| Popis: |
Nematodes are an important animal group that have shown remarkable adaptability, leading to their ubiquitous global distribution. Many are also major parasites of humans, plants and animals; therefore an understanding of nematode biology is of great importance both medically and agriculturally. The phylum is divided into three main taxa: the Chromadoria; the Dorylaimia and the Enoplia. Most studies on nematode biology tend to focus on the nematode model, Caenorhabditis elegans or other nematodes within the Chromadorea (clades III, IV and V). To date the Dorylaimia and the Enoplia (nematodes that occupy clades I and II respectively) have been poorly studied, but recent work on the Dorylaim parasite, Trichinella spiralis, is providing valuable data to help understand the evolution of the Nematoda. The recent publication of the T. spiralis genome has revealed nematode-specific features and has allowed for in depth comparative genomic analyses. These analyses have revealed that in some respects – such as highly conserved signalling pathways, T. spiralis seems to more closely resemble the arthropod outgroup than it does to other nematodes within the Chromadorea. I decided to see whether attributes seen in T. spiralis were conserved in other Dorylaimian nematodes, and extended this study to include two additional Dorylaim nematodes, the free-living Prionchulus punctatus and the parasitic Trichuris muris. T. muris was chosen as it shares a relatively recent ancestor with the lineage leading to the Trichinellidae, whereas P. punctatus, whilst still a Dorylaimian, exists outside of this group. We used transcriptome data from P. punctatus, to further our analysis and characterisation of the Wnt, Hedgehog and TGF-β signalling pathways and show that like T. spiralis, this nematode also has a much less derived set of signaling pathways compared to C. elegans and other members of the Chromadoria. Spliced Leader (SL) trans-splicing is a phenomenon that occurs throughout the nematode phylum; as such it is a trait that was most likely present in the ancestral nematode. Nematodes in the Chromadorea have been shown to use two types of SLs, first characterized in C. elegans; SL1 and SL2. Previous studies have shown that the Dorylaim T. spiralis uses a range of highly polymorphic SL sequences that have only limited similarity to C. elegans SL1 and SL2. In contrast, initial searches for SLs in P. punctatus have shown that it possesses clear SL2-like sequences. I IV investigated the nature of SL trans-splicing within the Dorylaimia. In this study I identified the SL sequences present in T. muris and showed that they are similar to the SLs found in P. punctatus, which is unexpected given that T. muris is more closely related to T. spiralis. This indicates that the complement of SLs found in T. spiralis is derived relative to other nematodes. In C. elegans, it was found that SL trans-splicing is involved in the processing of polycistronic transcription units known as operons, into monocistronic mRNAs. To date, operons have only been seen within Chromadorean nematodes, but the presence of SL trans-splicing in the Dorylaimia implies that they may be present in these nematodes also. This thesis presents, for the first time, evidence for the presence of operons in the two Dorylaimian nematodes; T. spiralis and T. muris. We show that operons are likely to be an ancient feature of the Nematoda, with evidence for a conserved operon that spans throughout different nematode species within the phylum. |
