Electrophoretic variation in allelozymes related to function or structure?

Abstract

ATTEMPTS to explain variation at enzyme loci in terms of the function of the protein molecule^1–5 have shown that enzymes characterised by a single physiological substrate are less variable than enzymes which have been shown in vitro to act on a number of substrates⁵ and that enzymes regulating the flux through a pathway are more variable than enzymes with no regulatory function^3,4. The explanation for this is that different enzymatic forms are favoured in different environments. What distinguishes this hypotheses from others concerning the maintenance of protein variation is that it is to a large extent independent of mutation rate, population size, gene flow among populations and whether or not the population is at equilibrium. For these reasons it can easily be tested. Johnson⁴ and Gillespie and Langley⁵ analysed separately data from Drosophila, small mammals and man. Although the collection of data on small mammals and man is more or less complete, both papers, and that by Gillespie and Langley⁵ in particular, ignored a number of Drosophila studies. I have collected electrophoretic data referring to 61 species and semispecies of Drosophila (Fig. 1) and carried out an analysis identical to that of Johnson⁴, supporting the observation that enzymes with multiple substrates are more variable than enzymes with single substrates. Also, enzymes with variable substrates are more polymorphic than regulatory enzymes, which in turn are more polymorphic than non-regulatory enzymes.