SWEENEY, A; JOHNSEN, S; Duke University; Duke University: Evolution of Complex Optics in Squid Lenses

Squid lenses are composed primarily of S-crystallins, structural proteins derived from multiple duplications of the enzymatic liver protein glutathione s-transferase. In some squid species, lenses are sophisticated optical instruments, with a graded refractive index that corrects for spherical aberration and forms sharp images. Our analyses of S-crystallin isoform distribution, protein density distribution, optics and protein evolution in squid lenses give new insight into the physical and evolutionary mechanisms generating their sophisticated optics. Phylogenetic analyses and SDS-PAGE of squid lens S-crystallins show that possibly independent evolutionary radiations of S-crystallin have generated optical complexity. There was an increased rate of S-crystallin gene duplication and evolution in some squid species relative to others, and these differences in rate may be correlated with different optical quality in each species. SDS-PAGE analysis of dissected lenses several species showed that these various isoforms are arranged concentrically within the lens. We have also measured the concentric density gradients of S-crystallins in the lenses of the same squid species using protein concentration assays and digital image analysis. With this information, aspects of the molecular packing arrangement of S-crystallins in different regions of the lens were inferred. The relative optical quality of several species of squid lenses was determined with a camera-like apparatus that used squid lenses as the optical element. Differences seen in S-crystallin evolution, distribution and density were correlated with differences in optical quality of the lenses in several species.