Mitogenomic characterization and phylogenetic position of the oldest living vertebrate species - the Greenland shark (Somniosus microcephalus)

Abstract

The Greenland shark (Squaliformes, Somniosus microcephalus) is the largest fish living in Arctic waters, but little is known about its biology. This species lives for at least 272 years and is listed as a near threatened species on the IUCN´s Red list of Threatened Species. As S. microcephalus is the oldest living vertebrate species, it is important to strive for its conservation. The aim of the study was to sequence and provide the first characterization of the S. microcephalus mitogenome, in order to accurately determine the phylogenetic position of this elusive species. Mitochondrial DNA (mtDNA) is a widely used tool for phylogenetic analysis, as it is not subjected to recombination (maternal inheritance) and is relatively easy to amplify. Using next generation sequencing, the size of the S. microcephalus mitogenome was estimated to 16,730 bp. The mitogenome was composed by 13 protein-coding genes, 2 rRNA genes, 22 tRNA genes, and a control region (D-loop). This composition resembles what have been observed for other vertebrate mitogenomes. In the comparative phylogenetic analysis based on the mitogenomes of 17 related shark species, S. microcephalus was positioned as a sister species of the Pacific sleeper shark (Somniosus pacificus). The single genes provided more incongruent topologies for phylogenetic reconstructions than when the mitogenome was used. Divergence time estimates confirmed that S. microcephalus and S. pacificus diverged 3.5 million years ago (Mya). Less than 1 % of nucleotide difference and a recent indication of gene flow between these close related species, suggested to be a single species. The results suggested a possible continuous distribution of the Somniosus subgenus (S. microcephalus, S. pacificus and S. antarcticus) across the globe. The availability of S. microcephalus mitogenome will contribute to aid further studies of phylogeography, population structure and conservation genetics in this species and sleeper sharks in general.