Electrophoretic and Functional Enzymic Evolution in Four Species of Eastern Pacific Barracudas From Different Thermal Environments

The role of enzyme variants (isozymes and allozymes) in environmental adaptation and evolution has been a major focus of recent evolutionary studies (Lewontin, 1974; Ayala, 1976; Hedrick et al., 1976; Somero, 1978). This question has generally been approached from two distinct experimental levels in terms of the evolutionary distances between the species (enzymes) being compared. On the one hand, the enzymes of widely different organisms, e.g., mammals and ectothermic species, have been compared (Low et al., 1973; Borgmann et al., 1975; Borgmann and Moon, 1975; Johnston and Walesby, 1977; Somero, 1978; Somero and Siebenaller, 1979). On the other hand, many attempts have been made to determine the selective advantage of different allelic enzyme variants (allozymes) in populations of a single species exposed to different environmental, e.g., thermal, conditions (Koehn, 1969; Merritt, 1972; Place and Powers, 1979). Whereas both classes of studies have led to insights about the basic patterns of molecular evolution, the role of enzyme variants in fine-scale environmental adaptation remains controversial. There exists a third, and complementary, approach for resolving the uncertainties about fine-scale environmental adaptation. This involves examination of enzymes of closely related congeneric species which have similar ecologies, but which live in habitats differing in temperature, pressure (Siebenaller and Somero, 1978, 1979; Somero and Siebenaller, 1979) or some other variable apt to affect enzyme function. Congeneric species may provide just enough evolutionary divergence to Dermit the role of minor-yet selectively significant-enzyme functional differences to be viewed in sharp relief. Thus, unlike a situation involving different populations of a single species, in which fixed allelic differences in different subpopulations may not be established for reasons of evolutionary time or gene flow among groups, congeners inhabiting different environments and possessing different fixed allelic forms of a particular class of enzyme may provide an excellent study system for resolving the importance of enzyme variants in relatively fine-scale environmental adaptation. Of the physical factors which often vary between environments of closely related species, temperature is one of the most important, physiologically and biochemically. There are numerous cases of species replacements over thermal gradients in the marine environment, especially with ectothermic organisms (Hubbs, 1948; Ekman, 1953). As temperature is one of the few environmental parameters which directly affects organisms at the molecular level, and has been demonstrated to be a strong selective force in protein evolution (Somero, 1978), it would seem that closely related congeneric species inhabiting different thermal environments might provide an excellent opportunity for investigating the effects of small environmental differences on functional molecular evolution. The barracudas (genus Sphyraena) of the eastern Pacific provide an excellent study system of this type. The four Sphyraena species in the eastern Pacific are similar ecologically and morphologically.. All are schooling, pelagic, predatory fishes, and two of the species, S. argentea and