Evolution of Bivalve Transmissible Cancers

Cancer cells accumulate mutations that allow them to grow uncontrollably and eventually acquire the ability to metastasize, that is, spread to other parts of the body. Transmissible or contagious cancers, which are particularly frequent among bivalves, are large-scale metastases in which the cancer cells spread to other individuals beyond the body from which they originated them (Chapter 1). In common cockles, two phenotypically different contagious cancer lineages have been described by means of nuclear and mitochondrial DNA variation in a single Spanish location. In this thesis, we report the prevalence rates of 36 populations and 6,719 cockles alongside the distribution range of the species, and we unravel and characterize multiple mitochondrial horizontal transfers by studying the evolutionary history of healthy and cancer individuals, further describing various co-infections of two contagious cancer lineages affecting a single individual (Chapter 2). RNA revealed the same histogenesis for two independent cancer lineages pointing to the potential cancer susceptibility of haemolymph (Chapter 3). Finally, to investigate the limits of marine contagious cancers, we collected 345 warty venus clams for which we described a contagious cancer present in two distant locations that originated in a different species, the striped venus clam (Chapter 4). In summary, this doctoral thesis advances in the understanding of bivalve transmissible cancers providing novel insights and a robust evolutionary framework of mitochondrial horizontal transfer.2023-09-0