Abstract
Intronic sequences represent a large fraction of most eukaryotic genomes, and they are knownto play a critical role in genome evolution. Based on the conserved location of introns, conservedsequence within introns, and direct experimental evidence, it is becoming increasingly clear thatintrons perform important functions such as modulating gene expression. Here, we demonstratethat the positions of 69% (862/1246) of human introns in 343 orthologous genes are conserved in thestarlet sea anemone Nematostella vectensis, a phylogenetically basal animal (phylum Cnidaria; classAnthozoa). This degree of intron concordance greatly exceeds that between humans and three moreclosely related animals: fruitfly (14%), mosquito (13%) and nematode worm (19%). Surprisingly, the fruitfly and mosquito, two members of the order Diptera, share only 43% of intron locations, fewer than the percentage of cumulative introns shared between human and sea anemone (47%), despite sharing a much more recent common ancestor. Our analysis indicates (1) that early animalgenomes were intron-rich, (2) that a large fraction of introns present within the human genomelikely originated early in evolution, before the cnidarian-bilaterian split, at least 600 million yearsago, and (3) that there has been a high degree of intron loss during the evolution of the protostomelineage leading to the fruitfly, mosquito, and nematode. These data also reinforce the conclusionthat there are functional constraints on the placement of introns in eukaryotic genes.
