Abstract
Sea turtles are the only extant chelonian representatives that inhabit the marine environment. One key to successful colonization of this habitat is the adaptation to different energetic demands. Such energetic requirement is intrinsically related to the mitochondrial ability to generate energy through oxidative phosphorylation (OXPHOS) process. Here, we estimated Testudines phylogenetic relationships from 90 complete chelonian mitochondrial genomes and tested the adaptive evolution of 13 mitochondrial protein-coding genes of sea turtles to determine how natural selection shaped mitochondrial genes of the Chelonioidea clade. Complete mitogenomes showed strong support and resolution, differing at the position of the Chelonioidea clade in comparison to the turtle phylogeny based on nuclear genomic data. Codon models retrieved a relatively increased dN/dS (ω) on three OXPHOS genes for sea turtle lineages. Also, we found evidence of positive selection on at least three codon positions, encoded by NADH dehydrogenase genes (ND4 and ND5). The accelerated evolutionary rates found for sea turtles on COX2, ND1 and CYTB and the molecular footprints of positive selection found on ND4 and ND5 genes may be related to mitochondrial molecular adaptation to stress likely resulted from a more active lifestyle in sea turtles. Our study provides insight into the adaptive evolution of the mtDNA genome in sea turtles and its implications for the molecular mechanism of oxidative phosphorylation.
Highlights
Sea turtles are the only extant chelonian representatives that inhabit the marine environment
Because maximum likelihood (ML) and Bayesian approaches for both datasets yielded highly similar topologies, subsequent analyses were conducted using the Bayesian tree of dataset II (Fig. 1)
We found evidence for positive selection at the coding level for several sites in ND4 and ND5 genes for different sea turtles species, highlighting a site within a functional domain of the ND4 gene with selection signal shared by all species of the Chelonioidea clade
Summary
Sea turtles are the only extant chelonian representatives that inhabit the marine environment. Research on the molecular evolution of mtDNA related to high altitude has received special attention[6,13,14,15,16], mainly because evaluating selective pressures of environmental temperature and oxygen availability on mtDNA molecular changes could provide key insights on mitogenome adaptive evolution[14]. Sea turtles comprise seven extant species grouped into two sister families, Dermochelyidae (one species) and Cheloniidae (six species)[22] These species present adaptations to marine environmental challenges, such as flippers, high salt excretion by modified lachrymal g lands[23], hydrodynamic. It is reasonable to hypothesize that mitochondrial gene adaptations may have had an important role in the adaptive success of sea turtles lineages in the marine environment, leaving molecular footprints in their mitogenomes
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