Adaptive evolution of chloroplast genome structure inferred using a parametric bootstrap approach

Area of Science:

• Genomics
• Molecular Evolution
• Bioinformatics

Background:

• Genome rearrangements impact gene order and cluster configuration across genomes.
• Land plant chloroplast DNAs (cpDNAs) exhibit conserved gene content and order, suggesting selective constraint or slow rearrangement rates.
• The role of natural selection versus random processes in shaping cpDNA gene order remains unclear.

Purpose of the Study:

• Investigate the influence of natural selection on chloroplast genome gene order.
• Test if directional selection drives the clustering of functionally related genes in Chlamydomonas reinhardtii cpDNA.
• Compare observed gene order to predictions from random rearrangement models.

Main Methods:

• Inferred ancestral gene orders.
• Simulated genome rearrangements using a random breakage model with varying inversion and transposition ratios.
• Employed a novel parametric bootstrap approach to assess gene clustering and sidedness.

Main Results:

• Chlamydomonas reinhardtii cpDNA exhibits significantly higher "sidedness" (adjacent genes on the same strand) than random simulations (p < 0.0001).
• Functionally related genes are more clustered in C. reinhardtii than in randomly rearranged genomes (p < 0.0001).
• Evidence suggests co-transcription of neighboring genes contributes to observed gene clusters.

Conclusions:

• Both selective maintenance and directional selection contribute to chloroplast gene order.
• Natural selection plays a significant role in shaping cpDNA gene organization.
• Findings provide insights into the evolutionary forces governing chloroplast genome architecture.