Co-evolution of components of phototransduction cascades. A, Components of the phototransduction cascades in ciliary and microvillar (rhabdomeric) photoreceptors. From Terakita et al (103). B, Sequence of evolution for the components of phototransduction cascades proposed by Plachetzki et al (27). Bottom left: The ancestral (non-photosensitive) G-protein cascade utilized a GPCR that coupled ultimately to a CNG (cyclic nucleotide-gated) channel. Third from left: The mechanism in the ancestral light-sensitive cell was similar. Top right: Early in the bilaterian lineage two variants of this scheme arose, both of which have been preserved in protostomes and deuterostomes. (Note that the obscured rear diagram for protostomes is the same as the front diagram for deuterostomes, and vice versa.) The ancestral mechanism was retained only slightly modified in the ciliary variety of photoreceptors, that expressed C-opsins. But the microvillar variety of photoreceptors, that expressed R-opsins, instead coupled via a Gq G-protein (and unknown intermediaries) to a TRP (transient receptor potential) channel. Additional abbreviations: I, intermediary molecules; Ic, ciliary intermediaries such as phosphodiesterase (PDE) and guanylyl cyclase (GC); Ir, rhabdomeric intermediaries such as phospholipase C (PLC), DAG (diacylglycerol) and PIP2 (phosphatidylinositol-4,5-biphosphate). From Plachetzki et al (27).