Turtle and tortoise mitogenomes under contrasting positive selection pressure

Testudinoidea represents an evolutionarily unique taxon comprising both turtles and tortoises. Turtles and tortoises are closely associated groups that inhabit contrasting habitats associated with unique physio-ecological challenges, which enable distinct adaptive evolutionary strategies. To compare the pattern and strength of Darwinian selection and physicochemical evolution in both turtle and tortoise mitogenomes, we used divergence and selection analyses. We analysed codon models and changes in structural and biochemical properties in mitochondrial protein coding genes (PCGs) of oxidative phosphorylation (OXPHOS) respiratory system. We detected strong evidence of positive selection in turtles whereas weak selection pressure were observed in tortoises. In turtles, highest gene and site-wise positive selection was detected in Platysternidae family. Among turtles, NAD2, NAD4 genes of OXPHOS Complex I, and COB gene of Complex III showed more positively selected sites, indicating convergent evolution. Across Testudinoidea, we observed pervasive selection in NAD3 and lineages specific selection in NAD2. This study revealed specific mitogenome sites responsible for physiochemical changes influencing positive selection in turtles and tortoises. Hence, our study provides insight into the selection pressure shaping distinct evolutionary pathways in two closely related reptiles which are often overlooked.

Automated summary

This study analyzed the mitochondrial genomes of turtles and tortoises, and found strong evidence of positive selection in turtles and weak selection pressure in tortoises. The highest positive selection was detected in the Platysternidae family of turtles. Several genes, including NAD2, NAD4, and COB, showed more positively selected sites in turtles, indicating convergent evolution. Across Testudinoidea, NAD3 exhibited pervasive selection and NAD2 showed lineage-specific selection. This study identified specific mitochondrial genome sites responsible for physicochemical changes influencing positive selection in turtles and tortoises.