Mémoire
| Résumé : | Genome sizes of eukaryotic organisms vary considerably within species. One might expect that more complex organisms (or what is considered as such) should have larger genomes than less complex organisms. However, no relationship has yet been discovered between the genome size and organismal complexity. There does not seem to be a correlation between genome size and the number of protein-coding genes either. This is known as the ‘C-value paradox’. The discovery of non-coding DNA has explained a part of this apparent paradox was explained because a large genome does not necessarily correlate with a large amount of coding DNA.Gammarid amphipods are a very widely distributed family of crustaceans, occupying a very large variety of habitats. From freshwater, to brackish and marine, they can be found in all types of water habitats. They play a key role in the detritus and the microbial cycle. Becausethey are so widely distributed, they seemed like a good choice as study organism to determine if genome size could be correlating with habitat. For this, species from three habitats (freshwater, brackish water and marine) were collected and/or chosen from an existing collection of preserved specimens and their genome sizes were analysed. Furthermore, their genomes were sequenced and phylogenetic trees were build based on two markers (28S, a nuclear marker, and COI, a mitochondrial marker) in order to understand their relatedness and place their genome sizes in a phylogenetic context.The results of this study indicate that the genome sizes vary among habitats. Brackish water species present smaller genomes than freshwater and marine species. The concatenated phylogenetic tree of both markers supports these results, because genome sizes vary more between clades than among clades.If all this information (genome sizes and phylogenies) are taken into consideration, the following scenario might explain these observations: the gammarid amphipods, originating in a marine habitat, colonised brackish water habitats where selection was stronger and smaller genomes were favoured, leading to a decrease in genome sizes. |
