The molecular basis of adaptive evolution of squirrelfish rhodopsins

Abstract

The wavelengths of maximal absorption (λmax) of the rhodopsins of nine squirrelfishes (N. sammara, N. argenteus, S. punctatissimum, S. microstoma, S. diadema, S. xantherythrum, S. spiniferum, N. aurolineatus, and S. tiere) and two soldierfishes (M. violacea and M. berndti) vary between 481 and 502 nm. Phylogenetic and mutagenesis analyses suggest that the common ancestor of these pigments had a λmax value of approximately 493 nm, and the contemporary λmax values were generated mostly by amino acid replacements E122M, F261Y, and A292S. The probability of observing all these amino acid replacements at specific branches of the phylogenetic tree is only 2.5 × 10 -9; it is highly unlikely that these changes have occurred by neutral evolution. Because of a close association between the λmax values of these pigments and the wavelengths of light available to the corresponding species, the excess number of amino acid changes at specific branches in the phylogenetic tree strongly suggests that the rhodopsins have undergone adaptive changes at various stages of the holocentrid evolution.