differential rearing were that the bright lines did not profit from enrichment, but the dull ones did; the dull rats were not adversely affected by impoverishment, but the bright ones were. Numerous other studies have shown similar differential responses in a variety of phenotypes to environmental manipulations by groups of mice or rats of differing genotypes.
Another striking recent example of gene-environment interaction is provided by the study of quantitative trait loci (QTLs) affecting longevity in Drosophila flies. QTLs are loci that remain anonymous at present, but whose approximate chromosomal locations are known. Vieira et al. (2000) sought evidence for the effect of such loci on the length of life under five different environmental conditions of rearing. The extraordinary result was that 17 QTLs were identified, but none was pertinent to all environments. Some were effective in one sex only and in one environment; others were effective in both sexes in a specific environment, but the same allele was associated with longer lives in one sex and shorter lives in the other sex; some were effective in one sex in two environments, but with the same allele associated with longer lives in one environment and shorter lives in the other. All of the genetic variance was involved in genotype x sex interactions, genotype x environment interactions, or both.
Within the general domain of coaction of genes and environmental factors, there are several lines of investigation that convincingly demonstrate that environments not only can interact in a statistical sense with genetic factors, but can also actually influence which genes are expressed. In an oversimplified explanation, some environments can turn genes on and off. Certain subdomains of this research are of particular potential relevance for the present topic, dealing with the effects of stress of various sorts on gene expression. For instance, an extensive body of literature (summarized, for example, by Hoffman and Parsons, 1991) describes observations that suggest that stressful environments often increase the heritability—the proportion of phenotypic variance attributable to the collective influence of a polygenic system—of a wide variety of phenotypes in a wide array of organisms. A major body of data dealing with specific genes concerns the “heat-shock” proteins that are produced in Drosophila after exposure to a high-temperature environment. These proteins appear to protect other proteins in the organism from damage or destruction by the stressful environment. An example from mammals is the increase in the levels of specific RNAs in the adrenal glands of rats following immobilization stress (McMahon et al., 1992). Biobehavioral influences are clearly implicated by a study showing that classical Pavlovian conditioning—pairing foot-shock and an auditory stimulus—can result in a previously neutral feature of the environment acquiring the capability of eliciting a stress-related expression of a particular mRNA in regions of the brain of rats (Smith et al., 1992). These lines of research are perhaps particularly relevant