Time since divergence between the blue whiting sister species Micromesistius poutassou and M. australis

Abstract

The Micromesistius genus consists of two commercially important species, M. poutassou found in the Northern Hemisphere and M. australis in the Southern Hemisphere. Genetic variation between and within the species was studied. A 579 base pair fragment of the mitochondrial COX Igene was analysed in 279 individuals of M. poutassou and 163 of M. australis. M. poutassou was sampled throughout most of its range, while M. australis was sampled from the New Zealand population. Reciprocal monophyly was achieved and there were four fixed differences between the species. The taxonomic status could not be confirmed or rejected by the genealogical species concept. The time since divergence was estimated to approximately 2 million years (My) with an externally derived clock rate by Bayesian methods and with the use of genetic distances.Congruent estimates were obtained with an additional data set consisting of cyt b sequences from GenBank with a different clock rate. The divergence time of 2 My was in disagreement with previous hypotheses regarding the historical biogeography based on fossil findings, which suggested the divergence to be much older. This would imply a very slow rate of evolution in Micromesistius. An alternative hypothesis that appeared more likely was that Micromesistius has penetrated into the southern hemisphere at least two times. Paleoceanographic and paleoclimaticrecords indicated that conditions that would increase the likelihood for transequatorialdispersals were evident ˜2–1.6 million years ago. The COX I lineages of both species werefound to coalesce in the Pleistocene. Glacial cycles appears to have been an important factorin shaping the genealogies, based on evidence from neutrality tests for population growth andfrom signatures in the genetic diversity. The findings of private haplotypes and clear haplotypefrequency differences were, along with pairwise Fst values, indicating that M. poutassou from the Mediterranean Sea are genetically isolated from M. poutassou in the Barents Sea and west of the British Isles. This is in agreement with previous studies using other genetic markers.