Length, GC content and selective constraint distributions across intron positions in T. parva. Distribution of length (A), GC content (B) and selective constraint (C) in introns of different ordinal numbers are depicted by boxplots. The last class averages across introns of position equal to or larger than 5. Graph description as in legend of Figure 2. In the boxplot of length distribution, a log scale is used for the vertical axis since intron lengths span a large range. Similar results are found in T. annulata (not shown).