Overfed Hydras Can Pass Tumours to Their Clone Offspring

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

A study of this unusual process has both uncovered details of how it occurs in hydras and expanded understanding of transmissible forms of cancer more broadly.

Although tumours are an unavoidable risk of multicellular life, only a small number of cancers are known to spread between individuals. The best-known examples are two cancers affecting the Tasmanian devil; another occurs in dogs, while 11 have been recorded in bivalves.

Researchers want to establish how contagious cancers arise, both to prevent such a phenomenon from somehow emerging in humans and to help safeguard other animals from the severe suffering caused by Tasmanian devil facial tumour disease.

Hydras offer a model for transmissible cancer

However, every previously known case 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 another 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 a freshwater creature related to jellyfish and sea anemones, regarded as ‘immortal’ in many respects. It reproduces asexually by making small buds, which detach and develop into physically separate yet genetically identical animals.

Its tendency to produce tumours in laboratory conditions, combined with this cloning process, makes it promising for studying the genetics of cancer formation. Tissot and her team aimed to demonstrate how useful this model could be for examining 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.”

The team gathered 50 hydras from Lake Montaud in France and established them in laboratory conditions.

Overfeeding triggered tumours in Hydra oligactis

To ensure a high rate of tumour formation and budding-and consequently improve the prospects of tumour transmission-the researchers fed some polyps excessive quantities of brine shrimp larvae five times a week. This replicated the conditions that had caused tumour development in the earlier experiment.

After two months, the researchers selected 19 overfed hydras swollen with tumours. They collected the animals’ buds and raised them in identical conditions. This was continued across five ‘generations’ of clonal buds that formed tumours, with buds from affected individuals selected over their tumour-free peers to produce the subsequent generation.

To establish that tumours were inherited from parental hydras rather than simply developing independently in every generation, the researchers also examined descendants of cancer-free animals. 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 confirmed that tumours can be induced in H. oligactis and that their transmission rate can rise over time.

Clonal offspring changed their reproductive behaviour

By the fifth generation, hydras with transmissible tumours started displaying altered life-history traits relative to tumour-free counterparts. Before tumours could form, they devoted more effort to asexual reproduction; once tumours appeared, however, budding slowed.

This appeared to coincide with another shift: bud mortality became higher following the onset of tumours.

“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 transmissible cancers may be rare chiefly because environmental conditions suitable for their spread are uncommon. In this study, at least, tumour cells appeared to have little difficulty acquiring transmissibility.

If that is correct, 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.