Putative neoplastic disorders in mussels (Mytilus edulis) from Southern Vancouver Island waters, British Columbia

Traits of a mussel transmissible cancer are reminiscent of a parasitic life style

Some cancers have evolved the ability to spread from host to host by transmission of cancerous cells. These rare biological entities can be considered parasites with a host-related genome. Still, we know little about their specific adaptation to a parasitic lifestyle. MtrBTN2 is one of the few lineages of transmissible cancers known in the animal kingdom. Reported worldwide, MtrBTN2 infects marine mussels. We isolated MtrBTN2 cells circulating in the hemolymph of cancerous mussels and investigated their phenotypic traits. We found that MtrBTN2 cells had remarkable survival capacities in seawater, much higher than normal hemocytes. With almost 100% cell survival over three days, they increase significantly their chances to infect neighboring hosts. MtrBTN2 also triggered an aggressive cancerous process: proliferation in mussels was ~ 17 times higher than normal hemocytes (mean doubling time of ~ 3 days), thereby favoring a rapid increase of intra-host population size. MtrBTN2 appears to induce host castration, thereby favoring resources re-allocation to the parasites and increasing the host carrying capacity. Altogether, our results highlight a series of traits of MtrBTN2 consistent with a marine parasitic lifestyle that may have contributed to the success of its persistence and dissemination in different mussel populations across the globe.

Metal influence on metallothionein synthesis in the hydrothermal vent mussel Bathymodiolus thermophilus

The present study reports on the metallothionein expression in the hydrothermal vent mussel Bathymodiolus thermophilus. Metallothioneins (MT) are proteins involved in intracellular metal regulation and conserved throughout the animal kingdom. The hydrothermal vent environment presents peculiarities (high levels of sulfides and metals, low pH, anoxia) that may have driven associated species to develop original evolutionary ways to face these extreme living conditions. Mussels were exposed to different metal solutions at the atmospheric pressure. The MT mRNA levels and MT contents were measured in gills and mantles of each exposed mussel. The intracellular metal distribution was estimated in fractions obtained after the centrifugation of tissue homogenates. A few of the tested metals (Ag, Cu, Cd, Hg and Zn) were able to significantly induce MT mRNA levels. Silver was the only one that produced a significant increase of the MT protein level in both mantle and gills. The gills always presented higher MT protein levels than the mantle did, while their MT mRNA levels were similar. Our data show that MT mRNA and MT protein levels do not follow a clear relationship in the gills and mantle of B. thermophilus and we assume that a posttranscriptional control occurs in these mussels.

Disseminated neoplasia in blue mussels, Mytilus galloprovincialis, from the Black Sea, Romania

Disseminated neoplasia, also called leukemia or hemic neoplasia, has been detected in 15 species of marine bivalve mollusks worldwide. The disease is characterized by the presence of single anaplastic cells with enlarged nuclei and sometimes frequent mitosis, in hemolymph vessels and sinuses. The neoplastic cells gradually replace normal hemocytes leading to the increased mortality of animals. The neoplasia reaches epizootic prevalences in blue mussels, Mytilus trossulus, in some areas, whereas prevalences in Mytilus edulis are generally very low. Mytilus galloprovincialis was suggested to be resistant to the disease although very low prevalences were documented from Spain in the Atlantic Ocean and Italy in the Mediterranean Sea. A case of disseminated neoplasia was discovered in M. galloprovincialis from among 200 specimens studied from the coast of the Romanian Black Sea. Histological preparation revealed the presence of large anaplastic cells with lobed nuclei. This observation extends the geographic range of marine bivalve mollusks with disseminated neoplasia to include the Black Sea.

The ecotoxicological significance of genotoxicity in marine invertebrates

Attention is drawn to the goals of genetic ecotoxicology, in particular, the need to relate genotoxicity in individuals to population and community level consequences. The evidence for pollutant-induced genotoxicity in marine invertebrates is reviewed. Neoplasia is apparently rare in marine invertebrates and only limited evidence is available to suggest that chemical genotoxins act as causative agents. It is unknown why marine invertebrates exhibit low tumour incidences and are much more tolerant of ionising radiation than their vertebrate counterparts. The importance of the genotoxic disease syndrome is highlighted. Disentangling phenotypic manifestations of genotoxic damage and that due to direct metabolic toxicity provides a major challenge for the future. Further work is required to assess the significance of interspecific and interindividual variability in susceptibility to genotoxicity, especially with regard to the evolution of resistant populations and communities of marine organisms at contaminated sites. Only by addressing the issues highlighted above can proper risk assessments of genotoxic agents be performed to minimise threats to human and ecosystem health.

Alternate pathogenesis of systemic neoplasia in the bivalve mollusc Mytilus

The proliferative disease systemic neoplasia, also termed hemic neoplasia or disseminated sarcoma, was studied in four Puget Sound, Washington populations of the bay mussel (Mytilus sp.). Using flow cytometric measurement of DAPI-stained cells withdrawn from the hemolymph, DNA content frequency histograms were generated for 73 individuals affected by the disease. The cells manifesting systemic neoplasia were found to exist as either of two separate types, characterized by G0G1 phase nuclear DNA contents of either approximately 4.9 x haploid (pentaploid form) or approximately 3.8 x haploid (tetraploid form). The two disease forms were found to coexist in all four mussel populations sampled, with overall relative prevalences of 66% pentaploid form, 29% tetraploid form, and 5% exhibiting both disease forms simultaneously. These findings represent the first unequivocal demonstration of multiple cell types in a bivalve neoplasia. The two forms appear to represent separate pathogenetic processes rather than sequential stages of a single pathogenesis. Two cell cycling parameters associated with proliferative activity were employed to compare the alternate forms: (i) the percentage of cells assigned to the DNA Synthesis (S) phase of the neoplastic cell cycle, and (ii) the proportion of neoplastic cell mitotic figures in hemocytological preparations. Mean values for both parameters were significantly higher for mussels with the tetraploid form of the disease, suggesting a higher rate of proliferation relative to the pentaploid form. Qualitatively, cells of the tetraploid form contained slightly lower nuclear and cytoplasmic volumes compared to those of the pentaploid form. An observed wide variation in neoplastic cell nuclear size within either disease form may reflect the distribution of cells in the G0G1, S, and G2M phases of the cell cycle. Potential etiologic relationships between the two forms are discussed.

Impaired defense mechanisms in bay mussels, Mytilus edulis, with hemic neoplasia

Immunocompetence of bay mussels, Mytilus edulis, with hemic neoplasia was investigated with an in vitro yeast phagocytosis assay and by in vivo clearance from the blood of injected Cytophaga sp. bacteria. The yeast phagocytosis assay was conducted with hemocytes maintained in 90% plasma. Neoplastic hemocytes, characterized by enlarged nuclei and scant cytoplasm, failed to phagocytose yeast cells. In contrast, greater than 90% of hemocytes from unaffected animals and morphologically normal hemocytes from mussels with the disease phagocytosed yeast. Substitution of normal plasma with that from a mussel with advanced disease (essentially 100% neoplastic hemocytes) did not affect the phagocytic capability of normal hemocytes. Conversely, normal plasma did not enhance the phagocytic capabilities of neoplastic cells. Mussels with advanced disease showed reduced bacterial clearance; control or lightly affected mussels (less than 11% neoplastic hemocytes) cleared greater than 90% of injected bacteria in 4 hr, while mussels with advanced disease cleared 44-83%. These experiments indicate that mussels with advanced hemic neoplasia have compromised defense systems. This may account for the reported mortality in mussels and other bivalve molluscs with hemic neoplasia.