Sequence conservation in intron ordinal groups. Introns were grouped by their ordinal positions. Introns containing repeats were removed and for each remaining intron 300 bps from the 5’ as well as the 3’ end were removed to minimize interference from splicing signals (see Methods). Box plot analysis is performed for the proportions of conserved sites in introns grouped by ordinal positions from 1st introns to 20th introns. The proportions of conserved sites in first introns are represented by darker gray colors than those in the other downstream introns. The figure shows that first introns have the highest proportions of conserved sites and the proportion decreases monotonically with increasing ordinal number, stabilizing at 4th intron group. ‘*’ indicates p < 2.2e-16. Note that fewer introns are collected from the higher ordinal positions indicated by narrower box width.