BALANCING SELECTION IN A DESERT STREAM‐DWELLING FISH, POECILIOPSIS MONACHA

A Comparison of Isogenic Homozygous Clone and Wildtype Zebrafish (Danio rerio): Survival and Developmental Responses to Low pH Conditions

The value of bioassays as analytical methods for assessing the potency of particular stressors on live animal models depends on the precision of their results, which are greatly influenced by the choice of test subjects. The genetic makeup of experimental subjects varies, and, as such, so will their responses to the test environment. Genetic diversity of test populations may contribute to statistical variability; therefore, the use of genetically similar subjects may enhance the utility of bioassays. This study addresses the efficacy of using isogenic homozygous zebrafish (Danio rerio) as subjects for bioassays. Stress responses (acidic conditions) were compared during early development for gynogenetically produced isogenic homozygous line of zebrafish (C32) and wildtype (WT) zebrafish. Experiments evaluated early life stage milestones after exposure to low pH in water of a different electrolyte composition. Because the isogenic homozygous clonal (IHC) fish possessed far less genetic variability than the WT fish tested, it was predicted that the IHC fish would exhibit less variability in their response to stress. Although we found no significant differences in the variability between the responses of the IHC and WT fish, pH and water hardness level had a differential effect on the two groups. Simple strain differences may be the probable cause of the response differences to environmental stress. Factors that may affect stress response, such as heterogeneity, co-adapted gene complexes, and domestication, are discussed. Our findings and review of recent zebrafish literature stress the need for researchers to carefully consider breeding histories and trait characteristics for each potential test subject to maximize the sensitivity of the assay.

Geographic distributions of Idh-1 alleles in a cricket are linked to differential enzyme kinetic performance across thermal environments

Background

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, 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.

Results

We 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.

Conclusion

This 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.

Molecular genetic variation and individual survival during population crashes of an unmanaged ungulate population

Theoretical models of the effect of population bottlenecks on genetic variation assume that individuals are removed at random from the population. We investigated this assumption in a naturally regulated, unstable population of Soay sheep (Ovis aries). During rapid population declines or 'crashes', individuals were not removed at random with respect to genotype: we found associations between individual survival and certain genotypes at five polymorphic protein or microsatellite DNA loci (Ada, Got, Tf, MAF18 and OPACAP). Some loci appeared to show simple associations with survival whereas others had more complex interactions with crash year or age: all displayed different patterns of association between the sexes. Simple overdominance was not a general feature of our data; it seems likely that fluctuating selecting, countervailing selection in different fitness components or frequency-dependent selection may explain the pattern and complexity of the associations shown at different loci. Our study cannot distinguish between selection acting at these loci or at other, closely linked loci. However, our empirical study implies that the molecular genetic outcome of population bottlenecks in natural populations does not always follow theoretical expectations based on the random removal of genotypes. Bottlenecks in which individuals are removed at random are distinct from bottlenecks in which there is scope for selection via non-random survival of individuals.