High genetic diversity of bivalve transmissible neoplasia in the blue mussel Mytilus trossulus Gould from the subarctic Sea of Okhotsk

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 cytological and histological techniques 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 have been 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.

Whole-organism responses to constant temperatures do not predict responses to variable temperatures in the ecosystem engineer Mytilus trossulus

Understanding and predicting responses of ectothermic animals to temperature are essential for decision-making and management. The thermal performance curve (TPC), which quantifies the thermal sensitivity of traits such as metabolism, growth and feeding rates in laboratory conditions, is often used to predict responses of wild populations. However, central assumptions of this approach are that TPCs are relatively static between populations and that curves measured under stable temperature conditions can predict performance under variable conditions. We test these assumptions using two latitudinally matched populations of the ecosystem engineer Mytilus trossulus that differ in their experienced temperature variability regime. We acclimated each population in a range of constant or fluctuating temperatures for six weeks and measured a series of both short term (feeding rate, byssal thread production) and long-term (growth, survival) metrics to test the hypothesis that performance in fluctuating temperatures can be predicted from constant temperatures. We find that this was not true for any metric, and that there were important interactions with the population of origin. Our results emphasize that responses to fluctuating conditions are still poorly understood and suggest caution must be taken in the use of TPCs generated under constant temperature conditions for the prediction of wild population responses.

First description of a widespread Mytilus trossulus-derived bivalve transmissible cancer lineage in M. trossulus itself

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.

The root of the transmissible cancer: first description of a widespread Mytilus trossulus-derived cancer lineage in M. trossulus

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.