| Summary: | <p>Abstract</p> <p>Background</p> <p>Geographic clines within species are often interpreted as evidence of adaptation to varying environmental conditions. However, clines can also result from genetic drift, and these competing hypotheses must therefore be tested empirically. The striped ground cricket, <it>Allonemobius socius</it>, is widely-distributed in the eastern United States, and clines have been documented in both life-history traits and genetic alleles. One clinally-distributed locus, isocitrate dehydrogenase (<it>Idh</it>-1), has been shown previously to exhibit significant correlations between allele frequencies and environmental conditions (temperature and rainfall). Further, an empirical study revealed a significant genotype-by-environmental interaction (GxE) between <it>Idh</it>-1 genotype and temperature which affected fitness. Here, we use enzyme kinetics to further explore GxE between <it>Idh</it>-1 genotype and temperature, and test the predictions of kinetic activity expected under drift or selection.</p> <p>Results</p> <p>We found significant GxE between temperature and three enzyme kinetic parameters, providing further evidence that the natural distributions of <it>Idh</it>-1 allele frequencies in <it>A. socius </it>are maintained by natural selection. Differences in enzyme kinetic activity across temperatures also mirror many of the geographic patterns observed in allele frequencies.</p> <p>Conclusion</p> <p>This study further supports the hypothesis that the natural distribution of <it>Idh</it>-1 alleles in <it>A. socius </it>is driven by natural selection on differential enzymatic performance. This example is one of several which clearly document a functional basis for both the maintenance of common alleles and observed clines in allele frequencies, and provides further evidence for the non-neutrality of some allozyme alleles.</p>
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