Relaxed natural selection alone does not permit transposable element expansion within 4,000 generations in...

Area of Science:

• Microbiology
• Genomics
• Evolutionary Biology

Background:

• Insertion sequences (ISs) are mobile genetic elements found in bacterial genomes.
• ISs are common in bacteria but rare in free-living species due to negative fitness effects.
• ISs frequently proliferate in intracellular bacteria, influencing genomic evolution.

Purpose of the Study:

• To test hypotheses that IS expansion in intracellular bacteria is driven by relaxed natural selection.
• To investigate the roles of small effective population sizes (N(e)) and nutrient-rich environments in IS proliferation.
• To understand the genomic evolution of bacteria under different selective pressures.

Main Methods:

• A laboratory evolution experiment using Escherichia coli K-12.
• Propagation of 12 bacterial populations over 4,000 generations.
• Exposure to four conditions: large vs. small N(e) and nutrient-rich vs. minimal medium.

Main Results:

• Relaxed natural selection, whether due to small N(e) or nutrient-rich conditions, was insufficient to promote IS element expansion.
• No IS element expansion was observed in any experimental population under the tested conditions.
• These findings challenge the hypothesis that relaxed selection alone drives IS proliferation.

Conclusions:

• Relaxed selection is not sufficient to explain IS element expansion in bacteria.
• IS expansion in intracellular bacteria may require enhanced transposition rates, possibly triggered by environmental stress.
• Future research should explore the interplay between transposition rates, environmental factors, and host genetics in IS dynamics.