KINETIC AND ELECTROPHORETIC DIFFERENTIATION OF LACTATE DEHYDROGENASES OF TELEOST SPECIES-PAIRS FROM THE ATLANTIC AND PACIFIC COASTS OF PANAMA.

The important role of molecular adaptations in evolution has been demonstrated in studies of enzymic and respiratory proteins of organisms living in different habitats. Investigations of homologous proteins of warmand cold-adapted ectotherms, for example, have elucidated many of the critical molecular properties that are involved in adaptation to different environments (Somero, 1978). However, even though substantial information is available on adaptive differences among homologous proteins of widely different species, e.g., those belonging to different families and genera, there remains much to be learned of the role of fine-scale adaptations, as might be important in populations of a single species that inhabit slightly different environments. Substantial amounts of protein variation, both between populations of a single species and between closely related species, have been demonstrated with electrophoretic techniques (reviewed in Powell, 1975), and the existence of allelic variants which cannot be distinguished by standard electrophoretic methods has also been reported in comparisons of populations and species (Bernstein et al., 1973; Johnson, 1975; Siebenaller and Somero, 1978). The role of these electrophoretically detectable and cryptic allelic variants in molecular evolution has become an important focus of contemporary evolutionary study (Lewontin, 1974). Is the observed allelic variation of functional significance in many cases? Or is this variation generally lacking in selective importance? A closely related question, and one serving as a major focus of the present study, is what degree of environmental variation is required to select for functionally different proteins? One approach to addressing these questions experimentally is to study a given class of enzyme in several populations of closely related organisms that are separated by a common faunal barrier into habitats that differ in an environmental factor such as temperature. Comparisons of protein homologues of these populations may provide especially good insight into the role of fine-scale molecular adaptation. However, in order for such a comparison to be meaningful, several conditions must be met. First, one must know in advance, from broadly comparative studies of the protein in question, what values are to be expected for the functional trait(s) under investigation. Only with this strong empirical base can the observed allelic variations be interpreted. Second, the populations studied must have existed in the different habitats for long enough periods of time for the accumulation of new protein variants. Third, functional analysis of the allelic variants must be performed under in vitro conditions which simulate, as closely as possible, the in vivo conditions to which the proteins are normally exposed. An excellent opportunity for an investigation which meets these criteria is provided by the Panama land bridge, which arose approximately 3.1 million years ago (Keigwin, 1978), isolating a presumably very similar fauna of tropical marine shore fishes in significantly different thermal environments. During the dry season (December to May), strong offshore winds I Current address: Southwest Fisheries Center, NMFS, P.O. Box 271, La Jolla, California 92038.