The mechanisms behind stress: from populations to genes in nematodes

The increasing presence of abiotic stress factors in ecosystems over the past few decades has become an issue of major concern. The growing awareness of the detrimental effects that processes such as climatic change or chemical contamination can have on ecosystems and the species that inhabit them has been the driving force of research that focus on environmental risk assessment approaches. These approaches are often based on naive and descriptive models, disregarding to a great extent the underlying mechanisms of stress response. In this thesis I have addressed the need for a fundamental understanding of the stress response mechanisms in order to develop new rationales for risk assessment procedures.. I focus on the mechanisms underlying the effect of different abiotic stress factors such as those imposed by toxic compounds and temperature changes on various species of nematodes. Nematodes are a relevant study object to this respect because they play an important role in biological soil processes and they can easily be studied under laboratory conditions. The study cross cuts through a range of different organizational levels; from the level of populations which holds the greatest relation to the protection goal itself (a population or ecosystem), to the most basic level of gene expression where the initial stress response takes place.Whole life cycle toxicity tests were performed using various toxic compounds with nematodes having different life history strategies. A mechanistic model based on the Dynamic Energy Budget (DEB) theory was used to analyze the data. In Chapter 2, the data from two strains of Caenorhabditis elegans with different reproductive strategies (hermaphroditic and sexual), in exposure to pentachlorobenzene and carbendazim, were used for the analysis of the temporal dynamics of a widely used summary statistic in effect assessment; the ECx(where x is a time unit). The analysis revealed that the behavior in time of this parameter depended on the life cycle trait and the characteristics of the compound thus hampering the comparability between different traits, compounds and time points. This suggests that the correct interpretation and comparability of toxicity data generated with current ECx approaches would greatly benefit from the incorporation of ECx or LCx versus time curves, or alternatively focusing on parameters that do not suffer from these problems (i.e. NEC, population growth rate which are time invariant). In Chapter 3 I focused on the differences in the response of diverging life history strategies to a toxic stressor (in this case cadmium), and investigated how these differences are reflected at the level of population growth rate. The results show that certain traits respond differently to stress depending on the life history strategy (hermaphroditic or sexually reproducing C. elegans ). Overall, sexually reproducing individuals appeared to be more sensitive to cadmium than hermaphrodites. The different responses of the two life history strategies were clearly reflected at the population level where sexually reproducing individuals yielded lower populations growth rates than that of hermaphrodites. The differences in the life history strategies of organisms even within one species can therefore lead to very diverse responses implying that toxicity parameters generated for a particular life history scenario might not hold for another. In order to gain further insight into the mechanisms of these toxic stress scenarios, in Chapter 4 another nematode species ( Acrobeloides nanus ) was used to dissect the physiological mode of action of the three previously mentioned compounds, and to model their effects at the population level. The resulting physiological modes of action of these compounds were compared to those observed in C. elegans and it appeared that the modes differed in every case ??? which case??. This indicates that the life history characteristics in each case had a clear influence on the resulting physiological mode of action of the compounds and consequently lead to very different effects on the population growth rates. Classifications of compounds according to their mode of action, currently based mainly on their chemical structure, should also consider that interactions with the life history characteristics of the organism may lead to different mechanisms of toxic action.In Chapter 5 I studied the effects of temperature as the abiotic stress factor at the gene expression level. We used a 79???80 recombinant inbred line (RIL) panel derived from a C. elegans N2 x CB4856 cross to extract RNA from each RIL grown at 16°and 24°C and subsequently hybridized it to whole genome microarrays (approximately 22,000 transcripts). We then used QTL mapping to detect expression linkage patterns across the genome for each temperature. Using different analysis approaches (separate and combined analysis of the temperatures) we found that 28% of the detected expression QTL were temperature sensitive and comprised mainly by trans QTL. These QTLxT interactions were in many cases below detection level in genome wide analyses. These results present the first validation of thephenomenon of " expression linkage plasticity " in response to different environmental conditions, showing that the control of gene expression is sensitive to environmental cues such as temperature changes.