MITOCHONDRIAL GENOME REVEALS CONTRASTING PATTERN OF ADAPTIVE SELECTION IN TURTLES AND TORTOISES

Testudinoidea represents an evolutionarily unique taxon comprising both turtles and tortoises. The contrasting habitats that turtles and tortoises inhabit are associated with unique physio-ecological challenges hence enable distinct adaptive evolutionary strategies. To comparatively understand the pattern and strength of Darwinian selection and physicochemical evolution in turtle and tortoise mitogenomes, we employed adaptive divergence and selection analyses. We evaluated changes in structural and biochemical properties, and codon models on the mitochondrial protein-coding genes (PCGs) among three turtles and a tortoise lineage. We used mitochondrial PCGs that constitute the crucial oxidative phosphorylation (OXPHOS) respiratory system, a critical metabolic regulator which assumes key significance in energy regulation of ectotherms. We detected strong evidence of positive selection along the turtle lineages: Geoemydidae, Emydidae, and Platysternidae, but relatively weak signals in tortoises. The Platysternidae turtles revealed the highest gene and site-wise positive selection. In turtles, positively selected sites were prevalent in NAD2 and NAD4 genes in OXPHOS Complex I, and COB gene of Complex III, indicating convergent adaptive evolution. Besides, NAD3 was the only subunit that showed adaptive selection in both turtles and tortoises, expressing its relevance for all Testudinoidea. Structural and functional analysis revealed many sites and physiochemical changes in important conserved as well as biomedically significant regions, suggesting the influence of adaptive pressure on mitogenome functions. Hence, our study furnished novel evidence of contrasting evolutionary selective pressure acting on closely related groups such as turtles and tortoises with unique habitat preferences and associated eco-physiological challenges.