Fig. 1.

Vertebrate intron size. A-C, distributions of log₂ intron size in teleosts (A), mammals (B) and Sauria (C). Colour of lines indicate log genome size from blue (small) to purple (large); x-axis, log₂ intron size, y-axis density. Distributions for teleosts are much more varied than for mammals or Sauria, but are generally bimodal with an antimode at 2⁸ bases (indicated by vertical line). The distributions of mammals and Sauria also suggest bimodality with a peak of shorter introns at 105 bp. Distributions for Sauria separate naturally into two groups of different genome size ranges; these correspond to Aves (birds) and non-Aves species, with Aves species (blues) having both smaller genomes and introns. D, log₂ genome size (x-axis) plotted against the fraction of introns shorter than 2⁸ base pairs (y-axis). Colour indicates taxonomic membership as indicated. E, points plotted as in (D), but divided into first introns (filled circles, rank = 1) and other introns (rank > 1). Pairs of points for each species are joined by vertical lines. First introns are much less likely to be smaller than 2⁸ bp in teleosts