Genomic evidence is increasingly underpinning that hybridization between taxa is commonplace, challenging our views on the mechanisms that maintain their boundaries. Here, we focus on seven catadromous eel species (genus Anguilla) and use genome-wide sequence data from more than 450 individuals sampled across the tropical Indo-Pacific, morphological information, and three newly assembled draft genomes to compare contemporary patterns of hybridization with signatures of past introgression across a time-calibrated phylogeny. We show that the seven species have remained distinct for up to 10 million years and find that the current frequencies of hybridization across species pairs contrast with genomic signatures of past introgression. Based on near-complete asymmetry in the directionality of hybridization and decreasing frequencies of later-generation hybrids, we suggest cytonuclear incompatibilities, hybrid breakdown, and purifying selection as mechanisms that can support species cohesion even when hybridization has been pervasive throughout the evolutionary history of clades.
Conflict of interest statement
The authors declare no competing interests.
Molecular phylogeny and evolution of the freshwater eel, genus Anguilla.Mol Phylogenet Evol. 2001 Sep;20(3):450-9. doi: 10.1006/mpev.2001.0959. Mol Phylogenet Evol. 2001. PMID: 11527470
Molecular phylogeny and evolution of the freshwater eels genus Anguilla based on the whole mitochondrial genome sequences.Mol Phylogenet Evol. 2005 Jan;34(1):134-46. doi: 10.1016/j.ympev.2004.09.003. Mol Phylogenet Evol. 2005. PMID: 15579387
Surgeons and suture zones: Hybridization among four surgeonfish species in the Indo-Pacific with variable evolutionary outcomes.Mol Phylogenet Evol. 2016 Aug;101:203-215. doi: 10.1016/j.ympev.2016.04.036. Epub 2016 Apr 30. Mol Phylogenet Evol. 2016. PMID: 27143241
Do we protect freshwater eels or do we drive them to extinction?Springerplus. 2014 Sep 16;3:534. doi: 10.1186/2193-1801-3-534. eCollection 2014. Springerplus. 2014. PMID: 25279325 Free PMC article. Review.
The on-again, off-again relationship between mitochondrial genomes and species boundaries.Mol Ecol. 2017 Apr;26(8):2212-2236. doi: 10.1111/mec.13959. Epub 2017 Jan 27. Mol Ecol. 2017. PMID: 27997046 Free PMC article. Review.