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Herein, we use genetic data from 277 sleeper sharks to perform
coalescent-based modeling to test the hypothesis of early Quaternary
emergence of the Greenland shark (Somniosus microcephalus) from ancestral
sleeper sharks in the Canadian Arctic-Subarctic region. Our results show
that morphologically cryptic somniosids S. microcephalus and Somniosus
pacificus can be genetically distinguished using combined mitochondrial
and nuclear DNA markers. Our data confirm the presence of genetically
admixed individuals in the Canadian Arctic and sub-Arctic, and temperate
Eastern Atlantic regions, suggesting introgressive hybridization upon
secondary contact following the initial species divergence. Conservative
substitution rates fitted to an Isolation with Migration (IM) model
indicate a likely species divergence time of 2.34 Ma, using the
mitochondrial sequence DNA, which in conjunction with the geographic
distribution of admixtures and Pacific signatures likely indicates
speciation associated with processes other than the closing of the Isthmus
of Panama. This time span coincides with further planetary cooling in the
early Quaternary period followed by the onset of oscillating
glacial-interglacial cycles. We propose that the initial S.
microcephalus–S. pacificus split, and subsequent hybridization events,
were likely associated with the onset of Pleistocene glacial oscillations,
whereby fluctuating sea levels constrained connectivity among Arctic
oceanic basins, Arctic marginal seas, and the North Atlantic Ocean. Our
data demonstrates support for the evolutionary consequences of oscillatory
vicariance via transient oceanic isolation with subsequent secondary
contact associated with fluctuating sea levels throughout the Quaternary
period—which may serve as a model for the origins of Arctic marine fauna
on a broad taxonomic scale.
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