Insertion-sequence-mediated mutations both promote and constrain evolvability during a long-term experiment with...

Area of Science:

• Microbiology
• Evolutionary Biology
• Genetics

Background:

• Insertion sequences (IS) are mobile genetic elements found in bacteria.
• Their long-term impact on bacterial evolution and fitness is not fully understood.
• It remains unclear if IS elements act as genomic parasites or drivers of adaptation.

Purpose of the Study:

• Investigate the dynamics of IS elements in Escherichia coli over 50,000 generations.
• Determine the contribution of IS elements to genomic evolution and bacterial fitness.
• Assess whether IS elements promote or constrain bacterial evolvability.

Main Methods:

• Long-term experimental evolution of Escherichia coli populations.
• Monitoring of IS element dynamics and copy number.
• Analysis of mutation frequencies and their impact on fitness.

Main Results:

• IS elements accounted for ~35% of high-frequency mutations in ancestral rate populations.
• IS-mediated mutations were less frequent in mutator populations.
• One population showed an IS150 expansion linked to early beneficial mutations but later experienced reduced IS activity and lower fitness.
• Higher IS activity generally became detrimental to adaptation over evolutionary time.

Conclusions:

• IS-mediated mutations can both facilitate and limit bacterial adaptation.
• IS elements play a complex role in bacterial evolution, influencing evolvability.
• The balance of IS activity is crucial for long-term bacterial fitness and adaptation.