Summary

We measured the effects of temperature on cytosolic malate dehydrogenases (cMDHs) from the shell muscle of five species of eastern Pacific abalone, genus Haliotis, found at different latitudes and/or tidal heights. The apparent Michaelis-Menten constant (Km) of coenzyme (nicotinamide adenine dinucleotide, NADH) was conserved within a narrow range (11–21 micromolar) at physiological temperatures for all species. However, elevated temperatures perturbed the Km of NADH for cMDHs of the two species living at higher latitudes and/or lower tidal heights [Haliotis rufesens (red) and H. kamtschatkana kamtschatkana (pinto)] to a much greater extent than for cMDHs of congeners from lower latitudes and/or higher tidal heights [H. fulgens (green), H. corregata (pink) and H. cracherodii (black)]. The apparent Arrhenius activation energies for the cMDHs of these five species showed a similar interspecific divergence. Furthermore, green, pink and black abalone have cMDHs that are more resistant to thermal denaturation than are cMDHs of red and pinto abalone. Native gel electrophoresis showed that cMDHs of red and pinto abalone had identical mobilities, whereas cMDHs of green, pink and black abalone are distinct both from each other and from that of the two cold-adapted species. These data suggest that cMDHs from the abalone species living in warm habitats are adapted to function optimally at higher temperatures than are the cMDHs of the two species living in cooler habitats. The relationships suggested by these criteria are in agreement with other studies that used morphological and molecular indices to predict abalone phylogeny. These results therefore provide further evidence that interspecific variation in protein structure and function may be driven by natural selection based on only small (i.e. several degrees Celsius) differences in average body temperature, and that such selection is an important element of the mechanisms of species formation and the maintenance of biogeographic patterning