Characterization of the complete mitochondrial genome of Macrotocinclus affinis (Siluriformes; Loricariidae) and phylogenetic studies of Siluriformes.

The vertebrate mitochondrial genome is typically circular molecules made up of 14,000 to 16,000 bp, including 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), two ribosomal RNA genes (12 s rRNA and 16 s rRNA) and a control region. Compared with nuclear DNA, mitochondrial DNA has a higher mutation rate, so it is one of the most effective and reliable molecular markers in fish phylogeny. Macrotocinclus affinis was the only species in Macrotocinclus (it was classified as Otocinclus in the past) and currently lacks genetic information. Most of the current researches are based on the mitochondrial Cytb gene and RAG1 and RAG2 nuclear genes to study the phylogenetic analysis of Siluriformes. So, the study provides the characteristic features of the Macrotocinclus affinis mitochondrial genome and this is the first time that the phylogenetic relationship of Siluriformes has been reconstructed based on COI. We aimed to sequence the entire mitochondrial genome of Macrotocinclus affinis using conventional PCR techniques and to clarify its phylogenetic status in Siluriformes by using the COI sequence of mitochondria. In this study, we sequenced the whole mitochondrial genome of this species yielding a 16,632 bp circular assembly composed of the typical vertebrate mitochondrial features. It contains 13 protein-coding genes, two rRNA genes, 22 tRNA genes, a putative control region, and one origin of replication on the light-strand. The overall base composition includes A (30.07%), T (24.43%), C (29.43%) and G (16.01%). The genome composition is A + T biased (54.5%), and exhibits AT-skew (0.1036) and GC-skew (-0.2962). Moreover, the 13 PCGs encode 3850 amino acids in total. The result of the phylogenetic tree supports Macrotocinclus affinis has a closest relationship with Otocinclus cf. hoppei far. These results will help to understand the characteristics of the mitochondrial genome of Macrotocinclus affinis and provide molecular basis for the evolutionary relationship of Loricariidae.