Extensive gene flow within sibling species in the deep-sea Beryx splendens

Molecular markers allow insights into the population biology and ecology of deep-sea organisms, which are usually hardly accessible to direct observation and poorly known. Such a study was undertaken here for the deep-sea fish Beryx splendens, a species of growing interest to fisheries. B. splendens populations were sampled on seamounts and continental margins in the southwestern Pacific (New Caledonia, New Zealand, southeastern Australia) and in the northeastern Atlantic. Two hundred and fifty individuals were characterised by their single-strand DNA conformation (SSCP) of a z 360-base-pair (bp) fragment of the mitochondrial cytochrome b gene, amplified by the polymerase chain reaction (PCR). Two major SSCP haplotypes were observed in New Caledonia, a and w, whose frequencies were negatively correlated along a north-tosouth cline. All SSCP haplotypes in the total sample were sequenced on 273 bp. The phylogenetic tree of B. splendens haplotype sequences, rooted by two B. decadactylus sequences, showed that a and w belong to distinct mitochondrial clades, A and W, which are separated by z 4–6 % nucleotide divergence. Thirty individuals from New Caledonia were characterised by their DNA fingerprint from arbitrary-primed PCR. The distribution of individual-pairwise similarity indices was strongly bimodal. The larger similarity values all corresponded to comparisons within a clade (A or W) while the lower values were all between clades. Therefore, there was a strict association between the mitochondrial type and the DNA (presumably, nuclear DNA) fingerprint of an individual. Altogether, these results point to the existence of two biological species (sp. A and sp. W) within the current taxon B. splendens. No within-species differentiation was detected at the regional scale (New Caledonia). A remarkable result is that the three cytochrome b haplotypes of northeastern Atlantic B. cf. splendens sp. A were also the three commonest in the southwestern Pacific populations of this species. Such a level of homogeneity in the distribution of haplotypes suggests there is, or recently has been, gene flow at the inter-oceanic scale. © 2000 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS Beryx splendens / geographic structure / systematics / cytochrome b / DNA fingerprints Résumé – Flux géniques élevés chez deux espèces jumelles identifiées chez le poisson de profondeur Beryx splendens. Les marqueurs génétiques moléculaires permettent d’aborder la biologie et l’écologie des populations d’organismes des grands fonds marins, peu accessibles à l’observation directe et en général mal connus. Une telle étude a été ici entreprise chez le poisson de profondeur Beryx splendens, espèce d’intérêt halieutique croissant, dont les populations ont été échantillonnées sur les * Correspondence and reprints: [email protected] 315 C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 323 (2000) 315–325 © 2000 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S0764446900001244/FLA