Molecular Phylogeography Of Two Scomber Species In Northwestern Pacific

Scomber species are epipelagic and pelagic fishes with gregarious behavior.Scomber mackerel fisheries are very important in many regions throughout theirdistributions. Despite its economic importance for international fisheries, thetaxonomy and phylogenetic relationships of Scomber have long been controversial. Inthe present study, phylogenetic analysis of Scomber was conducted usingmorphological, mitochondrial and nuclear DNA sequence data to clarify the currenttaxonomic classification, and to assess phylogenetic relationships and theevolutionary history of this genus. Additionally, we estimated the population structureand phylogeographic pattern of Scomber japonicus and Scomber australasicus inNorthwestern Pacific using morphological, mitochondrial DNA sequence andmicrosatellite DNA markers to infer the relative role of historical vicariant events,environmental factors and life history traits in shaping and maintaining the populationstructure pattern of these two species.1. The taxonomy, and phylogeny and evolution of Scomber(1) The phylogenetic relationships of three Scomber species was inferred byusing morphological and otolith shape analysis. The shape of Scomber scombrusotolith is totally different from those of S. japonicus and S. australasicus. The resultsof multivariate analysis consistently showed S. japonicus is morphologically similarto S. australasicus, whereas large difference was observed between S. scombrus andthe other two Scomber species based on morphological characters and otolith shapevariable. (2) A molecular phylogenetic analysis of Scomber was conducted based onmitochondrial (COI, Cyt b and control region) and nuclear (5S rDNA) DNA sequencedata in multiple perspective. The present study produced a well-resolved phylogenythat strongly supported the monophyly of Scomber. We confirmed that S. japonicusand S. colias are genetically distinct. Although morphologically and ecologicallysimilar to S. colias, the molecular data showed that S. japonicus has a greatermolecular affinity with S. australasicus, which conflicts with the traditional taxonomy.This phylogenetic pattern was corroborated by the morphological data. Thepresent-day geographic range of each species was compared with the resultantphylogeny to evaluate possible hypotheses for its diversification and range expansion.In view of the unique geographic distributions of Scomber species, we hypothesizedthat speciation in the genus was primarily based on historical vicariance and adaption.2. Population genetic structure and molecular phylogeography of S. japonicus(1) Morphological characters and otolith shape variable were compared bymultivariate analysis for S. japonicus populations in Northwestern Pacific. Significantdifferences were found in many characters among eight S. japonicus populations.However, no geographical trend was observed. In addition, comparative analysis ofotolith shape variable from different length groups indicated that otolith shape of S.japonicus varied with growth rate.(2) Molecular phylogeography of S. japonicus in Northwestern Pacific wasinvestigated using mitochondrial control region sequences. Two significantly distinctlineages were detected, which might be isolated during glacial periods of Pleistocene.However, the relative frequency of individuals occupying the two major lineages didnot differ significantly among sample locations. It is supposed that the presentdistribution of haplotypes is indicative of secondary contact with subsequent highgene flow over the sampled range. Both mismatch distribution analyses and neutralitytests suggested a late Pleistocene population expansion for S. japonicus.(3) Genetic differentiation of S. japonicus in Northwestern Pacific was surveyedat eight microsatellite loci. Microsatellite analyses revealed relative high genetic diversity for S. japonicus due to high mutation rate of microsatellite DNA as well aslarge effective population size of this species. Significant population genetic structurewas detected among S. japonicus populations, which might be caused by thecomplicated interaction between marine currents and biological characteristics of S.japonicus.3. Population genetic structure and molecular phylogeography of S. australasicus(1) Morphological traits and truss network characters were measured toinvestigate morphological difference among S. australasicus populations.Multivariate analysis consistently found significant differences between China andJapan populations, which may be caused by marine currents in these two areas.(2) The first hypervariable region (HVR-1) of the mitochondrial DNA controlregion was analyzed for samples collected from five locations in Northwestern Pacific.NJ tree and median-joining network revealed no significant phylogeographic structure.No significant genetic structure was detected among populations of S. australasicusanalyzed. It is likely a consequence of extensive gene flow caused by passive larvaltransport and/or extremely high vagility in adult as well as population expansionduring late Pleistocene.(3) Weak but significant genetic structure was found in four S. australasicuspopulations in Japan’s Pacific coastal waters based on microsatellite analyses. Smallbut significant genetic differentiation between the middle of north Pacific populationand other populations might be attributed to the limited gene flow due to Kamchatka.In accordance with mtDNA result, microsatellite diversities were relatively high anddid not show any geographical trends.