The Relationship Between Environmental Stress and Variance

Here we investigate increased variance about mean enzyme activity levels as an alternative biomarker of environmentally induced stress using the rice weevil, Sitophilus oryzae, and the common house mosquito, Culex pipiens. When reared on a toxic food (yellow split-pea), S. oryzae show very little change in mean activity in two detoxifying enzyme systems compared with enzyme activity on a relatively nontoxic food (wheat). When transferred between food types, again there was very little effect on mean enzyme activity, but the variance about mean activity increased significantly, especially when the animals were transferred to the more toxic food. Increased variance was also shown to persist following long-term genetic selection by the toxicant. In a pesticide-resistant strain of C. pipiens, a shift in the mean esterase activity was associated with high levels of variance. Furthermore, variance about the mean was not related to the number of alleles influencing the detoxifying enzyme system. These results are discussed in relation to the potential value of the use of variance about the mean as a biomarker alternative or complementary to mean enzyme activity.