Gene-flow in a mosaic hybrid zone: is local introgression adaptive?
Genome-wide scans of genetic differentiation between hybridizing taxa can identify genome regions with unusual rates of introgression. Regions of high differentiation might represent barriers to gene flow, while regions of low differentiation might indicate adaptive introgression-the spread of selectively beneficial alleles between reproductively isolated genetic backgrounds. Here we conduct a scan for unusual patterns of differentiation in a mosaic hybrid zone between two mussel species, Mytilus edulis and M. galloprovincialis. One outlying locus, mac-1, showed a characteristic footprint of local introgression, with abnormally high frequency of edulis-derived alleles in a patch of M. galloprovincialis enclosed within the mosaic zone, but low frequencies outside of the zone. Further analysis of DNA sequences showed that almost all of the edulis allelic diversity had introgressed into the M. galloprovincialis background in this patch. We then used a variety of approaches to test the hypothesis that there had been adaptive introgression at mac-1. Simulations and model fitting with maximum-likelihood and approximate Bayesian computation approaches suggested that adaptive introgression could generate a "soft sweep," which was qualitatively consistent with our data. Although the migration rate required was high, it was compatible with the functioning of an effective barrier to gene flow as revealed by demographic inferences. As such, adaptive introgression could explain both the reduced intraspecific differentiation around mac-1 and the high diversity of introgressed alleles, although a localized change in barrier strength may also be invoked. Together, our results emphasize the need to account for the complex history of secondary contacts in interpreting outlier loci.
Université Montpellier 2, 34095 Montpellier Cedex 5, France CNRS, Institut des Sciences de l'Evolution, ISEM Unité Mixte de Recherche 5554, 34200 SETE, France Department of Genetics, University of Cambridge, CB2 3EH Cambridge, United Kingdom christelle.fraisse@univ-montp2.fr.
Christelle
C
Fraïsse
Université Montpellier 2, 34095 Montpellier Cedex 5, France CNRS, Institut des Sciences de l'Evolution, ISEM Unité Mixte de Recherche 5554, 34200 SETE, France Department of Ecology and Evolution, Lausanne University, Biophore/Sorge, CH-1015.
Camille
C
Roux
Department of Genetics, University of Cambridge, CB2 3EH Cambridge, United Kingdom.
John J
JJ
Welch
Université Montpellier 2, 34095 Montpellier Cedex 5, France CNRS, Institut des Sciences de l'Evolution, ISEM Unité Mixte de Recherche 5554, 34200 SETE, France.
Nicolas
N
Bierne
D000222:Adaptation, Physiological / Q000235:genetics*; D000465:Algorithms; D000483:Alleles; D000818:Animals; D001483:Base Sequence; D002875:Chromosomes / Q000235:genetics; D005060:Europe; D051456:Gene Flow; D056426:Genetic Loci; D016678:Genome; D005843:Geography; D006824:Hybridization, Genetic; D007178:Inbreeding; D016013:Likelihood Functions; D008957:Models, Genetic; D009030:Mosaicism; D049878:Mytilus / Q000235:genetics*; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D011110:Polymorphism, Genetic; D013045:Species Specificity
Mytilus; approximate Bayesian computation; maximum likelihood; outlier loci; soft sweep