Molecular diversity of bivalve transmissible neoplasia of blue mussels in the Kola Bay (Barents Sea) indicates a recent migration of the cancer lineages between the North Pacific and Northern Europe

Bivalve transmissible neoplasia (BTN) is a leukemia-like cancer “metastasizing” by transmission of living cancer cells between molluscs. Blue mussels harbor two evolutionary lineages of BTN, Mtr BTN1 and Mtr BTN2, both derived from Mytilus trossulus . While Mtr BTN1 has been found only in M. trossulus in North Pacific, Mtr BTN2 parasitizes different Mytilus species worldwide, particularly in Western Europe. No targeted studies of BTN in Northern European mussels ( M. edulis , M. trossulus ) have been made. We searched for BTN in mussels from the Kola Bay (Barents Sea) with the help of flow cytometry of the hemolymph, qPCR with primers specific to cancer-associated alleles and sequencing of mitochondrial and nuclear loci. The species of the mussel hosts was ascertained genetically. Both Mtr BTN1 and Mtr BTN2 were present in our material, though their prevalence was low (∼0.4%). The only instance of Mtr BTN2 was found in M. trossulus . Mtr BTN1 occurred in M. trossulus and in a hybrid between M. trossulus and M. edulis . This finding indicates that Mtr BTN1 may potentially infect the latter species. The mtDNA haplotypes found in both lineages were nearly identical to those known from the North Pacific, but not from elsewhere. Our results suggest that they arrived in the Kola Bay fairly recently, probably with the maritime transport along the Northern Sea Route, and that the invasion was independent of that in Western Europe. A relatively young evolutionary age of Mtr BTN1 seems to suggest that it is an emerging disease in the process of niche expansion. ### Competing Interest Statement The authors have declared no competing interest.