Phase distribution of spliceosomal introns: implications for intron origin

Area of Science:

• Molecular Biology
• Evolutionary Biology
• Genomics

Background:

• The origin of spliceosomal introns is debated between the introns-early and introns-late hypotheses.
• Intron phase distribution is non-uniform, with a notable excess of phase-0 introns.
• Introns-early suggests pre-existing introns in the progenote, leading to phase-0 excess.
• Introns-late posits that nonrandom intron insertions explain the observed phase distribution.

Purpose of the Study:

• To test the introns-early versus introns-late hypotheses using intron phase distribution analysis.
• To infer the evolutionary trajectory of intron phase distribution.
• To determine if intron insertions can explain genome-wide intron phase patterns.

Main Methods:

• Analysis of intron phase distribution across 684 gene orthologs from seven eukaryotes.
• Inference of intron phase evolution using a maximum likelihood method.
• Testing intron insertion models against observed intron phase distributions in 10 eukaryotes.

Main Results:

• The inferred evolution of intron phase distribution showed a progressive increase in phase-0 introns.
• This evolutionary trend contradicts the predictions of the introns-early hypothesis.
• Observed intron phase distributions closely matched predictions from an intron insertion model, supporting introns-late.

Conclusions:

• The study provides strong evidence supporting the introns-late hypothesis for the origin of spliceosomal introns.
• The findings indicate that intron phase distribution patterns are a result of nonrandom insertion events during evolution.