Populations under stress : analysis on the interface between ecology and evolutionary genetics in nematodes

Human activities increasingly affect the natural environment. Consequently, many organisms are confronted with environmental conditions different from the ones they evolved in and experience stress. Environmental stress affects various levels of biological organization (e.g. cell, life-histories, population, community, ecosystem) and concerns various time-spans. Therefore, to understand the mechanisms behind stress responses and the consequences for natural systems requires incorporation of multiple scientific disciplines.In this thesis I incorporate the frameworks of ecology and evolutionary genetics to investigate the effects of long-term stress on natural populations. In addition, I provide the resources to study genetic basis of stress-related traits within QTL approach. Nematodes were selected as the research subjects because of their wide distribution, importance for major ecological processes, such as decomposition, and the ease of laboratory handlings and genetic analysis.The major part of the presented research focused on the long-term effects of copper and pH stress on natural populations of the parthenogenetic nematode Acrobeloides nanus . Chapters 2, 3 and 4 present an extensive analysis of the populations from an experimental field where copper and pH treatments were applied in a factorial design approximately 20 years ago. At first, I considered effects of the combined stressors on population biomass and functional parameters such as secondary production and biomass turnover rate. Despite the fact that multiple anthropogenic stressors are common in human-dominated environment, the knowledge of their possible synergistic effects on population-level parameters is very limited. The investigation reported in chapter 2 is possibly the first study using a controlled experimental system in the field to disentangle the interactive effects of stressors on population parameters. The results indicated negative synergistic effects of high copper level and low pH on population secondary production and biomass turnover in A. nanus . Surprisingly, the biomass of A. nanus showed no negative effects under the same conditions. Overall, these findings demonstrated the impact of interactive stress effects and suggested that functional population parameters might be more sensitive to combination of stressors than the measures such as biomass or population abundance. Consequently, I concluded that realistic risk assessment would benefit from the analyses of combined stressors effects and incorporation of functional population measures. In chapter 3, I focused on the question whether the observed changes in life-history traits in A. nanus were the result of adaptation to local copper and pH conditions. Reciprocal transplant- and reaction norm experiments indicated that the populations from two extreme stress treatments underwent adaptive divergence within approximately 20 years of exposure. I interpreted these results from the perspective of evolutionary biology of asexual species and concluded that they contradict the general view of low adaptive potential of asexual eukaryotes. They also demonstrate that the population structure and distribution of asexual species might be shaped by local adaptation events. In chapter 4, I tested whether the adaptive changes in life-history traits of A. nanus were accompanied by underlying changes in genetic architecture represented by genetic variance-covariance matrix ( G ). Within evolutionary genetics, the concept of G matrix is central as it provides the framework for the analysis of phenotypic evolution under the assumption of G stability over time. It is acknowledged that genetic drift or strong selection such as stress might lead to evolution of G matrices, however the time-spans and conditions promoting these changes in natural populations remain unknown. The comparisons of G matrices for life-history traits of A. nanus indicated a profound divergence of G structure as a response to divergent selection imposed by field treatments and pointed to a less distinct divergence of G matrices within the treatments that are likely to be attributed to drift. Because all the detected changes took place within 20 years of the existence of the experimental field, I concluded that genetic architecture might evolve rapidly in natural populations. These results suggest also that strong stress might enhance this process and that the observed high dynamics of G structure is likely to represent a general feature of asexual species.The second part of the presented research concerned the genetic bases of stress-related and other complex traits in Caenorhabditis elegans . Although there is a growing interest for incorporating QTL methods to the mainstream genetic approaches in this model species, the lack of powerful resources hampers the advancements within this field. In chapter 5, I reported the construction of a permanent, genome-wide library of near-isogenic lines (NILs) from two parental lines, the Hawaiian line CB4856 and theBristolline N2. All of the 91 NILs have a single, short and homozygous segment of the CB4856 genome introgressed into N2 background and in total the introgressed segments cover at least 95% of the genome length. In the same chapter, I presented the analysis of one of the stress-related traits in C. elegans , clumping behaviour, using both recombinant inbred lines (RILs) and selected NILs which resulted in detection of a novel locus responsible for natural phenotypic variation in this trait. Overall, I concluded that the properties of this library allow for more efficient and accurate QTL localization and facilitate gene identification.In chapter 6, I discussed the most relevant findings reported in this thesis and considered them in the context of fundamental problems (evolution of sexual reproduction) and applied issues (the use of clonal organisms for toxicity testing). I devoted also one section of this chapter to discuss the role of QTL mapping in stress-related research. Finally, I made some suggestions regarding future research directions and risk assessment strategies