Why is this topic chosen for today’s GEITP email? Well, “light” is an environmental signal, and “genes” within the genome that respond to this signal, are responsible for the “vision” phenotype (trait). And, during evolution, “vision” of course was extremely important in [a] finding food, [b] avoiding predators, and [c] finding mates for survival of the species. In fact, we know that “eyes” have evolved independently — somewhere between 40 and 65 times..!! The simplest “eyes” (e.g. those in microorganisms) simply detect whether the surroundings are light or dark. In higher organisms, there are two fundamental “designs”: one in protostomes [insects, mollusks, segmented worms, spiders); the other in deuterostomes [starfish, jellyfish, sea urchin, and all vertebrates (i.e. animals having a spine)]
When ancestors of cave fish, and certain crickets, moved into pitch-black caves, their eyes virtually disappeared over generations of non-usage. However, fish — at depths greater than sunlight can penetrate — have developed a type of vision that is highly sensitive to the faint glow produced by other deep-sea organisms. This “super-vision” is now known to represent an extraordinary increase in number of genes for rod opsins (retinal proteins that detect dim light). Those extra genes have evolutionarily diversified to produce proteins capable of capturing every possible photon at multiple wavelengths — which could mean that, despite the darkness, these deep-sea fish can actually see in color.
At a depth of 1000 meters, the last trace of sunlight is gone. But now researchers realize there exists a faint bioluminescence from flashing shrimp, octopus, bacteria, and even fish. Authors [see attached article & editorial] have studied deep-sea fishes’ opsin proteins and found that variation in the opsins’ amino-acid sequences can change the wavelength of light detected; hence, multiple opsins make color vision possible. One opsin, RH1, works well in low light. Found in the eye’s rod cells, RH1 enables humans to see in the dark — but only in black and white.
By inspecting 101 fish genomes, authors [see attached article & editorial] found that three deep-sea teleost (all ray-finned fishes — except primitive bichirs, sturgeons, paddlefishes, freshwater garfishes, and bowfins) lineages have independently expanded their RH1 gene repertoires. Among these, the silver spiny-fin has the most opsin genes in vertebrates (two cone opsins and 38 rod opsins). Spiny-fins express as many as 14 RH1 genes (including the most blue-shifted rod photopigments known) — which cover the (required) range of residual daylight, as well as the deep-sea bioluminescence spectrum. These data reveal molecular and functional evidence for recurrent evolution of multiple rod opsin–based vision in vertebrates. 😊
Science 10 May 2019; 364: 588-592 and pp 520-521 [editorial]