Overfed Hydras Can Develop Transmissible Tumours

Jellyfish-like animals known as hydras can grow tumours when they are overfed, and these tumours may then be inherited by their clonal offspring.

New research into this unusual process both explains more about how it works in hydras and broadens our knowledge of transmissible forms of cancer generally.

Transmissible cancer in hydras

Although tumours are an unavoidable hazard of multicellular life, cancers passed between individuals are fortunately rare. The best-known examples are the two cancers affecting Tasmanian devils; another occurs in dogs, while 11 have been recorded in bivalves.

Naturally, scientists want to establish how contagious cancers first arise. This could help prevent such cancers from somehow emerging in humans, while also offering hope of protecting other species from the severe harm caused by Tasmanian devil facial tumour disease.

However, every previously recognised example of transmissible cancer was already firmly established when it was discovered. To investigate how contagious cancers begin, evolutionary ecologist Sophie Tissot of the French National Centre for Scientific Research (CNRS) and her colleagues therefore needed a different method.

Fortunately, a freshwater relative of jellyfish and sea anemones had spontaneously formed tumours during a laboratory experiment 15 years earlier after being overfed.

Hydra oligactis is regarded as ‘immortal’ in several respects. It reproduces asexually by producing small buds, which detach and mature into physically separate animals that are genetically identical to the parent.

Its tendency to develop tumours in laboratory settings, combined with this cloning process, makes it a promising organism for examining the genetics behind cancer development. Tissot’s team aimed to demonstrate how useful this model could be for studying the evolution of transmissible cancers.

"Using Hydra oligactis, which exhibits spontaneous tumor development that in some strains became vertically transmitted, this study presents the first experimental observation of the evolution of a transmissible tumor," the authors write.

"This work, therefore, makes the first contribution to understanding the conditions of transmissible cancer emergence and their short-term consequences for the host."

How overfeeding produced hydra tumours

The researchers collected 50 hydras from Lake Montaud in France and established them in the laboratory.

To ensure high rates of tumour growth and budding, and therefore improve the likelihood of tumour transmission, they fed some of the polyps excessive quantities of brine shrimp larvae five times weekly. This recreated the conditions that had triggered tumour growth in the earlier experiment.

After two months, 19 overfed hydras had become swollen with tumours. The team selected these animals, collected their buds and raised those buds under identical conditions. They repeated the procedure across five ‘generations’ of clonal, tumour-forming buds, choosing affected buds rather than tumour-free peers to produce the following generation.

To confirm that tumours were inherited from parents rather than independently developing in every generation, the researchers also examined the descendants of cancer-free hydras. Despite all hydras being genetically identical, offspring of parents with tumours were four times more likely to develop tumours than offspring whose parents were tumour-free.

Across the experiment, the team verified that tumours can be induced in H. oligactis and that the rate of transmission can rise over time.

Effects on hydra reproduction and survival

By the fifth generation, hydras carrying transmissible tumours had begun to display altered life-history traits compared with tumour-free hydras. Before tumours could develop, they devoted more effort to asexual reproduction; once tumours appeared, however, budding slowed.

This shift appeared to coincide with another effect: bud mortality increased after tumours emerged.

"These modifications suggest an adjustment of life-history traits of the host to offset the tumor's costs by producing more buds when they are more likely to survive and remain tumor-free," Tissot and colleagues write.

The researchers suggest that the rarity of transmissible cancers may result more from a shortage of environmental conditions that allow them to spread. In this study at least, tumour cells seemed to have little difficulty acquiring transmissibility.

If that is the case, they conclude, "it is crucial to consider these aspects in the study of ecosystems disturbed by human activities, as they could potentially modify the conditions that favor the spread of transmissible cancers."

The research was published in Proceedings of the Royal Society B.