Microevolutionary dynamics and genetic erosion in pollution-affected Chironomus populations

Natural populations inhabiting heavily metal impacted freshwater ecosystems may face intense selection and genetic drift that conduct populations to severe reductions of genetic diversity, the so-called process of genetic erosion. Because the ability of populations to adapt to environmental change is directly related to the levels of genetic diversity, contaminant-driven genetic erosion may threaten the long-term survival of populations. The search for more robust and context-driven bio-indicators that add ecological relevance to the environmental risk assessments has increased interest in evolutionary toxicology and measures of genetic diversity. The research described in the present thesis was performed with the ultimate goal of understanding whether the levels of genetic diversity of the model ecotoxicological species Chironomus riparius (Meigen) may be used as ecological indicators of the health of freshwater systems. For that, an integrative study was undertaken investigating microevolutionary responses of C. riparius towards historical metal pollution. This included assessments of levels of genetic diversity as well as determination of genetic adaptation to metals and associated fitness costs. Genetic diversity was estimated based on the variation of seven microsatellite markers. Genetic adaptation and associated fitness costs were investigated through acute and chronic exposures to different environmental stressors, measurements of energy budget and biochemical mechanisms of tolerance to metals, after maintaining populations for several generations under standard laboratory conditions. Microevolutionary responses of C. riparius to metal pollution were, afterwards, compared with macroinvertebrate diversity and composition metrics. To draw general conclusions of causal-relationship between microevolutionary responses and pollution history, effects were investigated across multiple metal polluted and reference site. Globally, the levels of genetic diversity were globally high and there was a remarkable genetic homogeneity among all C. riparius populations in the field. However, C. riparius populations from metal polluted sites showed signs of genetic adaptation to metals as suggested by the increased tolerance to acute concentrations of metal and high basal levels of glutathiones and metallothioneins that likely enhance the fitness of populations to cope with metal toxicity. Furthermore, populations from metals contaminated sites had higher energetic costs when exposed to metals and one of the populations from contaminated sites showed also a poorer performance under control clean conditions. Finally, diversity and composition of macroinvertebrate communities from metal polluted sites was strongly affected and many sensitive taxonomic groups were eliminated and replaced by more opportunistic ones such as C. riparius. Overall, measures of genetic diversity of C. riparius natural populations do not seem to be particularly advantageous biomonitoring tools per se once they did not reflect the underlying microevolutionary responses of natural populations to historical metal pollution. This is likely because of the large population densities together with the highly dynamic nature of C. riparius in the field as we observed genetic erosion in population reared under laboratory conditions over relatively long periods of time. However, several lines of evidence indicate that measures of genetic diversity may accrue valuable information in several experimental situations: detection of interspecific hybridization; establishment of minimum levels of genetic diversity in laboratory-reared C. riparius populations; finally, integrative use of measures of genetic diversity in biomonitoring programs with more community-level focus. The results presented in this thesis aim to stimulate discussion on the suitability of C. riparius as a model species in evolutionary ecotoxicology studies as well as the sensitivity and robustness of genetic diversity measures as indicators of environmental quality in ecological risk assessment. CONTENTS Chapter I: General Introduction 1. General introduction . 3 1.1. Genetic diversity . 4 1.2. Effects of environmental contaminants on the genetic diversity of populations . 5 1.3. Molecular markers of genetic diversity . 7 1.3. 1. Application of neutral markers in evolutionary toxicology studies . 9 1.4. Chironomus riparius (Meigen) as a model evolutionary toxicology species . 13 2. Objectives and outline of the thesis . 14 References . 18 Chapter II: Evolutionary consequences of historical metal contamination for natural populations of Chironomus riparius (Diptera: Chironomidae) Abstract . 27