Evolutionary patterns in squamate mitogenomes: Are selective regimes associated with fossoriality and limblessness?

The snakelike phenotype is characterized by limb reduction and body elongation, and independently evolved in several vertebrate lineages. This phenotype is often interpreted as adaptive to fossoriality or use of complex habitats. Limblessness and fossoriality might impose different energetic requirements for locomotion, affecting selective rates on mitochondrial genes. Previous studies identified signals of differential selection in mitochondrial genes of limbless lizards and fossorial rodents. However, it remains unclear which of these factors most intensely shapes mitochondrial genome evolution in Squamata. Amphisbaenia is a key group to answer this question, as it is one of the largest lineages of limbless and fossorial squamates. Here we report a new complete mitochondrial genome of Amphisbaena alba and address the relationships between limblessness and fossoriality in the evolution of mitochondrial genes in Squamata. The full length of the A. alba mitochondrial genome was 16,800 bp (13 protein-coding genes, 22 transfer RNAs, two ribosomal RNAs and the control region). We performed selective tests, allowing different rates for clades with limbless and fossorial species separately. Fossorial species have significant changes in selective rates in more mitochondrial genes than the limbless species, a result suggesting fossoriality as a prevalent factor shaping selective pressures on mitochondrial genes.