Genetic features of bivalve transmissible neoplasia in blue mussels from the Kola Bay (Barents Sea) suggest a recent trans-Arctic migration of the cancer lineages.

Ecology and biogeography of bivalve transmissible neoplasia (BTN) are underexplored due to its recent discovery and a challenging diagnostics. Blue mussels harbour two evolutionary lineages of BTN, MtrBTN1 and MtrBTN2, both derived from Mytilus trossulus. MtrBTN1 has been found only in M. trossulus from North Pacific. MtrBTN2 parasitizes different Mytilus spp. worldwide. BTN in M. trossulus in the Atlantic sector has never been studied. We looked for BTN in mussels from the Barents Sea using flow cytometry of cells, qPCR with primers specific to cancer-associated alleles and sequencing of mtDNA and nuclear loci. Both MtrBTN1 and MtrBTN2 were present in our material, though their prevalence was low (~0.4%). All cancers parasitized M. trossulus except one, MtrBTN1, which was found in a hybrid between M. trossulus and M. edulis. The mtDNA haplotypes found in both lineages were nearly identical to those known from the Northwest Pacific but not from elsewhere. Our results suggest that these two lineages may have arrived in the Barents Sea in recent decades with the maritime transport along the Northern Sea Route. A young evolutionary age of MtrBTN1 seems to indicate that it is an emerging disease in the process of niche expansion. Comparing the new and the published sequence data on tumour suppressor p53, we proved that the prevalence of BTN in mussels can reach epizootic levels. The finding of diverse recombinants between paternally and maternally inherited mtDNAs in somatic tissues of M. trossulus was an unexpected result of our study.

Species identification based on a semi-diagnostic marker: Evaluation of a simple conchological test for distinguishing blue mussels Mytilus edulis L. and M. trossulus Gould.

Cryptic and hybridizing species may lack diagnostic taxonomic characters leaving researchers with semi-diagnostic ones. Identification based on such characters is probabilistic, the probability of correct identification depending on the species composition in a mixed population. Here we test the possibilities of applying a semi-diagnostic conchological character for distinguishing two cryptic species of blue mussels, Mytilus edulis and M. trossulus. These ecologically, stratigraphically and economically important molluscs co-occur and hybridize in many areas of the North Atlantic and the neighboring Arctic. Any cues for distinguishing them in sympatry without genotyping would save much research effort. Recently these species have been shown to statistically differ in the White Sea, where a simple character of the shell was used to distinguish two mussel morphotypes. In this paper, we analyzed the associations between morphotypes and species-specific genotypes based on an abundant material from the waters of the Kola Peninsula (White Sea, Barents Sea) and a more limited material from Norway, the Baltic Sea, Scotland and the Gulf of Maine. The performance of the "morphotype test" for species identification was formally evaluated using approaches from evidence-based medicine. Interspecific differences in the morphotype frequencies were ubiquitous and unidirectional, but their scale varied geographically (from 75% in the White Sea to 15% in the Baltic Sea). In addition, salinity-related variation of this character within M. edulis was revealed in the Arctic Barents Sea. For every studied region, we established relationships between the proportions of the morphotypes in the populations as well as between the proportions of the morphotypes in samples and the probabilities of mussels of different morphotypes being M. trossulus and M. edulis. We provide recommendations for the application of the morphotype test to mussels from unstudied contact zones and note that they may apply equally well to other taxa identified by semi-diagnostic traits.