Worms under stress: unravelling genetic complex traits through perturbation

The genetic architecture of an organism could be considered ‘the most amazing piece of engineering’ existing in nature. Looking from a certain distance, the genetic complexity of an organism could be described as an immense jigsaw puzzle. As in a real jigsaw, the connection between two pieces will suggest or bring us to the third one and so on. The genetic responses to perturbation reveal interactions between many alleles of which effects are not that noticeable under optimal conditions. In the general introduction of this thesis (chapter 1), the reader can find a brief overview of the model organism<em>Caenorhabditis elegans </em>and how this nematode has increasingly taken a crucial position in relevant biological studies over the last few decades. Moreover, the outline of this thesis is extensively described in this section. My thesis brings together examples of perturbation in <em>C. elegans</em> at three different levels: phenotypic perturbation, as a result of genetic variation in Recombinant Inbred Lines (RILs), genetic perturbation, performed by genome combinations (RILs), introgressed fragments of exogenous genome in introgression lines (ILs) and induced mutations, and environmental perturbation (i.e. heat stress conditions). We have combined these perturbations in order to increase the potential of these tools in investigating genetic variation in <em>C. elegans</em>. Our RILs and ILs are combinations of two of the most divergent genomes of <em>C. elegans</em> strains these are Bristol N2 and CB4856 (Hawaii). The translation of this knowledge to research in human disease is of great interest since many complex diseases are regulated by small effect genes that occur during stress or aging. This thesis expounds how <em>C. elegans</em> has gained a prominent position as a model organism for studying the genetic of complex disease pathways. Here I have presented different studies and concluded that environmental stress perturbation experiments in <em>C. elegans</em> represents a crucial implement in the way to increase our understanding of the genetics of human diseases such as Huntington’s disease, Parkinson’s disease, Alzheimer’s disease or cancer. We performed an exhaustive analysis of the current experiments and new insights that used <em>C. elegans</em> as a model organism for complex human diseases. This thesis contains a detailed study where we analyze the past, discuss about the present and suggest new possibilities for the future of <em>C. elegans</em> as a model organism (chapter 2). The experimental sections of the thesis consist of quantitative genetic analyses of genomic regions associated with phenotypic variations observed under a certain perturbation. We use genotypic and phenotypic data measured in our RILs and ILs together with QTL mapping techniques to estimate the position of candidate loci, which are associated with a phenotypic variation. In <em>C. elegans</em>, β-catenin gene<em>bar-1</em>plays an important role in vulva development whereas in humans it plays an important role in cancer progression.In our study in <em>bar-1 </em>mutant RILs, we constructed different combinations of two genomes in <em>C. elegans </em>strains. The lack of β-catenin was included to the combination of the two genomes. These mutant RILs point us to the presence of polymorphic modifiers of vulva development in <em>C. elegans</em> and pave the way towards the development of a new tool to uncover cryptic variation in two genetic variants (chapter 3). To gain insight into the subtle effects of natural variants and more concretely into the genetics of heat-shock recovery, we exposed RILs to heat-stress. We investigated here the hormesis effects on life span and offspring, and described associated loci in <em>C. elegans </em>(chapter 4). Our observations showed that there is natural variation in hormesis effects on life span for heat-shock. As a combination of environmental and genetic perturbation, we studied the effects of heat stress in <em>C. elegans</em> combined with ILs (chapter 5). We demonstrate the importance of the chromosome <em>IV</em> and a concrete locus in heat stress response. In order to identify the role of a locus distal to chromosome<em>IV</em>, we selected a set of ILs containing a fragment of CB4856 (Hawaii) genome introgressed in a N2 (Bristol) background around this candidate locus previously described in other studies. We studied gene expression in normal and heat stress conditions by quantitative RT-PCR. Through this study we are able to confirm the role of the locus distal to chromosom<em>e IV</em> in heat stress response. In the general discussion (chapter 6) I discuss the final results of the thesis and put them into the perspective of the current progresses in genetics studies of <em>C. elegans</em> and how stress response is associated with variations in pathways and thus in cellular and molecular process in health and/or disease. Overall this thesis demonstrates at least a part of the incredible potential of <em>C. elegans</em> as a complex genetic model using quantitative genetic analyses, which complement more established forward genetic screens.