How squid have evolved to see in dim ocean water
In a new paper published in Science, researchers at the University of Pennsylvania provided a detailed look into how self-assembled squid lenses have evolved to adjust for light distortion, which allows them to see clearly in the dim waters of the open ocean.
In addition to contributing to the field of nanotechnology by outlining self-assembly at the nanometer length scale, the research could one day help scientists build improved artificial lenses that can focus light perfectly. It could also allow them to better understand cataracts, a clouding of the human eye lens that leads to blurred vision.The research was led by postdoctoral fellow Jing Cai and Alison Sweeney, an assistant professor in the Department of Physics & Astronomy in the School of Arts & Sciences. Graduate students James Townsend and Tom Dodson and physics professor Paul Heiney also contributed to the study.
A spherical lens with a constant material density fails to focus light into a point, which is called spherical aberration. Squid lenses manage to get rid of this unwanted property. The researchers’ main goal was to find the mechanism that allows the proteins that make the lens to self-assemble into this special optical design.
Sweeney likened the field of self-assembly to Lego bricks: it allows someone to specify the size of the bricks, the number of spaces where the bricks can interact with each other and how strongly they stick when they link together. With the right specifications, Sweeney said, someone could throw all the bricks into a paper bag, shake it and have the Legos assemble themselves into a castle without having to put individual bricks together by hand.
But the researchers weren’t simply interested in the self-assembly. In particular, Sweeney said, they wanted to find out how this process evolves so that Lego bricks that may have once formed a car could slowly begin to form a castle.
“In biology,” she said, “you're dealing with proteins, DNA and RNA, not Lego bricks. Our analog for Lego bricks in this study was the individual proteins that make up the refractive material in the lens. We found that squid have a really elegant optical design whereby having a very dense material in the center, made of proteins with lots of linkage sites, and a very sparse material on the edge, made of proteins with two linkage sites, they can form a spherical lens that doesn't suffer from spherical aberration. This work was a really nice case study that enabled us to talk about the evolution of self-assembly, or how evolution invents new materials through a random process, in detail, without needing to be speculative about it.”
To investigate the squid lens, the researchers used a technique called small angle X-ray scattering, in which an X-ray beam is projected through a sample and, by looking at how the rays scatter as they interact with the sample, researchers can learn about the structure of the material they’re studying.
“Wherever there is a higher density of electrons,” Cai said, “it will show some pattern in the X-ray. In the squid lens, places where there’s a higher concentration of water will have a different pattern from places where there's a higher concentration of protein. We can use X-rays to probe the protein distribution.”
Once the researchers were able to learn about the structure of the squid lens, they completed other supporting experiments, such as putting the proteins into different solutions to see how the proteins behave under different densities. These experiments allowed the researchers to understand how the proteins in every given region of the lens interacted to form this structure.
The researchers found that the proteins, which Cai describes as “bunny-shaped,” have two “bunny ears” with various lengths. The bunny ears link together to form a chain-like structure. On the periphery of the lens, the bunny ears are longer and only able to connect to one other protein. The bunny ears closer to the core are shorter and, as a result, proteins have higher connection numbers. This results in a lens with a core that is much denser than the periphery, enabling the lens to both remain transparent and focus light rays to a single point.
Although the squid lens self-assembles from only one type protein, Cai said, while the human lens assembles from three (making it more complicated to study), this research could also provide insights into human vision, such as why cataracts form. It could also lead to the development of artificial lenses that don’t suffer from spherical aberration.
At the most fundamental level, Cai said, it may also give access to a broader picture of how life forms and evolves in general.
“This is an elegantly simple example compared to other biological materials,” Cai said, “but it will help us eventually understand human beings from this self-assembling perspective, and how life has assembled into its current form.”
The results of this study, Sweeney said, show the rules for doing self-assembly at the nanoscale, which may play a huge role in nanotechnology.
“Nanotechnology is a huge and developing field of science,” she said, “but what's almost universally true is that we don't understand very well how to make proteins and nanoparticles behave in space. And that's almost what the whole field of nanotechnology is about; now that I have nanoparticles that I can control the size and properties of, how do I make them assemble into something that's useful to me? It’s a tricky problem because nanoparticles are sort of generically sticky and clump to one another. It’s hard to get them to behave.
By investigating the squid lens, the researchers were able to find an evolved set of rules to make nanoparticles behave in space in any arbitrary way that one might like them to.
“We now have the rules for shaking a paper bag to make the Lego castle,” Sweeney said, “but now we can work at the nanometer length scale. We’re getting much closer to being able to say, ‘Here are the rules for building life,’ in a way that we haven’t been able to in the past.”
This research was supported by National Science Foundation grants MRSEC DMR11-20901 and NSF-1351935, the Packard Foundation Fellowship for Science and Engineering, the Sloan Foundation, the Kaufman Foundation and the Department of Defense NDSEG graduate fellowship program.