Ecology of vertical tumor transmission in the freshwater cnidarian Hydra oligactis

Transmissible tumors are increasingly regarded as a new form of parasitic life, but relatively little is known about the ecology and evolution of their interactions with their host. In this work, we provide new insights into transmission dynamics of vertically transmitted tumors in the freshwater cnidarian Hydra oligactis. First, we found tumoral hydra to be infectious at any age, regardless of whether they were in their asymptomatic or symptomatic phases, with the bacteriome composition remaining constant during both phases. Interestingly, tumor transmission increased with the number of tentacles, particularly for hydras with supernumerary tentacles. Additionally, tumors developed earlier in the offspring from parents with more advanced tumors. Furthermore, despite being direct descendants of tumoral polyps, some hydras never developed tumoral phenotype. The latter exhibited a distinct bacteriome composition, reduced lifespan and a lower tentacle number increase over time. Interestingly, the tumor phenotype expression in these hydras appears to be able to skip generations, as transmission occurred at any age from parents to offspring. We discuss these results in the context of current knowledge on the evolutionary ecology of host-transmissible tumor interactions as well as parasite-host interactions and suggest avenues for further research.