From simple eyespots up to complex refractive and compound eyes, most animals are equipped with some form of visual structure that allows them to navigate the world; but not sponge larvae. Lacking conventional eye structures, rings of primitive light-sensitive cells guide larval sponges toward the blue light or shadow cues that indicate potential settlement sites. Todd Oakley and an international team of collaborators say, ‘With pigment adjacent to photoreceptors, the sponge ring structure meets a minimal definition of an eye.’
However, the Amphimedon queenslandica genome lacks one vital visual component: a gene for a light-sensitive opsin pigment – which is essential for vision in other animals – suggesting that the sponge’s unique eyes might have evolved a completely novel light-detection mechanism. Curious to find out whether the sponge’s simple eyes evolved independently of other eye structures, Oakley and his co-workers decided to try to identify components of the sponge’s visual system (p. 1278).
Knowing that another group of light-sensitive proteins – known as cryptochromes – is produced by plants, insects and mammals, the team searched the sponge genome for evidence of the gene and discovered two examples. Next, the team made RNA probes to determine where the genes were expressed in the embryonic and larval sponges, and found that one of the two cryptochromes, Aq-Cry2, was produced near the sponge’s simple eye cells. However, the team had to prove that the protein was cryptochrome and not a very similar protein, photolyase, which repairs damaged DNA in response to UV and blue light. So, the team expressed both genes – to produce the proteins that they encoded – and then tested whether the proteins could repair damaged DNA. Neither could. And when the team tested which wavelength of light the proteins absorbed, they found that Aq-Cry2 absorbed 450 nm light most strongly; the same wavelength that the larvae respond most strongly to.
The team says, ‘These results are consistent with a hypothesis that opsin-less sponge eyes utilize cryptochrome, along with other proteins, to direct or act in eye-mediated phototactic behaviour.’ However, they also point out that although Aq-Cry2 cryptochrome has all of the necessary attributes for responding to blue light, it is not clear exactly how the protein functions to direct larvae to their settlement sites. Ultimately, the team is keen to determine how much sponge ring eyes share in common with other eye structures to learn whether sponges evolved their novel photodetection system completely independently or from other more conventional eye structures.