However, there is no clear explanation of how the elaborate camera eyes of humans and octopuses evolved from the prototype eye. However, the expression of Pa圆 in the camera eye of the squid, a member of the same phylum, supports the prediction that Pa圆 controls the development of the octopus eye. Pa圆 expression has not yet been observed in the octopus. Within molluscs, it has been shown that the scallop, ear shell, and squid all express Pa圆 ( Tomarev et al. It has previously been reported that Pa圆, a “master control” gene for the development of the eye, is highly conserved across species. This view has been changed, however, by Gehring and Ikeo ( 1999), who maintain that the expression of the common master regulator Pa圆 in both types of eyes indicates the divergence of these two types of eyes from a single prototype eye present in the common ancestor of cephalopods and vertebrates. Each camera eye has different evolutionary origins. The phylogenetic evidence of convergent evolution of camera eyes between humans and octopuses. Convergent evolution is the process by which independently evolved features that are superficially similar to each other can arise through different developmental pathways ( Lauder 1981). Therefore, the eyes of humans and octopuses have been described as a typical example of convergent evolution ( Fig.
Despite the differences in direction of visual cells, focusing mechanism, ability to detect polarized light and encoding genes for crystallins, the camera eyes of human and octopus are believed to have independently evolved after the divergence of the two lineages during the Precambrian period because both humans and octopuses have structural similarities in their camera eyes, as shown in Figure 1 ( Harris 1997). Insects, members of Lophotrochozoa, have compound eyes. Other molluscs have various types of eye, such as the concave mirror eye and the pinhole eye. The single-lens camera eye is found in vertebrates and in two groups of molluscs, octopus and squid. These organs are highly diverse in structure, ranging from small groups of light-sensitive cells to highly sophisticated and complex structures that register precise images in some groups of arthropods, molluscs, and vertebrates. Much is known about the photoreceptive organs of various animals ( Salvini-Plawn and Mayr 1977 Osorio and Bacon 1994). The evolution of the eye is one of the most complicated and interesting stories for molecular biologists and molecular evolutionists. It suggests that a larger number of conserved genes and their similar gene expression may be responsible for the convergent evolution of the camera eye.
We found that 1019 out of the 1052 genes had already existed at the common ancestor of bilateria, and 875 genes were conserved between humans and octopuses. To trace the evolutionary changes that are potentially responsible for camera eye formation, we also compared octopus-eye ESTs with the completed genome sequences of other organisms. On the contrary, when we compared octopus eye ESTs with human connective tissue ESTs, the expression similarity was quite low. Comparing these 1052 genes with 13,303 already-known ESTs of the human eye, 729 (69.3%) genes were commonly expressed between the human and octopus eyes. We sequenced 16,432 ESTs of the octopus eye, leading to 1052 nonredundant genes that have matches in the protein database. To study the molecular basis of convergent evolution of camera eyes, we conducted a comparative analysis of gene expression in octopus and human camera eyes. It has been known as a typical example of convergent evolution. The extent to which the ability contributes to their swift changes in appearance, however, remains unknown.Although the camera eye of the octopus is very similar to that of humans, phylogenetic and embryological analyses have suggested that their camera eyes have been acquired independently.
These findings, reported online today in The Journal of Experimental Biology, suggest that the light-sensing ability may have originated with an ancestral mollusk, which over time cephalopods have drafted to facilitate their unique behavior. Although most other mollusks, including scallops and snails, lack the cephalopods' skill in shapeshifting, they can sense light with their skin, too. Now, two new studies have found evidence of rhodopsin-a light-sensitive protein usually in retinas-in the skin of cephalopods. Cephalopods such as squid, octopus, and cuttlefish use these chromatophores to dramatically change the color, shape, and texture of their skin, and scientists began to wonder if they could sense light as well. When researchers shone a bright light on it and then removed the light, chromatophores-tiny, circular pigment-filled structures-embedded in the skin would expand and then relax. Several decades ago, scientists noticed that octopus skin did something strange.
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