Aim The objective of this study was to reveal the present population structure and infer the gene-flow history of the Indo-Pacific tropical eel Anguilla bicolor.
Location The Indo-Pacific region.
Methods The entire mitochondrial control region sequence and the genotypes at six microsatellite loci were analysed for 234 specimens collected from eight representative localities where two subspecies have been historically designated. In order to infer the population structure, genetic differentiation estimates, analysis of molecular variance and gene-tree reconstruction were performed. The history of migration events and population growth was assessed using neutrality tests based on allelic frequency spectrum, coalescent-based estimation of gene flow and Bayesian demographic analysis using control region sequences.
Results Population structure analysis showed genetic divergence between eels from the Indian and Pacific oceans (FST = 0.0174–0.0251, P < 0.05 for microsatellites; ΦST = 0.706, P < 0.001 for control region), while no significant variation was observed within each ocean. Two mitochondrial sublineages that do not coincide with geographical regions were found in the Indian Ocean clade of a gene tree. However, these two sublineages were not differentiated at the microsatellite markers. The estimation of mitochondrial gene-flow history suggested allopatric isolation between the Indian and Pacific oceans, and a possible secondary contact within the Indian Ocean after an initial population splitting. Bayesian demographic history reconstruction and neutrality tests indicated population growth in each ocean after the Indo-Pacific divergence.
Main conclusions Anguilla bicolor has diverged between the Indian and Pacific oceans, which is consistent with the classical subspecies designation, but is apparently genetically homogeneous in the Indian Ocean. The analysis of gene-flow and demographic history indicated that the two mitochondrial sublineages observed in the Indian Ocean probably represent the haplotype groups of relict ancestral populations. A comparison with a sympatric congener suggested that absolute physical barriers to gene flow may not be necessary for population divergence in eels.