Elevated Rates of Molecular Evolution Genome-wide in Mutualist Legumes and Rhizobia

Area of Science:

• Evolutionary Biology
• Molecular Evolution
• Symbiosis Research

Background:

• Antagonistic species interactions are predicted to increase molecular evolution rates.
• The effect of mutualism on evolutionary rates is less clear, with mixed theoretical predictions.
• Legume-Rhizobia symbiosis provides a model system to study mutualistic interactions.

Purpose of the Study:

• To investigate how mutualism influences molecular evolution rates in legumes and rhizobia.
• To compare evolutionary rates between mutualistic and non-mutualistic species and their close relatives.
• To determine if symbiosis-specific genes evolve differently from the rest of the genome.

Main Methods:

• Transcriptome assembly for 12 legume species.
• Calculation of dN/dS ratios for orthologous genes across species.
• Analysis of published bacterial genomic data for rhizobia and related species.
• Comparative analysis of symbiotic and non-symbiotic clades within the Ensifer genus.

Main Results:

• Mutualistic legumes exhibit higher genome-wide molecular evolution rates than non-mutualistic legumes.
• Symbiotic rhizobia generally show higher dN/dS values than their non-symbiotic relatives.
• Symbiotic lineages within Ensifer have elevated genome-wide evolutionary rates, but not for genes on the pSymB plasmid.

Conclusions:

• Mutualism is associated with increased genome-wide molecular evolution in both plants and bacteria.
• Genes directly involved in symbiosis may evolve more slowly, indicating potential evolutionary constraint or stasis.
• Symbiosis can drive overall genomic evolutionary rates while core symbiosis genes remain conserved.