Convergent selection pressures drive the evolution of rhodopsin kinetics at high altitudes via non-parallel mechanisms.

Convergent evolution in response to similar selective pressures is a well-known phenomenon in evolutionary biology. Less well understood is how selection drives convergence in protein function, and the underlying mechanisms by which this can be achieved. Here, we investigate functional convergence in the visual system of two distantly related lineages of high-altitude adapted Andean and Himalayan catfishes. Statistical analyses revealed in the two high-altitude lineages, a parallel acceleration of evolutionary rates in rhodopsin, the dim-light visual pigment. However, the elevated rates were found to be accompanied by substitutions at different sites in the protein. Experiments substituting Andean- or Himalayan-specific residues significantly accelerated the kinetic rates of rhodopsin, destabilizing the ligand-bound forms. As found in cold-adapted enzymes, this phenotype likely compensates for a cold-induced decrease in kinetic rates, properties of rhodopsin mediating rod sensitivity and visual performance. Our study suggests that molecular convergence in protein function can be driven by parallel shifts in evolutionary rates but via nonparallel molecular mechanisms. Signatures of natural selection may therefore be a powerful guide for identifying complex instances of functional convergence across a wider range of protein systems.