Two lineages of bivalve transmissible neoplasia affect the blue musselMytilus trossulusGould in the subarctic Sea of Okhotsk

Abstract

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.

Marine transmissible cancer navigates urbanised waters, threatening to spillover

“…mussels are affected by two BTN lineages known as MtrBTN1 and MtrBTN2 that originate from two different M. trossulus founder hosts. MtrBTN2 is distributed worldwide and has crossed the species barrier several times to infect four Mytilus species: M. trossulus in East Asia [12,13], M. chilensis in South America [9], and M. edulis and M. galloprovincialis in Europe [9,14]. The oldest MtrBTN2 sample is 14 years old.…”

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

“…The only MtrBTN2 found in the Kola Bay bore the mtDNA haplotype nearly identical to a common Northwest Pacific one and noticeably different from the haplotypes of this lineage elsewhere, in particular, in Western Europe (Figure 4). In terms of cell ploidy, MtrBTN2 in the Kola Bay (ploidy 5n) was also similar to that in the Northwest Pacific (range 4.3-5.8n) but not in Western Europe (8.4-11.4n) (Figure 3; Burioli et al, 2019;Skazina et al, 2022).…”

“…In all the following analyses, the procedures were the same as in our previous studies (Skazina et al, 2021(Skazina et al, , 2022, unless indicated otherwise.…”

“…(3) The available, albeit limited, phylogenetic data indicate that MtrBTN1 is evolutionarily young (Yonemitsu et al, 2019; Figure 4, Figure 5). It may be an emerging infection (sensu Morse, 1995) Metzger et al, 2016, Yonemitsu et al, 2019, Skazina et al, 2021, Skazina et al, 2022 S1. IDs of mussels with MtrBTN are in bold.…”

“…Direct COI sequencing ("COI test for heteroplasmy", hereafter referred to as COI-test) is a rapid method of MtrBTN diagnostics suggested by Skazina et al, 2022. Using M. trossulus from the Northwest Pacific, they have shown that heteroplasmy related to MtrBTN infection could be detected and known cancer alleles identified on the high quality chromatograms. In order to take into account both heavily and weakly infected individuals, parallel sequencing of the hemolymph, the tissue most affected by the disease, and the least-affected muscle tissue is recommended (Skazina et al, 2022).…”