VISUAL SYSTEM: STATE OF THE ART 43 



Thus, they studied several teleosts and the bonnethead shark, Sphyrna 

 tiburo, reporting the absence of a crystallin in bony fish as had been pre- 

 viously suggested by Cobb and Koenig (1968). However, others (i.e, Rabaey 

 19656) have identified small amounts of a crystallin in the fish lens. In fact, 

 Mehta and Lerman (1971) investigated the properties of dogfish (Mustelus) 

 a. crystallin. Electrophoretic mobility of shark a was significantly slower 

 than that of mammals and one bird. In addition, the amino acid pattern of 

 dogfish ot was quite different and did not cross-react in an immune assay 

 with any other species tested. Therefore, dogfish a crystallin is funda- 

 mentally different from that of higher vertebrates. 



Cobb et al. (1968) were eventually able to isolate a component in the lens 

 of Sphyrna with a sedimentation coefficient of mammalian a crystallin. The 

 authors reported that about 70% of the native soluble protein in the shark 

 lens exists as a low molecular weight fraction equivalent to y crystallin. They 

 also found an intermediate weight fraction apparently corresponding to j3 

 crystallin. 



Analyses of lens crystallins have been used as tools in both population and 

 taxonomic studies. These investigations are based on occurrence of species- 

 specific lens proteins. However, at least two nongenetic sources of variation 

 exist: protein changes through ontogeny and changes occurring between 

 death of the animal and protein extraction. 



Peterson and Smith (1969) investigated intraspecific variation in the 

 ocular lens proteins of the sandbar shark, Carcharhinus milberti. Expected 

 ontogenetic changes were shown by comparison of adult lens proteins to 

 those of near-term fetal sharks. The adults were collected from different 

 islands in the Hawaiian chain and possibly represent separate populations. 

 The individual protein pictures were quantitatively and qualitatively hetero- 

 geneous: no less than 10 different electrophoretic patterns were described. 

 The authors related the different patterns to different groups of sharks, 

 suggesting that this is a practical method of identifying separate breeding 

 populations. 



Lenses of young mammals cooled to less than 10°C become reversibly 

 opaque due to precipitation of cold-precipitable protein fraction (CPP) 

 (Lerman and Zigman 1965). Upon warming, the lens again becomes trans- 

 parent. The importance of CPP relates to ontogenetic changes in the lens: 

 along with the albuminoid fraction, the most marked changes in protein 

 concentration occur in the CPP fraction. In mammals, CPP apparently con- 

 sists of a single component with a sedimentation rate differing from that of 

 all three crystallins. Normally, the phenomenon of cold cataract does not 

 occur in the elasmobranch lens. However, Zigman et al. (1964) observed 

 that treating mammalian lenses with 0.3 M urea tended to inhibit CPP for- 

 mation. Reasoning that concentration of urea in elasmobranch tissues 

 (0.25 M) might be a factor blocking formation of CPP, they extracted the 

 water-soluble lens proteins of the dogfish, Mustelus, and removed the urea 

 by dialysis. Cooling the dialysed preparation to below the freezing point 

 of water precipitated a CPP fraction. Replacing the urea after warming again 

 prevented CPP formation. As in the rat, dogfish CPP decreases markedly 



