LAUSR

Pleistocene glacial cycles associated with geological and climatic changes have been suggested to affect the distribution and abundance of marine organisms in the north-western Pacific. In addition to historical processes, several contemporary forces are associated with spatial distributions and genetic structuring of marine species. Herein, we gathered partial mitochondrial control region sequences of chub mackerel Scomber japonicus from 14 localities over most of its geographical range in the north-western Pacific to infer the effect of Pleistocene climatic fluctuations on its historical demography, and to assess the role of historical process and contemporary factors in shaping present-day patterns of genetic differentiation within this species. Phylogeographic patterns revealed two distinct lineages that originated in vicariance during the Middle Pleistocene. However, there was no evidence of phylogeographic partitioning of haplotypes over the sampled range. Population structure analyses indicated a high degree of genetic homogeneity among chub mackerel populations. Demographic analyses indicated both lineages experienced Late Pleistocene population expansion. The observed genetic pattern of chub mackerel is consistent with a scenario of the survival of this species in at least two allopatric glacial refugia during the glacial maxima of the Pleistocene followed by massive dispersals throughout the north-western Pacific and species-specific ecological processes facilitating contemporary gene flow.