<p>The bottom and top of the box is the 25<sup>th</sup> percentile (the lower quartile) and 75<sup>th</sup> percentile (the upper quartile) of lengths, respectively; the line in the box is the 50<sup>th</sup> percentile (the median). The lowest and highest datum is the minimum and maximum not considered outliers, respectively, and outliers are not plotted. ‘+’denotes average length of introns with the same position. ‘*’ denotes that the maximum length of the first introns is more than 20000 bp which is not displayed in figure. The situations of introns whose position beyond 80 are not shown since the numbers of sequences are less than 30, hereinafter.</p
