Natural selection and evolution of mitochondrial genome in terrestrial and aquatic turbellarians

Mitochondrial energy metabolism may be directly influenced by natural selection pressures in response to the energy demands of an organisms specific ecological niche. Here, we hypothesized that the mitochondrial genome of turbellarian animals was influenced by the metabolic requirements of various habitats We used selection pressure and phylogenetic independent contrasts (PIC) analyses to detect the selection pressure of protein-coding genes in the mitochondrial genome of turbellarian living in aquatic and terrestrial habitats. The results indicated turbellarians in aquatic habitats experienced greater selection pressure, resulting in the evolution of several genes at a lower {omega} value compared to their terrestrial counterparts. The NAD4 was identified as having positive selection sites in all site models analyzed. Among all genes, the equilibrium constant (ionization of COOH) was the most frequently detected amino acid characteristic with significant positive selection changes, followed by hydropathy and molecular volume. The equilibrium constant (ionization of COOH) was found to be the amino acid characteristic with the greatest magnitude of change in NAD4. This suggests that it plays a crucial role in the adaptive evolution of turbellarians to varying habitats. The study found a significant positive correlation (R = 0.61, p < 0.05) between the {omega} value of turbellarian and their habitats by PIC analysis. These findings shed light on the adaptive evolution of turbellarian mtDNAs and their influence on the oxidative phosphorylation molecular mechanism. 0. Significance statementThe adaptive evolution of mitochondrial genes is closely related to environmental temperature, high energy metabolism demand, altitude, and oxygen availability. Mitochondrial energy metabolism may be directly influenced by natural selection pressures in response to the energy demands of an organisms specific ecological niche. Here, we investigated the evolution of mtDNA PCGs in turbellarians across different habitats, and identified purifying selection as the main evolutionary pattern of the turbellarian mitochondrial genome. Additionally, we found that NAD4 played an important role in the adaptive evolution of turbellarians to different habitats.