Evolution of photoreceptors: insights from amphioxus

The topic of eye evolutionwas touchedas early as at the very beginningof the evolutionarytheoryitself.In his book'On theOrign of Species'CharlesDarwinexpresses concemsfor theexplanationof theocclurenceof complexeyesby naturalselection.Eventhe simplest'proto-eye'composedof a singlephotoreceptorcellanda shadingpigmentrequires properfrrnctioningof dozensof genesinvolvedin the phototansductioncascade,signal transmissionand pigmentation.Photoreceptorcells can be classifiedaccordingto their morphology as ciliary or rhabdomeric.Primary observationsrevealedthe rhabdomeric photoreceptorsbeing predominantlypresentin the eyes of invertebrates,whereasthe vertebrateandcnidarianseyesemploytheciliary type.Key componentsof thephotoreceptor cells arevisualpigments(opsins)andassociatedGa subunitsresponsiblefor absorptionof photonandthefirst stepof aphotoffansductioncascade.To properlyorchesftatetheassembly of thesestructuralgenesmediatingbiochemicalprocessesinto a functionalorgan,a tight regulationby a dedicatedsetof tanscriptionfactorsis required.Oneof themostremarkable andbeststudiedexamplesof geneswith anevolutionaryconservedrole in eyeorganogenesis is a paired-homeoboxgenePax6.However,availableliteraturepointsto othertranscription factors participatingin photoreceptordevelopmentacrossanimal phyla, for examplea..