The complete mitogenome of two Australian lampreys: mordacia mordax and mordacia praecox

As one of only two surviving outgroups to all jawed vertebrates, lampreys (Petromyzontiformes) can provide important information in our understanding of early vertebrate evolution. However, with many phylogenetic aspects of lamprey evolution still uncertain, the ability to use contemporary lampreys in this role depends on robust phylogenetic hypotheses regarding the interrelationship of the three lamprey families, as well as the relationship between lampreys and hagfishes. To achieve this, complete mitogenome data of Southern Hemisphere lampreys is required. Another contentious issue in lamprey taxonomy is the status of paired species. Whilst many studies have focused on Northern Hemisphere species pairs, this study is the first to compare Mordacia mordax and Mordacia praecox, two lamprey species endemic to Australia and the only species pair in the Southern Hemisphere. The complete mitochondrial genome of Mordacia mordax and Mordacia praecox was determined twice independently, in a single shotgun sequencing run on an Ion Torrent PGM, and using a combination of Sanger sequencing of short range PCR products and Roche 454 GS Junior pyrosequencing of long range PCR products. Both of the mitogenomes contain the 37 typical vertebrate genes. Their gene order and contents are identical to those of previously described lamprey mitogenomes, with the exception of a novel tandem repeat array located between Cyt b and tRNA proline. The tandem repeat array, referred to as NCIII, contains pseudogenes of tRNA proline and phenylalanine, indicating that it has arisen by tandem duplication of the tRNA proline – phenylalanine region. Characterisation of NCIII revealed that the number of repeat copies was polymorphic between individuals of both species, and was a source of both intra-individual and inter-individual variation. Consistent with other studies of lamprey species pairs, the mitogenome of M. mordax and M. praecox are nearly identical. Phylogenetic analyses were carried out using the newly determined mitogenomes, together with five additional lamprey species and two hagfishes. Most tree topologies obtained strongly support the hypothesis that Petromyzontidae plus Geotriidae are a clade whose sister group is Mordaciidae. Additionally, lamprey divergence times were estimated by a temporally-calibrated phylogenetic analysis that included 20 vertebrate mitogenomes and was done using nine well-established fossil calibration points. The recovered topology strongly supported the hypothesis that lampreys separated from hagfishes about 409 MYA, and that lamprey divergence involved the early radiation of Mordaciidae (about 132 MYA), followed by the monophyletic divergence of Geotriidae plus Petromyzontidae about 85 MYA. Taken together, the results in this study provide robust hypotheses regarding the interrelationship of lamprey families and the relationship between hagfish and lampreys, whilst providing an estimation of their divergence times