The genes that make squid eyes also make your legs

PROVIDENCE—Looking a squid in the eye is eerily like looking in a mirror. Squids, octopuses, and other cephalopods are on a very different part of the tree of life from vertebrates. But both have evolved sophisticated peepers that rely on a lens to focus light and provide excellent vision. This independent evolution of such complexity has puzzled biologists for centuries and has prompted searches for clues about how this might have come about.

Evolutionary developmental biologists have now discovered that the genes that guide the initial formation of legs in us and other vertebrates also guide the formation of the squid’s lens (seen in cross section of eye above). The find is yet another example of how nature recruits genes used for one purpose to do another job for the body.

The squid lens forms as extra-long membranes jutting out for specialized eye cells overlap to form a tight ball. Our lenses are actually degraded cells themselves packed with a clear protein. To learn how the squid lenses form, these researchers carefully tracked where, when, and which genes turn on and off as embryos of Doryteuthis pealeii, a squid commonly served as fried appetizers, develop.

They were surprised to see the gene regulatory network for sprouting limbs at work in a developing eye. But when they added a chemical called WNT that naturally inhibits this network in many organisms to squid embryos, the lens failed to form, they reported here last month at Evolution 2019, an annual meeting of evolutionary biologists, confirming this limb network is also a lens network. Next, the researchers will peer into exactly what each of these genes is doing to make the lens form.