Spliceosomal introns are parts of primary transcripts that are removed by RNA splicing. Although introns apparently do not contribute to the function of the mature transcript, in vertebrates they comprise the majority of the transcribed region increasing the metabolic cost of transcription. The persistence of long introns across evolutionary time suggests functional roles that can offset this metabolic cost. The teleosts comprise one of the largest vertebrate clades. They have unusually compact and variable genome sizes and provide a suitable system for analysing intron evolution.
We have analysed intron lengths in 172 vertebrate genomes and show that teleost intron lengths are relatively short, highly variable and bimodally distributed. Introns that were long in teleosts were also found to be long in mammals and were more likely to be found in regulatory genes and to contain conserved sequences. Our results argue that intron length has decreased in parallel in a non-random manner throughout teleost evolution and represent a deviation from the ancestral state.
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 ( < 256 bp). However, a non-random subset of introns has resisted this process across the teleosts and these are more likely have functional roles in all vertebrate clades