Fig. 6.

Non–Ca²⁺-conducting Ca_V1.1 channels position higher teleost fishes as most derived group concerning the evolution of skeletal muscle EC coupling. In Ca²⁺ influx-dependent EC coupling (22, 23) of early prochordates, such as tunicates or amphioxus, RyR opening is induced via rapidly activating Ca²⁺ currents (Ca²⁺ current) (24), and Ca_V channels are arbitrarily arranged in clusters (Ca_V clusters) opposite RyR (25). Later, hagfish and lampreys developed direct Ca_V1.1-RyR1 coupling (24, 26, 27) with Ca_V1.1 arranged in groups of four (tetrads) (25). In tetrapods, the tetrads changed their disposition within the array, and a slowly activating Ca²⁺ current is still observed, despite that EC coupling is independent of the Ca²⁺ influx. The Ca²⁺ inward current completely vanished in teleost fishes, and EC coupling is solely induced via Ca_V1.1-RyR1 protein–protein interaction. This is true for both distinct Ca_V1.1α_1S isoforms (α_1S-a and α_1S-b) that are present in higher teleosts, concordant with the hypothesis of a whole-genome duplication (WGD) at the basis of the teleost linage (28). Selectivity filter glutamates are boxed in gray, marker amino acids indicating non-Ca²⁺ conductivity in black. Phylogenetic tree is drawn after the Tree of Life web project (21).