Alternatively and constitutively spliced exons are subject to different evolutionary forces.

Area of Science:

• Evolutionary biology
• Molecular evolution
• Genomics

Background:

• Debate exists on whether alternatively spliced exons (ASEs) evolve faster than constitutively spliced exons (CSEs).
• Previous studies show conflicting evidence regarding the evolutionary rates of ASEs versus CSEs.
• ASEs are generally considered under weaker selective constraints than CSEs.

Purpose of the Study:

• To investigate the evolutionary rates of alternatively spliced exons (ASEs) compared to constitutively spliced exons (CSEs).
• To determine whether ASEs or CSEs exhibit faster evolution at both amino acid and synonymous substitution levels.
• To explore potential mechanisms driving observed evolutionary rate differences.

Main Methods:

• Retrieved over 5,000 human-mouse orthologous exons.
• Calculated synonymous (KS) and nonsynonymous (KA) substitution rates for these exons.
• Compared evolutionary rates between ASEs and CSEs across different mammalian species.

Main Results:

• ASEs exhibit higher nonsynonymous (KA) values and KA/KS ratios than CSEs, indicating faster amino acid evolution.
• The majority of ASEs show lower synonymous (KS) values than CSEs, approaching neutral rates.
• CSEs display accelerated synonymous substitution rates, potentially linked to codon usage bias.
• Observed trends were consistent across human-rat and mouse-rat comparisons.

Conclusions:

• ASEs evolve faster at the amino acid level, likely due to weaker selective constraints and faster functional adaptation.
• CSEs show accelerated synonymous substitution rates, suggesting distinct evolutionary pressures.
• Findings are robust across different mammalian species, supporting broad applicability.