Nevertheless, the structure–function mechanisms responsible for these differences have not been experimentally investigated in an evolutionary context in any cetacean pigment. 1998 Fasick and Robinson 2000 Newman and Robinson 2005). Compared with most terrestrial mammals (including their nearest living relatives, the hippopotamids) cetaceans have a reduced number of visual pigments (reviewed in Bowmaker 2008 and Jacobs 2013), and the ones they do possess show evidence of spectral shifts ( Fasick et al. Marine environments present a particular challenge to vision because light intensity is reduced, and the spectrum is narrowed and blue-shifted relative to terrestrial environments ( Warrant and Locket 2004). Because light detection is often essential for behaviours such as predator avoidance, mate selection, and foraging, its evolution has been directly linked to survivorship and reproductive fitness ( Nilsson 2013). Recent studies that exploit cetacean sequence data have correlated large-scale patterns of positive selection and gene loss with key aquatic adaptations including limb development, diving physiology, echolocation, and dim-light vision ( Gatesy et al. The molecular mechanisms underlying these major evolutionary changes are only just beginning to be understood, largely due to advances in cetacean whole-genome sequencing projects ( Lindblad-Toh et al. Our study demonstrates that combining computational and experimental methods is crucial for gaining insight into the selection pressures underlying molecular evolution.Ĭetacean vision, evolution of protein structure and function, codon substitution model, d N/ d S, clade model, adaptive evolution, opsins, visual ecology, absorption spectra, dim-light vision, site-directed mutagenesis IntroductionĬetaceans are the only mammals that have evolved a fully aquatic lifestyle and diversified into all major aquatic ecosystems, a remarkable transition that has been characterized by extreme morphological and physiological adaptations ( Uhen 2010 Gatesy et al. This suggests that foraging dives might be a selective regime influencing cetacean rhodopsin divergence, and our experimental results indicate that spectral tuning may be playing an adaptive role in this process. Only the model partitioning according to depth was significant.
#Bsw shift select series#
We then investigated patterns of ecological divergence that may be correlated with rhodopsin functional variation by using a series of clade models that partitioned the data set according to phylogeny, habitat, and foraging depth zone. Using codon-based likelihood models, we also found significant evidence for positive selection in cetacean rhodopsin sequences, including on spectral tuning sites we experimentally mutated. The S292A mutation had the largest effect, and was responsible for the majority of the spectral difference between killer whale and bovine (terrestrial) rhodopsin. To investigate the spectral effects of amino acid substitutions thought to correspond with absorbance shifts relative to terrestrial mammals, we used the orca gene as a background for the first site-directed mutagenesis experiments in a cetacean rhodopsin. In this study we use in vitro expression methods to experimentally characterize the first step of the visual transduction cascade, the light activation of rhodopsin, for the killer whale. Recent sequencing of cetacean genomes has made it possible to begin exploring the molecular basis of these adaptations. Cetaceans have undergone a remarkable evolutionary transition that was accompanied by many sensory adaptations, including modification of the visual system for underwater environments.
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