| The aetiology and the mechanism of haemic neoplasia are mainly unknown but many causative factors are suggested such as pollution by anthropogenic substances, pathogens and genetics. The aim of this thesis is to contribute to a better understanding of haemic neoplasia development in the soft-shell clam (Mya arenaria). First of all, it seems that a transmission between diseased and healthy clams is possible. The aim of the chapter 1 was to determine whether haemic neoplasia can be transmitted between haemic neoplasia-positive clams from North River (Prince Edward Island, Canada) and haemic neoplasia-negative clams from different populations from Eastern Canada: Anse Saint-Etienne (QC), Metis Bay (QC), Havre-aux-Maisons lagoon (QC), Kouchibouguac National Park (NB) and Barasway Bay (NF). The hypothesis for this experiment was the implication of a virus in haemic neoplasia transmission. The results obtained in this experiment showed that after a cohabitation of 62 days, none of the HN-negative clams developed the disease. The fact that no transmission was obtained suggests that a possible virus alone cannot explain the development of HN. However, the action of the virus could be amplified by other factors like pollution by anthropogenics substances. The aim of the chapter 2 was to determine if, under chronic exposure, two major pesticides (chlorothalonil and mancozeb) which are used in potato production could induce haemic neoplasia. For this experiment, the commercial formulation was used. Long term exposure to fungicide Bravo 500RTM did not induce high tetraploidy levels on negative clam from North River and the analysis of the digestive gland and the mantle did not reveal any detectable level of chlorothalonil. In the Manzate 200 DFRTM, some clams revealed high level of tetraploid cells but no difference were observed between the treatments and the control. The analysis of the digestive gland and the mantle for manganese did not highlight any significant difference in tissue concentration. Fungicides tested in this experiment did not induce haemic neoplasia although a strong association was made in literature. At this point, we decide to use gene expression involved in contamination instead of tetraploidy status as an early indicator of cell cycle disruption. The aim of chapter 3 was to investigate the effect of a mixture of benzo [a] pyrene, chlorothalonil and mancozeb on p53, ubiquitin and aryl hydrocarbon receptor gene expression levels. The hypothesis was that carcinogenic compounds (pesticides and polycyclic aromatic hydrocarbon) induced gene expression in haemocytes of M. arenaria . The results showed a significantly high expression of AhR after 72 h of exposure. P53 gene expression seems to be up-regulated by the mixture after 48 h, however not significantly; but the level of p53 mRNA is down-regulated by the xenobiotics between 48 and 72 h after exposure. However, there was no significant difference for ubiquitin gene expression level during exposure, while it followed the p53 gene expression pattern. The results obtained from this thesis support the multifactorial aetiology hypothesis. A potential virus alone and pollution alone cannot explain the development of haemic neoplasia in soft shell clam. If tetraploidy remains effective to define haemic neoplasia population status, our work showed that this indicator becomes less effective in the experimental model that we used. Tetraploidy shows cellular transformation, which precedes molecular events; as a consequence, its interest in the experimental model is limited. In this perspective, it seems that elucidation of the molecular mechanisms involved in haemic neoplasia development could contribute significantly to the disease understanding.;Keywords : Mya arenaria, haemic neoplasia, flow cytometry, chlorothalonil, mancozeb, benzo[a]pyrene, p53, aryl hydrocarbon receptor, ubiquitin, housekeeping genes. |
