Evolution of vertebrate intron size

Lars Martin Jakt, Nord University

 

The position of introns in coding transcripts are remarkably well conserved across the eukaryotes. However, intron size correlates well with genome size suggesting that introns can be used as a proxy for the study of the evolution of genome size. We have analysed the evolution of intron size within the vertebrates, whose genome sizes range from a few hundred megabases to tens of gigabases. In general intron size correlates well with genome size and our analysis suggests a divergence early in vertebrate evolution with intron size increasing and decreasing in lineages leading to the tetrapods and teleosts respectively. Introns in the major teleost lineages appear to have

decreased in size in parallel suggesting either an evolutionary advantage of small genomes or shared molecular mechanisms predisposing teleosts to genome size reduction. We also observed a bimodal distribution of intron lengths in the majority of the teleosts studied. Introns that are long across the teleosts are also more likely to be

long in tetrapod species, to contain conserved sequences and are enriched for specific gene categories.

 

Our observations indicate an accelerated rate of intron size evolution in the teleosts and that teleost introns can be divided into two classes by their length. Teleost intron sizes have evolved primarily as a side-effect of genome size evolution and small genomes are dominated by short introns. However, a non-random subset of introns has resisted

this process across the teleosts and these are more likely to have functional roles throughout the vertebrates.