Mitochondrial genome sequencing of marine leukemias reveals cancer contagion between clam species in the Seas of Southern Europe

Clonally transmissible cancers are tumour lineages that are transmitted
between individuals via the transfer of living cancer cells. In marine
bivalves, leukemia-like transmissible cancers, called hemic neoplasias,
have demonstrated the ability to infect individuals from different
species. We performed whole-genome sequencing in eight V. verrucosa clams
that were diagnosed with hemic neoplasia, from two sampling points located
more than 1,000 nautical miles away in the Atlantic Ocean and the
Mediterranean Sea Coasts of Spain. Mitochondrial genome sequencing of
tumour tissues from neoplastic animals revealed the coexistence of
haplotypes from two different clam species. Phylogenies estimated from
mitochondrial and nuclear markers confirmed this leukemia originated in C.
gallina (or a closely related taxa) and was later transmitted to V.
verrucosa, in which it survived as a contagious cancer. The analysis of
mitochondrial and nuclear gene sequences supports all the studied tumours
belonging to a single neoplastic C. gallina lineage that spread in the
Seas of Southern Europe.