Abstract
Stimulated by retinoic acid 6 (STRA6) is the receptor for retinol binding protein and is relevant for the transport of retinol to specific sites such as the eye. The adaptive evolution mechanism that vertebrates have occupied nearly every habitat available on earth and adopted various lifestyles associated with different light conditions and visual challenges, as well as their role in development and adaptation is thus far unknown. In this work, we have investigated different aspects of vertebrate STRA6 evolution and used molecular evolutionary analyses to detect evidence of vertebrate adaptation to the lightless habitat. Free-ratio model revealed significant rate shifts immediately after the species divergence. The amino acid sites detected to be under positive selection are within the extracellular loops of STRA6 protein. Branch-site model A test revealed that STRA6 has undergone positive selection in the different phyla of mammalian except for the branch of rodent. The results suggest that interactions between different light environments and host may be driving adaptive change in STRA6 by competition between species. In support of this, we found that altered functional constraints may take place at some amino acid residues after speciation. We suggest that STRA6 has undergone adaptive evolution in different branch of vertebrate relation to habitat environment.
Highlights
During the evolution of animal kingdom, sunlight has been the most potent selective force to control the evolution of living organisms [1].The evolution of photo-detection, giving rise to eyes, offers a kaleido-scopic view of selection acting at both the organ and molecular levels [2]
Alignment of amino acids for Stimulated by retinoic acid 6 (STRA6) protein Alignment of the STRA6 protein sequence across 89 vertebrate species showed that a 38 residues long segment was present in ferret and 11 or 12 amino acids in other species. This range is seated at residues 74 and 87 of the human STRA6 protein, which is located in first intracellular loops of STRA6 protein
Our results showed that the Primates, Chiroptera, Carnivora, Rodent, Cetartiodactyla and Afrotheria branch had significantly higher v value compared with other branches, suggesting the potential action of positive selection during early stages of the evolution of six mammalian phyla
Summary
During the evolution of animal kingdom, sunlight has been the most potent selective force to control the evolution of living organisms [1].The evolution of photo-detection, giving rise to eyes, offers a kaleido-scopic view of selection acting at both the organ and molecular levels [2]. The gradual evolution of novel molecular/developmental mechanisms in eye and vision allow animals to live in different light environments (aquatic/terrestrial/ subterranean/nocturnal) [3,4,5]. Vertebrate vision depends on light-dependent isomerization of a chromophore (11-cis-retinal) bound to the visual pigment opsin triggering the phototransduction cascade, and resulting in neural signals being sent to the brain. While the light-dependent reaction occurs in the photoreceptor cells, the enzymatic trans-to-cis reisomerization occurs in the cells of the retinal pigment epithelium (RPE), a monolayer epithelium adjacent to and partly enclosing the photoreceptor cells. All-trans-retinol bound to RBP (holo-RBP) is delivered to the RPE through choriocapillaris blood. The holo-RBP interacts with the basal side of RPE cells via STRA6 protein [7]. STRA6 is a cell-surface receptor for retinol-RBP that removes retinol from RBP and transports it across the plasma membrane [8]
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