CILIATES AS MODEL SYSTEMS TO STUDY MOLECULAR ADAPTATION AND ENVIRONMENTAL RESPONSES

Ciliates provide optimal model systems to study genome evolution, environmental adaptation and response to stress. Addressing evolution, and adaptation and gene regulation requires access to large sample sizes of genome or transcriptome sequencing. In this context, we analysed the genome and transcriptome of Euplotes focardii, a strictly psychrophilic ciliate isolated from Antarctic seawater samples. Comparative genome analysis of E. focardii and the mesophilic congeneric species E. crassus revealed rapid evolution and unusual plasticity of the programmed +1 ribosomal frameshifting, a standard feature of the genetic code that affects decoding over 3,000 genes in these genomes. Furthermore, approximately 5.5% of the obtained E. focardii genomic contigs were potentially of symbiotic bacterial origin suggesting that different organisms cooperate for environmental adaptation. The sequencing of the transcriptome revealed that the majority of the transcripts correspond to proteins involved in oxidoreductase activity, as reported for Antarctic fishes and krill. These results confirm that a major problem of Antarctic marine organisms is to cope with increased O2 solubility at low temperatures, and suggest that an increased defence against oxidative stress likely constituted an important evolutionary aspect that allowed the adaptation of Antarctic organisms in their oxygen-rich environment. While E. focardii provides a new system for genomic and transcriptomic studies, Tetrahymena thermophila has been largely investigated and Tetrahymena genome and functional genomic databases are available. Therefore, T. thermophila provides an optimal model system for studying molecular bases of environmental responses, since molecular data obtained in different environmental conditions can be easily compared. We recently used T. thermophila to elucidate the environmental effects of silver nanoparticles by analyzing T. thermophila’s gene expression profile after exposure to Collargol (protein-stabilized silver nanoparticles) and comparing with the effect of the soluble silver salt, AgNO3. Currently, silver nanoparticles are increasingly used as biocides and they can affect non-target organisms in the environment. Therefore, understanding the toxicity mechanisms is crucial. We found that genes involved in mRNA splicing and oxidation reduction appear down and up regulated, respectively, only in Collargol treated samples. This research provides evidence that silver nanoparticles might be toxic due to combined effects of soluble silver ions released from the particles and the particles themselves.