The use of microsatellites as a surrogate for quantitative trait variation in conservation
Conservation biologists are interested in maintaining genetic variation in small populations, with a view to maintaining fitness and the ability of the species to adapt to changing environmental conditions. The most important type of genetic variation is therefore that which affects fitness and reproduction, and is therefore subject to natural selection. Such fitness traits are often quantitative, i.e. are the result of a suite of loci, and are continuously variable. Microsatellite markers are a popular method of determining the level of variation present in a species’ genome. The assumption is made that microsatellites, which are neutral markers, behave in the same manner as quantitative traits. If this assumption were proved incorrect, then the use of neutral markers in conservation monitoring would have to be re-evaluated. In this study, experiments have been conducted using Drosophila melanogaster to test the assumption that variation in quantitative traits under stabilising selection declines at the same rate as heterozygosity in microsatellite markers, during a population bottleneck. Experimental population bottlenecks were of two effective population sizes (Ne), Ne=2 for one generation and Ne=60 for 35 generations. Based on the effective population size, we expected both types of bottlenecks to lose 25% of neutral genetic variation. Ten replicates of each bottleneck were maintained, along with four large control populations with Ne=320. In each population, heterozygosity (He) for eight microsatellite loci was compared with the heritability and additive genetic variance of two quantitative traits subject to balancing selection: fecundity and sternopleural bristle number. Microsatellite heterozygosity decreased in accordance with neutral predictions, whereas additive genetic variation in quantitative traits altered more than expected in both large and in bottlenecked populations relative to the initial sampling values, indicating that variation in quantitative traits was not being lost at the same rate as predicted by neutral theory. For most traits, the changes in additive genetic variance were congruent in all populations, large or bottlenecked. This congruence suggests that a common process was affecting all populations, such as adaptation. A mite infestation in early generations is a possible source of selective pressure. When bottlenecked populations were compared to the contemporaneous large populations (Ne = 320), the additive genetic variance of most traits was seen to have been lost in accordance with predictions from the loss of microsatellite heterozygosity. Loss of variation in microsatellites can thus be used to predict the loss of variation in quantitative traits due to bottlenecks, but not to predict the potentially much larger changes due to other processes such as adaptation. The effects of concurrent environmental stress and reduced population size were also evaluated. Endangered populations are often subject to environmental stress in addition to reduced population size, but the effect of stress on the additive genetic variance of fitness traits in organisms undergoing population bottlenecks is unknown. If the presence of stress alters the level of additive genetic variance in fitness traits, the viability of such populations could be substantially affected. The loss of microsatellite heterozygosity was not affected by the presence of a stress agent during a bottleneck. I found some significant effects of stress on the additive genetic variance of sternopleural bristles and fecundity; there was also a significant interaction between stress and the response to directional selection in sternopleural bristles. There was also an increase in the coefficient of variation of VA for sternopleural bristles. Stress may therefore affect the manner in which populations respond to selective pressures.
genetic markers, microsatellites (genetics)
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