The complete mitochondrial genome of the imperiled Bullnose ray Myliobatis freminvillei (Myliobatiformes: Myliobatidae) with comments on its phylogenetic position and claims of diversifying selection affecting protein coding genes in a closely related species.

The Bullnose ray Myliobatis freminvillei is a bentho-pelagic eagle ray that inhabits the north Gulf of Mexico and the Western Atlantic Ocean Coast, discontinuously, from Massachusetts, USA to Buenos Aires, Argentina. Myliobatis freminvillei is currently listed as vulnerable by the 2019 IUCN Red List of Threatened Species given that it is often captured as bycatch by artisanal and commercial fisheries, along the coasts of Argentina, Brazil, and Venezuela. This study, for the first time, assembled and characterized the mitochondrial genome of M. fremnvillei. The A+T rich mitochondrial genome of M. fremnvillei is 18,356 bp long and encodes 22 transfer RNA genes (tRNA), 2 ribosomal RNA genes (12S ribosomal RNA and 16S ribosomal RNA), 13 protein coding genes (PCGs), and also contains a non coding control region 2,617 bp long. Nonsynonomous codon usage with a preference for A+T rich codons was observed in all 13 PCGs. Leu (CTA), Ile (ATC), Phe (TTC), Thr (ACA), and Ala (GCC) were the most frequently used codons. Ka/Ks ratios estimated for all 13 PCGs exhibited values < 1, indicating strong purifying selection affecting all these genes. In contrast to the results of a previous study that claimed diversifying selective pressure in two mitochondrial PCGs of Mobula tarapacana, reanalysis of the Ka/Ks values for the same species indicated purifying selection in all 13 PCGs. Of the 22 tRNA genes, all have a cloverleaf secondary structure except tRNA-Ser1 which has a truncated dihydrouridine arm. In the control region, A+T rich microsatellites (n = 42) and short tandem repeats (n = 6) were identified, and the secondary structure of the same region contained numerous hairpin loops. Phylomitogenomic analyses supported the monophyletic status of the order Myliobatiformes and family Myliobatidae. The assembled mitochondrial genome will assist with conservation efforts in Myliobatis fremnvillei.