Convergent Evolution of Intron Structures Figure 3. Relation between branch point site consensus strength and intron number. (A) Percentage of introns with the most common BP-like hexamer across eukaryotes. Dark colored bars show the percentage of introns with the most common branch-point hexamer; light colored bars show the additional introns allowing for two-fold degeneracy at one of the six sites. The left red bar separates species with ‘strong’ BP motifs (.50% of introns) from species with weaker BP motifs. The right red bar separate extremely intron-poor species (, 0.15 introns per gene). Asterisks (*) mark the two exception to the correspondence between the two variables (strong BP/intron-poor vs. weak BP/intron-rich). (B) Logarithm of total intron number (Y-axis) and percentage of introns showing the most common potential branch point motif with one site two-fold degeneracy (X-axis) are plotted for all fully sequenced studied genomes. The species cluster in two different groups (with two exceptions) - intron-rich/weak BP consensus, intron-poor/strong BP consensus. The two intron-poor species not showing strong consensus branch points, G. thetha NM and C. parvum, are labeled. The line corresponds to a lineal regression analysis, showing the negative correlation (r = 20.75) between intron numbers and branch point consensus conservation. doi:10.1371/journal.pgen.1000148.g003 position -4 (compared to only 39.3% for other introns; p = 0.00004 by a Fisher’s Exact test), and 50.0% having an A at position -5 (compared to only 22.0% of other introns; p = 0.002 by a Fisher’s Exact test). However no clear general trend towards stronger boundaries for short BP-AG introns was observed: introns with a BP-AG distance of 7 nt did now show a stronger 39 consensus than PLoS Genetics | www.plosgenetics.org 6 August 2008 | Volume 4 | Issue 8 | e1000148