"Genome design" model: evidence from conserved intronic sequence in human-mouse comparison

Area of Science:

• Genomics
• Evolutionary Biology
• Molecular Biology

Background:

• Introns are generally shorter in housekeeping genes compared to tissue- or development-specific genes.
• Previous explanations for intron length variation include selection for economy, mutation bias, or genomic design.

Purpose of the Study:

• To investigate the evolutionary conservation and length variation of introns in human and mouse genomes.
• To determine the factors contributing to the observed differences in intron length between housekeeping and tissue-specific genes.

Main Methods:

• Utilized a rigorous local sequence alignment algorithm to analyze large-scale DNA sequences.
• Masked lineage-specific repeats to identify evolutionarily conserved intronic DNA.
• Controlled for factors such as between-species sequence identity, mutation rate, and GC content.

Main Results:

• Approximately 60-70% of human and mouse intron length, respectively, remained after masking repeats, indicating significant conserved DNA.
• Introns in tissue-specific genes exhibited a higher fraction and absolute length of conserved sequence compared to housekeeping genes.
• Intron length correlated with the number of functional domains in the encoded protein, and repeat content or indel balance did not explain length differences.

Conclusions:

• The greater length of introns in tissue-specific genes is likely related to functional complexity, possibly mediated by chromatin condensation.
• The evolution of noncoding DNA, including introns, is not entirely neutral and is influenced by functional constraints.
• Selection for economy or mutation bias alone cannot fully explain intron length variation.