A leukemia-like cancer of clams has been revealed to be caused by a clonal, transmissible cell line. Metzger et al. discovered that the Steamer retrotransposon is present at high copy number in the leukemic cells, and further investigation revealed that, startlingly, the retrotransposon integration sites were identical across the various samples tested. This and other genetic evidence strongly suggests that the cancer originated in a single organism and has since spread throughout the clam population. This phenomenon was once thought to be rare (only two other examples are known), but may be more common than was realized. -CMJ
Metzger, M et al. 2015.
Horizontal Transmission of Clonal Cancer Cells Causes Leukemia in Soft-Shell Clams. Cell. 161, 255-263. doi:10.1016/j.cell.2015.02.042