Fin whale (Balaenoptera physalus) mitogenomics: A cautionary tale of defining sub-species from mitochondrial sequence monophyly

Abstract The advent of massive parallel sequencingtechnologies has resulted in an increase of studies based upon complete mitochondrial genome DNAsequences that revisit the taxonomic status within and among species. Spatially distinct monophylyin such mitogenomic genealogies, i.e., the sharing of a recent common ancestor among con-specific samples collected in the same region has been viewed as evidence for subspecies. Several recent studies in cetaceans have employed this criterion to suggest subsequent intraspecific taxonomic revisions. We reason that employing intra-specific, spatially distinct monophylyat non-recombining, clonally inherited genomes is an unsatisfactory criterion for defining subspeciesbased upon theoretical ( genetic drift) and practical (sampling effort) arguments. This point was illustrated by a re-analysis of a global mitogenomic assessment of fin whales, Balaenopteraphysalus spp., published by Archer et al. (2013), which proposed to further subdivide the Northern Hemisphere fin whale subspecies, B. p. physalus . The proposed revision was based upon the detection of spatially distinct monophylyamong North Atlantic and North Pacific fin whalesin a genealogy based upon complete mitochondrial genome DNAsequences. The extended analysis conducted in this study (1676 mitochondrial control region, 162 complete mitochondrial genome DNAsequences and 20 microsatellite loci genotyped in 380 samples) revealed that the apparent monophylyamong North Atlantic fin whalesreported by Archer et al. (2013) to be due to low sample sizes. In conclusion, defining sub-species from monophyly(i.e., the absence of para- or polyphyly) can lead to erroneous conclusions due to relatively “trivial” aspects, such as sampling. Basic population genetic processes (i.e., genetic driftand migration) also affect the time to the most recent common ancestorand hence the probability that individuals in a sample are monophyletic.