The mitochondrial trn-cox1 locus: Rapid evolution in Pompilidae and evidence of bias in cox1 initiation and termination codon usage

Background. The translation initiation codon of the mitochondrial (mt) cytochrome oxidase I gene, cox1, frequently differs from conventional AUN triplets, and the termination codon is often incomplete. Methods. To investigate whether initiation and termination codon usage may depend on the rate of mt genome evolution, a fast evolving locus comprising cox1 and adjacent tRNA-encoding trn genes was sequenced from representatives of the major clades of spider wasps, Pompilidae (Hymenoptera), and then included in insect-wide analyses of codon usage in the context of mtDNA sequence stability. Results. The rapid evolution of Pompilidae trn-cox1 locus was particularly evident in the rearrangement of the trnC and trnY genes, the presence of a trn pseudogene in Psorthaspis, and the relatively high frequency of cox1 nucleotide substitutions. Such extensive genetic diversification, underscored by variability in the arrangement of trn genes even at the subfamily level, contrasted with the conventional form of cox1 initiation and termination codons. Similar preference for codon usage was identified for other fast evolving insect lineages unrelated to Pompilidae. Instead, conventional codons were often absent in slower evolving loci. Conclusion. Analysis of codon usage across a spectrum of evolutionary rates indicates the existence of positive correlation between the incidence of cox1 canonical initiation and termination codons and relative rate of the trn-cox1 locus evolution.