Transmissible cancer: the evolution of interindividual metastasis

Although transmissible cancers have been documented under laboratory conditions, naturally occurring transmissible contagious cancers with no underlying pathogen infections are rare in the wild. So far only three transmissible cancers have been identified. Here we review the biology of transmissible cancers observed under laboratory conditions and in the wild and discuss the evolutionary and ecological impact of the latter. We propose and discuss several key factors that are necessary for malignant cell transmission and successful establishment in the novel host environment, such as the ability to bypass/downregulate histocompatibility barriers, propagule number, phenotypic plasticity, as well as permissive host environment. We suggest a scenario in which transmissible malignant allografts engage in an evolutionary tug-of-war with their host similar to that observed in host–parasite interactions. Our analyses show that despite of their rarity, transmissible cancers provide fascinating examples of cancer evolution and ecology, which may have a major impact on the ecology and evolution of the affected host species as well as on the ecosystem in which it lives.