Viral diversity threshold for adaptive immunity in prokaryotes.

Area of Science:

• Microbiology
• Genomics
• Evolutionary Biology

Background:

• Prokaryotes utilize CRISPR-Cas systems for adaptive immunity against viruses and plasmids.
• CRISPR-Cas provides immunological memory by storing DNA fragments of invaders.
• While archaea predominantly have CRISPR-Cas, only about 50% of bacteria possess it.

Purpose of the Study:

• To investigate the reasons behind the lower prevalence of CRISPR-Cas systems in bacteria compared to archaea.
• To determine the influence of environmental factors, specifically temperature and viral mutation rates, on CRISPR-Cas presence.
• To model the coevolution of viruses and CRISPR-Cas systems in prokaryotic populations.

Main Methods:

• Comparative genomics analysis of hundreds of prokaryotic genomes.
• Mathematical modeling of virus-CRISPR coevolution, incorporating fitness costs and benefits.
• Stochastic modeling to simulate competition between CRISPR-Cas-positive and -negative prokaryotes.

Main Results:

• CRISPR-Cas systems are significantly more common in thermophilic prokaryotes than in mesophilic ones.
• Higher mutation rates in mesophilic prokaryotes correlate with lower CRISPR-Cas prevalence.
• Mathematical models predict that high viral mutation rates can lead to the purging of CRISPR-Cas systems from host populations.

Conclusions:

• Environmental temperature, rather than prokaryotic taxonomy, appears to be a key factor in CRISPR-Cas distribution.
• An accelerated viral mutation rate can exceed the adaptive capacity of CRISPR-Cas systems, leading to their loss.
• This viral diversity hypothesis offers a potential explanation for the reduced prevalence of CRISPR-Cas in mesophilic bacteria.