Theory of prokaryotic genome evolution

Area of Science:

• Microbial genomics
• Evolutionary biology
• Population genetics

Background:

• Prokaryotic genomes are typically small and gene-dense.
• Genome compactness is often attributed to adaptive streamlining via purifying selection.
• This streamlining is thought to eliminate nonfunctional DNA due to its energetic cost.

Purpose of the Study:

• To investigate the evolutionary forces shaping microbial genome size.
• To reconcile the observed positive correlation between selection strength and genome size with existing models.
• To determine if genome compactness is solely due to selection against non-functional DNA.

Main Methods:

• Fitting genome size distributions of prokaryotes to mathematical models of population evolution.
• Analyzing the correlation between protein sequence evolution rates (dN/dS) and genome size.
• Comparing model predictions with empirical genomic data.

Main Results:

• A positive correlation exists between protein sequence selection strength and microbial genome size.
• Models where gene acquisition is slightly beneficial best fit genomic data.
• Genome size appears to be an equilibrium between gene acquisition benefits and deletion bias.

Conclusions:

• New genes acquired by prokaryotic genomes are, on average, adaptive.
• Genome size reflects an equilibrium between diminishing gene benefits and deletion bias.
• Tight gene packing results from deletion bias and purifying selection against non-coding DNA.