NLM DIR Seminar Schedule

Abstract:

Prokaryotes can acquire antivirus immunity via two fundamentally distinct types of processes: direct interaction with the virus as in CRISPR-Cas adaptive immunity systems and horizontal transfer of defense genes which is the main route of transmission of innate immunity systems. These routes of defense evolution are not mutually exclusive and may operate simultaneously, but empirical observations suggest that at least in some bacterial and archaeal species, one or the other dominates the defense landscape. We hypothesized that the observed dichotomy is due to the different life-history tradeoffs characteristic of these organisms. To test this hypothesis, we analyzed a mathematical model of a well-mixed population of prokaryotes under a stochastically changing viral load. Optimization of the long-term growth rate of the population reveals two contrasting modes of defense evolution. In a stable, predictable and fluctuating, unpredictable environments with a moderate viral load, adaptive immunity and horizontal transfer of defense genes become the optimal routes of immunity acquisition, respectively. In the HGT-dominant mode, we observe a universal distribution of the fraction of microbes possessing different immune repertoires. Under very low virus load, the cost of immunity exceeds its benefits such that the optimal state of a prokaryote is carrying no defense systems at all. By contrast, under very high virus load, horizontal spread of defense systems dominates regardless of the stability of the environments. These findings seem to explain some consistently observed but enigmatic patterns in the spread of antivirus defense systems among prokaryotes such as the ubiquity of adaptive immunity in hyperthermophiles contrasting their patchy distribution among mesophiles.