A comparative study of the thermal stability of the vertebrate eye lens: Antarctic ice fish to the desert iguana.

We analyzed the thermal stability characteristics of the lenses of 12 vertebrate species in four vertebrate classes. In the selection of animals for comparisons, we controlled for the variable of phylogenetic relatedness by choosing closely-related species that naturally live under very different environmental temperatures, as well as distantly-related species that live under similar environmental temperature regimes. Further, we obtained animals over the range of temperatures in which vertebrates occur, -2 degrees C to 47 degrees C. Experiments in which whole lens transparency was measured under conditions of heat stress showed that animals naturally occurring in high temperature environments have lenses whose transparency is significantly more resistant heating. Studies of behavior of the crystallins during heat stress of the whole lens showed that: (1) a direct correlation exists between the resistance of the lens to thermal insult and both the preferred and maximum body temperature of the animal, (2) some crystallins are more resistant to thermal stress than others, and (3) the taxon-specific crystallins (delta-, tau- and rho- crystallins) were more labile than alpha-, beta- and gamma-crystallins.