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| The arms race between pathogens and hosts is a major evolutionary force. In this race, hosts have developed three qualitatively different types of defense mechanisms: (i) innate immunity systems, which recognize conserved pathogen features and evolve in a Darwinian way; (ii) adaptive immunity systems, which evolve during the life of the host and provide enhanced efficiency; (iii) host suicide, which prevents further spreading of the infection. Remarkably, these three mechanisms can be found in very different organisms, both unicellular and pluricellular. Moreover, defense mechanisms are usually found in association, e.g. innate and adaptive immune systems in vertebrates or CRISPR-Cas and cell suicide systems in prokaryotes.
This contribution presents a computational model aimed at understanding which types of defense mechanisms evolve under distinct ecological conditions. Variable efficiencies and costs for innate immunity, adaptive immunity and suicide were considered, as well as multiple spatial structures for the host population and pathogen diffusion rates. Simulations reveal a collection of evolutionary behaviors characterized by the acquisition of one, two or all three classes of defense mechanisms. Moreover, they show that population structure is critical for the evolution of cell suicide. All these hints help us understand the distribution of defense mechanisms observed in nature. |
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