Jellyfish may seem like simple blobs but some have surprisingly sophisticated features, including eyes. These are often just light-sensitive pits but species like the root-arm medusa have complex ‘camera’ eyes, with a lens that focuses light onto a retina. Not only are these organs superficially similar to ours, they’re also constructed from the same genetic building blocks.
Hiroshi Suga from the University of Basel has been studying the eyes of the root-arm medusa (Cladonema radiatum). His work strongly suggests that all animal eyes share a common origin, whether they belong to a human or an insect, an octopus or a jellyfish. The details may be different but they’re all under the control of closely related ‘master genes’ that themselves evolved from a common ancestor.
As you might imagine, growing an eye is a complicated business and involves a huge alliance of different genes, switching on and off in a coordinated way. But in humans and other animals, this alliance all comes under the control of a master gene called Pax-6. Pax-6 was discovered in 1994 by Walter Gehring, who also led the current Cladonema study. Faulty copies can cause serious eye problems in animals as diverse as flies and rodents. And activating the gene in the wrong part of the body can produce eyes where they really shouldn’t exist, like the leg of a fly.
Pax-6 is so important that it’s largely the same in very distantly related animals (the technical term is ‘conserved’). You can take the version of Pax-6 from a mouse and shove it into a fly, and it will still be able to trigger the development of an eye. Even though these misplaced eyes have been activated by a mouse gene, they have the compound structure of typical fly eyes. This underlies the role of Pax-6 as a conductor – its job is to coordinate an orchestra of other eye-producing genes.
Pax-6 is just one of a number of closely related Pax genes. Cladonema doesn’t have a direct equivalent of Pax-6 but it does have three Pax genes of its own, each belonging to a distinct lineage. Only one of these – Pax-A – is actually active in the eyes and Suga clearly showed it’s the jellyfish’s master eye gene. When he transferred it into a fruit fly, he managed to trigger the development of eyes on odd body parts.
Cladonema isn’t the only jellyfish with complex eyes. Another one called Tripedelia belongs to a different group of jellies altogether and it too has a master eye gene called Pax-B, which belongs to a different group to either Pax-A or Pax-6. These three groups of genes evolved shortly after the very dawn of animal evolution from a single ancestral gene that duplicated itself several times. Its copies diverged into the different Pax groups.
So three groups of animals build their eyes using related master eye genes: the hydrozoan jellyfish, represented by Cladonema, use Pax-A; the cubozoan jellies, represented by Tripedelia, use Pax-B; and the bilaterians, including humans and the vast majority of other animals, use Pax-6.
You could argue that this means animal eyes evolved independently at least three times. But Suga disagrees – if this was the case, you might expect the master genes to be recruited from different gene families. As it is, they’re all Pax genes. Instead, Suga thinks that the building blocks of all animal eyes share a common origin. It’s a view that runs counter to the common assertion that animal eyes evolved many times independently but it’s one that Gehring has been championing for years.
When the common ancestor of jellyfish and more complex animals initially evolved eyes, Suga thinks they were under the control of several different Pax genes from the various families. As the bilaterians, hydrozoans and cubozoans diverged from one another, their eye programs eventually fell under the control of single Pax genes from different families. This shared origin explains why genes from one Pax group can still perform the role of genes from the others, and why Cladonema’s Pax-A can produce eyes in a fly.
The evolution of Pax genes. 1) An ancestral gene duplicates itself to produce different classes of Pax genes. 2) The ancestral animal eye evolves under the control of several different classes of Pax genes. 3) In three different animal groups, the Hydrozoa and Cubozoa (both jellyfish) and the Bilateria, eye development comes under the control of species Pax genes. 4) Some of the Pax genes in Bilaterians have been altered.
Reference: PNAS http://dx.doi.org/10.1073/pnas.1008389107
The amazing ways in which animals see the world