Horizontal gene transfer has been long known in viruses and prokaryotes, but its importance in eukaryotes has been only acknowledged recently. Close contact between organisms, as it occurs between pathogens and their hosts, facilitates the occurrence of DNA transfer events. Once inserted in a foreign genome, DNA sequences have sometimes been coopted by pathogens to improve their survival or infectivity, or by hosts to protect themselves against the harm of pathogens. Hence, horizontal transfer constitutes a source of novel sequences that can be adopted to change the host-pathogen interactions. Therefore, horizontal transfer can have an important impact on the coevolution of pathogens and their hosts. 1. Introduction The evolution of pathogens and their hosts is often interpreted as an arms race: while hosts have developed multiple mechanisms to protect themselves, pathogens have generated diverse strategies to evade their hosts’ defenses. But pathogens and hosts have also evolved mechanisms that allow a mutualistic coexistence. During their coevolution, a relationship between pathogen and host has been established based on intricate and specialized molecular interactions. One of the possible outcomes of this long-standing and close relationship is the exchange of genetic material. Horizontal or lateral gene transfer (HT) is the nonsexual movement of genetic information between two organisms . These sequences can be modified and adapted (i.e., coopted, domesticated) during the evolution of the recipient species to improve their own survival. When HT occurs between hosts and their pathogens (in one direction or the other), the acquired sequences can be coopted to affect how the two organisms interact with each other. Pathogens have developed an impressive array of strategies to avoid host defenses, including the interference or disruption of the host defensive mechanisms and signaling cascades. For instance, vertebrate viruses can avoid detection and elimination by the host immune response by obstructing antigen presentation, blocking apoptosis, disrupting complement cascades, and mimicking or modulating cytokines and their receptors, among others [2, 3]. I will discuss how several of these strategies have been achieved through the acquisition and domestication of host genes. Hosts, in turn, can protect themselves from the deleterious effects of infections by two approaches: resistance and tolerance [4, 5]. Resistant traits reduce damage by limiting the pathogen growth and, therefore, the extent of the infection, sometimes eliminating the pathogen
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