Abstract
Transmissible cancers are a rare phenomenon where cancer cells are transmitted from one individual to another. Even though there are currently only fourteen known cases of transmissible cancers in natural environments, their ecological and evolutionary impacts are significant for both host species and ecosystems. Given the lack of knowledge surrounding these transmissible cancers, this thesis focuses on the conditions for the emergence and spread of these cancers, the influence of environmental variables affected by human activities on these dynamics, and their consequences for hosts and ecosystems.The first task was to complete the conceptual model of the « Perfect Storm », which theorizes the conditions for the emergence of transmissible cancers, by adding a second barrier to the first barrier of acquiring transmissibility. This second barrier consists of environmental factors that can influence their spread within host populations. Indeed, abiotic and biotic variables can affect transmissible cancer cells and/or hosts infected by these cells, thus influencing their dissemination in host populations. This work also suggests that human activities can have a significant impact on the emergence and spread of transmissible cancers by altering variables affecting both barriers.To go beyond a conceptual model, laboratory experiments were conducted using the model organism Hydra oligactis and/or the zebrafish Danio rerio. An experimental evolution study of tumors in Hydra oligactis revealed that the first barrier of the Perfect Storm (i.e., acquisition of transmissibility) does not seem to be a major obstacle since some spontaneous tumors are transmissible from their onset. Therefore, the rarity of transmissible cancers may, for certain species, be more due to the difficulty of overcoming the second barrier related to environmental conditions influencing the survival of transmissible tumor cells and/or their host. A third study showed that food availability could influence the emergence and spread of transmissible cancers by promoting the appearance and progression of tumors when it is increased.Finally, this thesis addresses the evolutionary and ecological consequences of transmissible cancers for their hosts by showing that the presence of these tumor cells in hosts is not insignificant, both for the hosts and surrounding species. Their presence, on the one hand, induces changes in the host’s life-history traits, which mitigate the cost imposed by the tumor, and on the other hand, makes the host more susceptible to becoming a super-spreader of symbionts, thus impacting pathogen dynamics. Understanding these impacts is crucial for assessing the risks these cancers pose to biodiversity and ecosystem health, particularly due to oncogenic pollution of anthropogenic origin.In conclusion, this thesis provides new information on the earliest stages governing the appearance of transmissible tumors and the conditions necessary for their propagation. This work also connects some fundamental aspects of cancer biology to ecological and evolutionary dynamics, highlighting the increasing importance of this research in the context of global and anthropogenic changes.