Two lineages of bivalve transmissible neoplasia (BTN), BTN1 and BTN2, are known in blue mussels Mytilus. Both lineages derive from the Pacific mussel M. trossulus and are identified primarily by their unique genotypes of the nuclear gene EF1α. BTN1 is found in populations of M. trossulus from the Northeast Pacific, while BTN2 has been detected in populations of other Mytilus species worldwide but not in M. trossulus itself. Here we examined M. trossulus from the Sea of Japan (Northwest Pacific) for the presence of BTN. Using hemocytology and flow cytometry of the hemolymph, we confirmed the presence of disseminated neoplasia in our specimens. Cancerous mussels possessed the BTN2 EF1α genotype and two mitochondrial haplotypes with different recombinant control regions, similar to that of common BTN2 lineages. This is the first report of BTN2 in its original host species M. trossulus. A comparison of all available BTN and M. trossulus COI sequences suggests a common and recent origin of BTN2 diversity in populations of M. trossulus outside the Northeast Pacific, possibly in the Northwest Pacific.

Integrins play a key role in the intermediation and coordination between cells and extracellular matrix components. In this study, we first determined the presence of the b integrin-like protein and its presumptive ligand, fibronectin-like protein, during development and in some adult tissues of the bivalve mollusc Mytilus trossulus. We found that b integrin-like protein expression correlated with the development and differentiation of the digestive system in larvae. Besides the presence of b integrin-like protein in the digestive epithelial larval cells, this protein was detected in the hemocytes and some adult tissues of M. trossulus. The fibronectin-like protein was detected firstly at the blastula stage and later, the FN-LP-immunoreactive cells were scattered in the trochophore larvae. The fibronectin-like protein was not expressed in the b integrin-positive cells of either the veliger stage larvae or the adult mussel tissues and the primary hemocyte cell culture. Despite the b integrin-and fibronectin-like proteins being expressed in different cell types of mussel larvae, we do not exclude the possibility of direct interaction between these two proteins during M. trossulus development or in adult tissues.

Using immunofluorescence phenotyping, the expression of αvβ3-like integrin was examined during neuronal and muscle differentiation in cell cultures derived from trochophore larvae of the mussel Mytilus trossulus. We have demonstrated that some mussel cells grown on fibronectin in vitro express the extracellular matrix (ECM) αvβ3 integrin-like receptor. At the same time, the distribution of αvβ3-like integrin is not ubiquitous, i.e. it depends on the cell type and the time of cultivation. Using immunohistochemical staining, we have found that only in some cells this integrin is co-localized with molluscan neuronal markers, neurotransmitters serotonin (5-HT) or Phe-Met-Arg-Phe-NH(2) neuropeptide (FMRFamide), and also with filament actin but not with paramyosin. Although we have previously shown that an integrin-dependent mechanism is involved in cell adhesion and differentiation of muscle cells of Mytilus, in this study, αvβ3-like integrin has not been found to participate in fibronectin adhesion of muscle cells but may be a linking agent between the ECM and the neuron-like cells.

Two lineages of bivalve transmissible neoplasia (BTN), BTN1 and BTN2, are known in blue mussels Mytilus. Both lineages derive from the Pacific mussel M. trossulus and are identified primarily by the unique genotypes of the nuclear gene EF1α. BTN1 is found in populations of M. trossulus from the Northeast Pacific, while BTN2 has been detected in populations of other Mytilus species worldwide but not in M. trossulus itself. The aim of our study was to examine mussels M. trossulus from the Sea of Japan (Northwest Pacific) for the presence of BTN. Using hemocytology and flow cytometry of the hemolymph, we confirmed disseminated neoplasia in our specimens. Cancerous mussels possessed the unique BTN2 EF1α genotype and two mitochondrial haplotypes with different recombinant control regions, similar to that of common BTN2 lineages. This is the first report of BTN2 in its original host species M. trossulus populations in West Pacific may be the birthplace of BTN2 and a natural reservoir where it is maintained and whence it spreads worldwide. A comparison of all available BTN and M. trossulus COI sequences suggests a common and recent, though presumably prehistoric origin of BTN2 diversity in populations of M. trossulus outside the Northeast Pacific.