hospital Drive NW, Calgary, Alberta, Canada
4 Laboratoire de Biologie intégrative des populations, Ecole Pratique des Hautes Etudes, 75005 Paris, France
5 Muséum National d’Histoire Naturelle, ISYEB, UMR-CNRS 7205, 75005 16 rue Buffon, 75005 Paris, France
BMC Evolutionary Biology 2014, 14:61 doi:10.1186/1471-2148-14-61Published: 28 March 2014
Hybridization, the interbreeding of diagnosably divergent species, is a major focus in evolutionary studies. Eels, both from North America and Europe migrate through the Atlantic to mate in a vast, overlapping area in the Sargasso Sea. Due to the lack of direct observation, it is unknown how these species remain reproductively isolated. The detection of inter-species hybrids in Iceland suggests on-going gene flow, but few studies to date have addressed the influence of introgression on genetic differentiation in North Atlantic eels.
Here, we show that while mitochondrial lineages remain completely distinct on both sides of the Atlantic, limited hybridization is detectable with nuclear DNA markers. The nuclear hybridization signal peaks in the northern areas and decreases towards the southern range limits on both continents according to Bayesian assignment analyses. By simulating increasing proportions of both F1 hybrids and admixed individuals from the southern to the northern-most locations, we were able to generate highly significant isolation-by-distance patterns in both cases, reminiscent of previously published data for the European eel. Finally, fitting an isolation-with-migration model to our data supports the hypothesis of recent asymmetric introgression and refutes the alternative hypothesis of ancient polymorphism.
Fluctuating degrees of introgressive hybridization between Atlantic eel species are sufficient to explain temporally varying correlations of geographic and genetic distances reported for populations of the European eel.