Assessing patterns of hybridization between North Atlantic eels using diagnostic single-nucleotide polymorphisms

Abstract

The two North Atlantic eel species, the European eel (Anguilla anguilla) and the American eel (Anguilla rostrata), spawn in partial sympatry in the Sargasso Sea, providing ample opportunity to interbreed. In this study, we used a RAD (Restriction site Associated DNA) sequencing approach to identify species-specific diagnostic single-nucleotide polymorphisms (SNPs) and design a low-density array that combined with screening of a diagnostic mitochondrial DNA marker. Eels from Iceland (N ¼ 159) and from the neighboring Faroe Islands (N ¼ 29) were genotyped, along with 94 larvae (49 European and 45 American eel) collected in the Sargasso Sea. Our SNP survey showed that the majority of Icelandic eels are pure European eels but there is also an important contribution of individuals of admixed ancestry (10.7%). Although most of the hybrids were identified as F1 hybrids from European eel female  American eel male crosses, backcrosses were also detected, including a first-generation backcross (F1 hybrid  pure European eel) and three individuals identified as second-generation backcrosses originating from American eel  F1 hybrid backcrosses interbreeding with pure European eels. In comparison, no hybrids were observed in the Faroe Islands, the closest bodies of land to Iceland. It is possible that hybrids show an intermediate migratory behaviour between the two parental species that ultimately brings hybrid larvae to the shores of Iceland, situated roughly halfway between the Sargasso Sea and Europe. Only two hybrids were observed among Sargasso Sea larvae, both backcrosses, but no F1 hybrids, that points to temporal variation in the occurrence of hybridization.

Population Genomics of Marine Fishes: Next-Generation Prospects and Challenges

“…By comparison, hybrids represented less than 0.5% in mainland Europe and were all late-generation backcrosses. These data suggest low but biologically significant gene flow (Pujolar et al, 2014b), which could explain the limited genetic differentiation between European and American eel (F ST ϭ 0.09; Als et al, 2011). Other good examples of marine fish species in which genetic differentiation could be hampered by hybridization are plaice (Pleuronectes platesa) and flounder (Platichthys flesus) in Northern Europe (Kijewska et al, 2009), redfish species (Sebastes spp.)…”

“…For instance, patterns of hybridization between the two North Atlantic eels, European (Anguilla anguilla) and American (A. rostrata) eel were recently investigated (Pujolar et al, 2014b). RAD sequences of both species were aligned and a total of 3348 diagnostic SNPs (F ST 0.95) were identified.…”

Exploring the phylogeography of a hexaploid freshwater fish by RAD sequencing

“…This method is popular and has been used in many studies since its inception (Figure S1). RAD sequencing has been used, particularly in fish, to identify population divergence (Boehm, Waldman, Robinson, & Hickerson, 2015; Ferchaud & Hansen, 2016; Larson et al., 2014), for SNP identification in polyploid fish (Hohenlohe, Amish, Catchen, Allendorf, & Luikart, 2011; Ogden et al., 2013; Palti et al., 2014), in phylogeographic studies (Macher et al., 2015; Reitzel, Herrera, Layden, Martindale, & Shank, 2013), for QTL analysis (Gagnaire, Normandeau, Pavey, & Bernatchez, 2013; Houston et al., 2012; Yoshizawa et al., 2015), for linkage mapping (Brieuc, Waters, Seeb, & Naish, 2014; Henning, Lee, Franchini, & Meyer, 2014), in hybridization studies (Hand et al., 2015; Lamer et al., 2014; Pujolar et al., 2014), for exploration of genome architecture and evolution (Brawand et al., 2014; Kai et al., 2014; Waples, Seeb, & Seeb, 2016), and in phylogenetic analyses (Gonen, Bishop, & Houston, 2015; Wagner et al., 2013). This methodology should be particularly suited to phylogeographic studies as the inference power from large numbers of markers may identify patterns that are not easily visible in traditional analyses based on relatively few loci (Davey et al., 2011).…”

Assessing pre- and post-zygotic barriers between North Atlantic eels (Anguilla anguilla and A. rostrata)

“…However, gene flow still occurs, as hybrids have been reported in several studies (Avise et al, 1990;Albert et al, 2006;Gagnaire et al, 2009;Pujolar et al, 2014a, b). Thus, although temporal and spatial separation at the spawning grounds likely constitutes prezygotic barriers, they are not absolute.…”