Is natural selection a plausible explanation for the distribution of Idh‐ 1 alleles in the cricket Allonemobius socius ?

Abstract.  1. Allozyme alleles in natural populations have been proposed as either neutral markers of genetic diversity or the product of natural selection on enzyme function, as amino acid substitutions that change electrophoretic mobility may also alter enzyme performance. To address these possibilities, researchers have used both correlative analyses and empirical studies. 2. Here, geographically structured variation of the enzyme isocitrate dehydrogenase ( Idh‐ 1) in the striped ground cricket Allonemobius socius Scudder (Orthoptera: Gryllidae) is examined. The distributions of Idh‐ 1 alleles appear to be related to environmental gradients, as allele frequencies showed significant relationships with mean annual temperature and precipitation. Specifically, the slowest mobility allele was more frequent at colder temperatures, while the converse occurred for the fastest mobility allele. 3. An exploratory experiment was performed to examine fitness effects of possessing different Idh‐ 1 alleles at two temperatures to test the hypothesis that the geographic structure of this locus may reflect environmental adaptation. Results showed that a significant interaction between temperature and Idh‐ 1 genotype affected the number of eggs laid, with success of homozygous individuals matching environmental expectations. 4. The above results show that (1) variation in the frequency of Idh‐ 1 alleles is significantly related to environmental gradients in the eastern U.S.A. and (2) alternative alleles of Idh‐ 1 appear to influence the egg‐laying ability of individuals differently depending on environmental temperature. Together, these results suggest that natural selection is a plausible mechanism underlying the distribution of Idh‐ 1 alleles in this species, although more detailed studies are needed.