Abstract
BackgroundGeographic 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, Allonemobius socius, 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 (Idh-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 Idh-1 genotype and temperature which affected fitness. Here, we use enzyme kinetics to further explore GxE between Idh-1 genotype and temperature, and test the predictions of kinetic activity expected under drift or selection.ResultsWe found significant GxE between temperature and three enzyme kinetic parameters, providing further evidence that the natural distributions of Idh-1 allele frequencies in A. socius are maintained by natural selection. Differences in enzyme kinetic activity across temperatures also mirror many of the geographic patterns observed in allele frequencies.ConclusionThis study further supports the hypothesis that the natural distribution of Idh-1 alleles in A. socius 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.
Highlights
Geographic clines within species are often interpreted as evidence of adaptation to varying environmental conditions
We found a significant genotype-by-environment interaction (GxE) between isocitrate dehydrogenase (Idh)-1 genotype and temperature on 3 measures of enzyme kinetic performance in A. socius
We hypothesized that natural selection may be maintaining a natural cline in Idh-1 allele frequencies (1.8, 2.0, and 2.2) in the cricket A. socius, due to both correlations between allele frequencies and environmental conditions and fitness differences between homozygotes of the various alleles across temperatures [14]
Summary
Geographic clines within species are often interpreted as evidence of adaptation to varying environmental conditions. An empirical study revealed a significant genotype-by-environmental interaction (GxE) between Idh-1 genotype and temperature which affected fitness. We use enzyme kinetics to further explore GxE between Idh-1 genotype and temperature, and test the predictions of kinetic activity expected under drift or selection. Individuals within populations are under selection pressure to adapt to their environment; these adaptations can be morphological, physiological, or behavioral in nature These diverse adaptations all have a molecular basis and the study of molecular adaptation to environmental conditions is an active area of research within evolutionary biology. BMC Evolutionary Biology 2009, 9:113 http://www.biomedcentral.com/1471-2148/9/113 itself to empirical study is the kinetic performance of different enzyme alleles (allozymes) under a range of environmental conditions, such as temperature. Amino acid substitutions may affect the function of the protein, altering optimal ranges for temperature, pH, or substrate concentration. Some substitutions may not significantly affect the function of the protein, and are selectively neutral, while others may improve enzyme function and be favored by selection
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