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Published on: July 3, 2016
Estimating the evolutionary fitness of specific synonymous codon changes
Vitor A C Pavinato1, Jody Hey1
1Department of Biology, Temple University, Philadelphia, PA 19122.
Molecular Biology and Evolution
|August 11, 2026
Summary
Natural selection on synonymous mutations in Drosophila melanogaster is weak, with most codon changes showing minimal impact. This study introduces a new method to accurately predict codon usage and gene expression based on these findings.
Area of Science:
- Evolutionary Biology
- Genetics
- Molecular Biology
Background:
- Synonymous mutations, which do not alter protein sequences, are known to be under natural selection in various species.
- Previous studies on selection strength for synonymous mutations in Drosophila melanogaster have yielded conflicting results, ranging from undetectable to strong selection.
- Understanding the forces shaping synonymous codon usage is crucial for deciphering genome evolution and gene expression regulation.
Purpose of the Study:
- To develop and apply a novel method for estimating the population selection coefficient (2Ns) for synonymous codon changes in Drosophila melanogaster.
- To investigate the strength and patterns of natural selection acting on synonymous codons.
- To assess the relationship between synonymous codon fitness, codon usage, gene expression, and mRNA secondary structure stabilization.
Main Methods:
- Utilized ratios of site frequency spectra for synonymous codon changes and matched neutral changes to estimate the population selection coefficient (2Ns).
- Avoided reliance on divergence data or codon frequencies, making the method less sensitive to demographic history.
- Employed a selection-mutation-drift model to predict codon usage based on estimated codon fitness.
Main Results:
- Natural selection on synonymous codons in Drosophila melanogaster is generally weak, with |2Ns| < 2.30 for all codon pairs and |2Ns| < 1 for 49% of changes.
- Estimated codon fitness strongly correlates with observed codon frequencies and accurately predicts codon usage when incorporated into a selection-mutation-drift model.
- Higher-fitness codons are preferentially used in highly expressed genes, although the effect of expression level on selection strength is modest.
- Evidence suggests selection favors codon changes that stabilize mRNA secondary structure.
Conclusions:
- The novel polymorphism-based approach provides a robust framework for understanding natural selection on synonymous sites.
- Synonymous codon usage is shaped by a combination of selection, mutation, and genetic drift, with selection playing a significant role.
- Selection on synonymous codons is influenced by gene expression levels and the stabilization of mRNA secondary structures.
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