Selective forces for the origin of spliceosomes

Area of Science:

• Molecular Biology
• Evolutionary Biology
• Genetics

Background:

• The evolutionary origin of eukaryotic spliceosomes remains debated, with theories including derivation from bacterial group II introns.
• Alternative hypotheses suggest a common, undefined RNA ancestor (proto-spliceosome) for both spliceosomes and group II introns.

Purpose of the Study:

• To propose a selection-based model for the evolution of spliceosomes, contrasting with neutral evolution theories.
• To explain the origin, retention, and loss of spliceosomes through positive and negative selection pressures.

Main Methods:

• The study employs a theoretical and comparative analysis of molecular mechanisms across different life forms.
• It integrates concepts from the RNA world, RNP world, and DNA-protein world to explain evolutionary transitions.

Main Results:

• Proto-spliceosomes likely originated in the RNA world for excising functional RNAs and joining non-coding sequences.
• Protein synthesis adoption led to increased proto-spliceosome complexity for trans-splicing and exon-shuffling.
• Spliceosomes were later adopted for cis-splicing of introns to produce mRNAs, with selection favoring spliceosome-dependent eukaryotes and spliceosome-negative prokaryotes.

Conclusions:

• Spliceosome evolution is primarily driven by positive and negative selection, not solely by constructive neutral changes.
• Selection explains the differential presence of spliceosomes in prokaryotes (energy saving) and eukaryotes (dependency).
• The model provides a straightforward explanation for the diverse roles and presence of spliceosomes throughout evolutionary history.