Alternative models for the evolution of coupled suicide and immunity. Two types of AI systems are considered: one whose activation can lead to cell death (suicide) with the probability q = 0.5, and another that only works as a standard immune mechanism (with no induction of additional suicide). Mutations between both types occur with a small probability. The figure shows the relative fraction of hosts with suicide-inducer AI in the stationary state (normalized by the fraction of hosts with any kind of AI, the value 0.5 has been assigned wherever hosts with AI represent less than 10% of the population). The evolutionary process was simulated in a structured population with different values of the innate and adaptive immune efficacies (pII and pAI), and a fixed value of the maximum suicide efficacy CD
= 0.3. In A, the maximum suicide efficacy only affects AI-independent suicide; in B, the maximum suicide efficacy limits all kinds of suicide, included suicide induced by AI. Panels C and D correspond to a modified scenario where the efficacy of AI depends on the total degree of suicide according to a logistic formula; in C there is no limitation on AI-induced suicide, whereas in D it is limited by CD
. The red region in panel D (but not in other panels) is also characterized by the absence of AI-independent suicide.