Like wings, eyes have evolved multiple times in different lineages of animals. According to a new study, humans and cephalopods evolved the same eyes through tweaks to the same gene -- even though their eyes arose independently of ours.
Many genes are responsible for making the eye. One holds instructions for making light-sensitive pigments, another provides information for making the lens, and then there are genes that orchestrate it all, telling various parts when and where they need to be assembled. These are called master control genes, and for eyes, the most important one Pax-6. The ancestral Pax-6 gene controlled the formation of a very simple eye: a few light-sensing cells working together in a primitive organism living in the Cambrian period.
Over time, the number of instructions that arose from a single Pax-6 gene increased, with today’s more complex version directing the formation of compound eyes in insects as well as the “camera-type eye” found in all vertebrates. That is, an enclosed structure with the iris and lens, a liquid interior, and an image-sensing retina. Cephalopods -- which include octopus, cuttlefish, and squid -- have a camera eye similar to ours, even though they belong to a different clade altogether.
So, a team led by Atsushi Ogura from Nagahama Institute of Bio-Science and Technology in Japan examined Pax-6 variations in the pygmy squid (Idiosepius paradoxus) and Japanese spear squid (Loligo bleekeri). Specifically, they analyzed RNA splicing. That’s when a piece of the code is removed and the ends are stitched together. In vertebrate eyes, developmental processes are controlled by four Pax-6 splicing variants, with each influencing different genes downstream.
They found five types of Pax-6 splicing variants used to create the camera eye in cephalopods. They acquired their Pax-6 splicing variants completely independently of vertebrates, though they used these variants in the same way to create a very similar camera eye. Pretty remarkable considering how we last shared a common ancestor 500 million years ago.
The work was published in Nature Scientific Reports in March.
[Via The Conversation]
Image: pacificklaus via Flickr CC BY-NC
