Catching cancer in the sea
Like the cancers in Tasmanian devils, transmissible cancers in cockles are spread by living cancer cells, which are passed from one animal to the next. It is thought that sea creatures catch cancer during filter feeding, as free-floating cells in the water pass by.
The cancers that infect cockles are termed bivalve transmissible neoplasia (BTN). The cancer cells accumulate in the animals’ fluids and organs, causing a disease much like leukaemia. Infection is usually fatal. Eight BTNs have been identified so far. One, in Cerastoderma edule, the common cockle, was first seen 40 years ago in Ireland.
Researchers at Universidade de Santiago de Compostela in Spain and the Sanger Institute studied BTN in common cockles, unravelling its genome and evolution for the first time.
Like in the Tasmanian devil study, the team first determined the reference genome sequence of the cockle species. The researchers then collected thousands of specimens at 36 locations from 11 countries, along the Atlantic coasts of Europe and North Africa.
Images L-R, Genomes and disease lab at CiMUS from Universidade de Santiago de Compostela, Spain, collecting cockles in Noia, Spain and cockle in seawater in the laboratory. Image credits: Alicia L. Bruzos.
The team then sequenced cancer cells from 61 animals with the disease. They compared the data and catalogued the genomic variations in the cancers.
Combining methods including microscopy, sequencing of whole genomes and sequencing the active genes, their study illuminates the evolutionary history of marine leukaemia.
While it is not possible to precisely estimate the age of cockle BTN, their work suggests that these cancers emerged centuries or even millennia ago.
Chiming with previous research, they found that the BTN tumour genomes are highly unstable. The number and size of chromosomes varied between the different samples of the same cancer. The team showed that this instability was probably activated by a whole-genome duplication, early in the cancer’s evolution. There is also evidence that the cells chromosome structure continues to be unstable during cell division.
This instability is in stark contrast with the transmissible cancers in Tasmanian devils, which show a stable genome in terms of chromosome number over time. The researchers conclude that a stable genome is probably not needed for the long-term survival of transmissible cancers.