VISUAL SYSTEM: STATE OF THE ART 45 



has been confined to Negaprion, Carcharhinus, and Scyliorhinus. Typically, 

 fresh frozen lens material was homogenized with Freund et al.'s (1937) 

 adjuvant and injected into adult chinchilla rabbits for periods up to 2 years. 

 Antibodies, concentrated by precipitating out the globulin fraction of the 

 antiserum, were tested with lens antigens by the methods of immunodif- 

 fusion (Ouchterlony 1949) and Immunoelectrophoresis (Scheidegger, 1955). 



Results on sharks revealed first that testing antidogfish sera with either 

 dogfish, bull shark, or lemon shark lens homogenates gave reactions that 

 were indistinguishable from one another. This shows that all sharks and 

 probably all cartilaginous fish possess the same lens antigens. No immune 

 reaction was observed when antishark serum was tested against squid or 

 lobster lens antigens. Thus, Manski et al. demonstrated that analogous lens 

 tissues are biochemically unrelated, supporting the notion of convergent 

 evolution of the invertebrate and vertebrate eye. Antishark sera tested 

 against lenses from all other vertebrate classes reacted to some extent. There- 

 fore all classes of vertebrates possess ancient lens proteins, at least one of 

 which is shared with early cartilaginous fishes. 



The full significance of this research is beyond the scope of this review, 

 but one crucial test can be discussed. To follow the evolution of crystallins 

 through the vertebrate phylogenetic series, Manski et al. absorbed lens anti- 

 gens in antisera from one group and tested with another. For example, if 

 they wished to determine what antigens were shared by, say, sharks and 

 teleosts but evolved after contribution of antigens by the cyclostomes, stand- 

 ard antishark serum was prepared. To this, cyclostome lens homogenate was 

 added, precipitating out all antigens specific to that group. Such a "reagent 

 serum" could now be tested with lens homogenates of many other verte- 

 brates. This Manski termed "systematic absorption" of antisera. Results of 

 these experiments demonstrated that at least four primitive crystallins 

 derived from the agnatha were retained by elasmobranchs and all other 

 vertebrates. At least six others that originated after the agnatha were trans- 

 ferred to higher vertebrates. Alpha and j3 crystallins remained relatively 

 unchanged during transfer through the vertebrate series, while y crystallin 

 did not transfer well. Analyses of all test combinations led to a provisional 

 molecular phylogeny for the fishes: "... agnatha were the first vertebrates 

 to appear. From the agnatha descended all jawed fish . . . one branch led to 

 the primitive choanicthyes (lung fishes), the other led to some other form of 

 primitive jawed fish ancestral to the bony actinopterygii (represented today 

 by the bichir) and to the . . . modern sharks." (Manski et al. 1967c) 



An interpretive problem with antigens studies is that protein fractions 

 with different electrophoretic mobility display identical immunological prop- 

 erties. Thus, Rabaey (1965a) warned that, for phylogenetic studies, it is not 

 desirable to analyze a single body fluid or tissue extract. 



Lens Metabolism— Several studies on lens metabolism in the dogfish 

 have been published by Lerman and his coworkers. Lerman's major concern 

 was the type of RNA found in the lens and its role in aging. Although the 

 experimental species was not identified in any of the cited research, we 



