There are increasing findings of the bivalve transmissible neoplasia derived from the Pacific mussel Mytilus trossulus (MtrBTN) in populations of different Mytilus species worldwide. The Subarctic is an area where this disease has not yet been sought despite the fact that Mytilus spp. are widespread there, and M. trossulus itself is a boreal species. We used flow cytometry of the hemolymph, hemocytology and histology to diagnose disseminated neoplasia in a sample of M. trossulus from Magadan in the subarctic Sea of Okhotsk. Neoplasia was identified in 11 of 214 mussels studied. Using mtDNA COI sequencing, we revealed genotypes identical or nearly identical to known MtrBTN ones in the hemolymph of most of the diseased mussels. Both MtrBTN evolutionary lineages have been identified, the widespread MtrBTN2, and MtrBTN1, so far only known from M. trossulus in British Columbia on the other side of the Pacific from Magadan. In addition, MtrBTN2 was represented by two common diverged mtDNA haplolineages. These conclusions were confirmed for selected cancerous mussels by molecular cloning of COI and additional nuclear and mtDNA genes. On the background of high genetic diversity, different cancers were similar in terms of ploidy (range 4.0 - 5.8n) and nuclear to cell ratio. Our study provides the first description of neoplasia and MtrBTN in mussels from the Sea of Okhotsk and from the Subarctic, of both MtrBTN1 and MtrBTN2 in the same mussel population, and the first direct comparison between these transmissible cancers.

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In this study, we focus on the specific contribution of β integrin-like protein to adhesion-mediated events in molluscan larval cells in culture that could not have been investigated within the whole animal. An analysis of disturbances to cell-substratum adhesion, caused by the integrin receptor inhibiting Arg-Gly-Asp-Ser (RGDS)-peptide, the Ca(2+)/Mg(2+)-chelators and the stress influence of freezing-thawing, reveals that all these factors resulted in the partial destruction of the integrin-extracellular matrix (ECM) interaction in culture and, in particular, changes in the distribution and relative abundance of β integrin-positive cells. The experiments, carried out on selected substrates, found that β integrin-positive cells demonstrate different affinities for the substrates. This finding further supports the assumption that epithelial differentiation in cultivated cells of larval Mytilus may be mediated by β integrin-like proteins via binding to laminin; direct binding to other components of the ECM could not be demonstrated. The mussel β integrin-positive cells are not involved in myogenic or neuronal differentiation on any of the substrates but part of them has tubulin-positive cilia, forming some epithelia-like structures. Our data indicate that β integrin-positive cells are able to proliferate in vitro which suggests that they could participate in renewing the digestive epithelium in larvae. The findings provide evidence that the distribution pattern of β integrin-like protein depends on the cell type and the factors influencing the adhesion.

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