Tuesday, December 4, 2007

The 'eyes' have it

Three reasons why cephalopod eyes are better than human eyes (I am sure there are more):

The joining of the optic nerve bundle to the retina itself in vertebrates causes a blind spot (no photoreceptor are located here). On the other hand, cephalopod optic nerves are attached at different points along the back of the eye (not the inner layer), eliminating the need for a 'bald spot' on the retina.


The photoreceptor in cephalopod eyes actually face towards the light! Vertebrate photoreceptor face away from the light and light must pass through other layer before hitting the photoreceptor.

Another interesting design plus is that squids very rarely get cataracts in the center of their eyes. What they found was that squids, and some other cephalopods (like octopods) have two types of genes responsible for making the proteins for the lenses in their eyes. These genes have an extra insertion, either short or long insertions, (which are basically like extra instructions) that are not found in our eyes. When these extra insertions are translated into the proteins, they give extra stability to the protein so they won’t unfold. Since cataracts are caused by the proteins unfolding (making an opaque part in the lens), squids are less likely to get cataracts.

Now, the long gene produces a more stable form of protein than the short gene. The ‘long gene’ proteins are found in the center of the squid eye, while the ‘short gene’ proteins are found in the edges. This means that the center of the eye is least likely to get a cataract. You may wonder why the squid does not just use all ‘long gene’ proteins. I don’t know, but there may be some energy costs associated with making the larger more stable protein, so that it is more cost effective to use the ‘short gene’ proteins on the edges where they don’t count (hypothesis).


Either way, they are better lenses than what we have.

Another cool thing is, that each group of ‘advanced’ cephalopods has their own special version of the two genes, but they work very similarly. This stuff was very well put together; I can’t wait to read the paper on this. It will probably end up in Science, if it has not been published already!

Here’s the original abstract…

SWEENEY, A*; JOHNSEN, S.

Evolution of High-Acuity Vision in Coleoid Cephalopods

Spherical lenses with a graded refractive index design are required for camera-like vision in aquatic animals. In cephalopods, these lenses are made of a group of closely related proteins collectively called S-crystallins. Our earlier work has shown that an adaptive radiation these S-crystallin genes and positive selection on the electrostatic properties of S-crystallin proteins led to a graded refractive index lens capable of forming high-resolution images in the squid Loligo opalescens. In the L. opalescens lens, S-crystallins with high charge stabilize the optical properties of regions of low refractive index in peripheral layers, and S-crystallins with lower charge are tightly packed in the high refractive index cortex. The mechanistic link between S-crystallin sequence, biochemistry and refractive index allows us to understand in molecular detail the optical evolution of a camera-like eye in cephalopods. To understand the transition from ancestral cephalopod vision to extant camera-like vision in coleoid cephalopods, we used techniques from molecular evolution, biochemistry, molecular dynamics, optical modeling and image analysis. We sequenced 600 S-crystallin genes from most major coleoid taxa, constructed a gene tree from these sequences and analyzed it for patterns of charge evolution. We also measured the optical quality of these lenses by calculating their modulation transfer functions (MTFs). Our gene tree suggests that high-resolution lenses evolved from a low-resolution ancestor multiple times within the coleoid cephalopods. Consistent with our gene tree data, our MTF data show that there is taxonomic variation in lens quality within coleoid cephalopods. We will discuss the correlations between independent adaptive radiations of S-crystallin molecules, high acuity vision in cephalopods and possible evolutionary scenarios in which these changes in visual acuity may have been occurring during the Jurassic radiation of squid.

1 comment:

prudence said...

thank you so much! what an interesting animal. i found this information extensively helpful in my research, and as i am only 13, particularly easy to read.