Complex selection on 5' splice sites in intron-rich organisms

Area of Science:

• Genomics
• Molecular Evolution
• Bioinformatics

Background:

• Eukaryotic genomes possess complex structures, including introns and repetitive elements, unlike streamlined prokaryotic genomes.
• The persistence of these seemingly suboptimal genomic features is debated, with hypotheses including optimality, neutral variation, or weak selection.
• Investigating splice sites offers a unique window into the evolutionary pressures shaping intron persistence.

Purpose of the Study:

• To test hypotheses explaining the maintenance of suboptimal genomic structures in eukaryotes.
• To analyze the evolutionary dynamics of 5' splice sites in intron-rich species.
• To determine the role of selection in maintaining intron boundary sequences.

Main Methods:

• Comparative analysis of 5' splice site sequences across three groups of closely related intron-rich species: primates, Drosophila, and fungi.
• Quantification of evolutionary rates of nucleotide changes at splice sites.
• Statistical assessment of selection pressures acting on consensus-to-variant and variant-to-consensus nucleotide substitutions.

Main Results:

• Intron-rich species exhibit less conserved 5' splice site consensus sequences compared to intron-poor species.
• Selection generally disfavors nucleotide changes from consensus to variant at 5' splice sites.
• Nucleotide changes from variant to consensus at 5' splice sites are not significantly favored or disfavored by selection.

Conclusions:

• The evolutionary pattern of 5' splice sites suggests that while changes away from consensus are selected against, changes back to consensus are neutral.
• This pattern is consistent with varying selective pressures across different introns, potentially due to compensatory mechanisms within genes.
• Weak selection may allow the persistence of suboptimal intron boundary sequences in eukaryotic genomes.