All eukaryotic genomes carry introns as parts of some gene structures and the introns are to be eliminated by a complex molecular machinery called the spliceosome comprising five snRNAs and more than 150 proteins [12]. Although the debate on the origin of introns, i.e., the intron-early versus intron-late hypothesis, has still not been completed, it is obvious that most spliceosomal introns have been gained after prokaryote-eukaryote divergence [345], and there has been no spliceosomal introns found in prokaryotic lineages so far. The intron has still been propagating in some eukaryotic lineages [6], whereas other lineages have experienced extensive losses of introns during evolutionary life history [2]. Primates have a higher density of intronic sequences than primitive eukaryotes that diverged earlier in eukaryotic life history, such as yeast, Drosophila and Caenorhabditis elegans. Intron sequences constitute approximately 25% of the human genome, which is 4~5 times the size of exons [7]. It has been noticed that the number of genes varies little between these eukaryotic species ranging less than 2-fold from 14,000 genes to 25,000 genes, whereas the size of introns greatly varies up to several fold, implicating that introns might have roles in determining species-specific characteristics and complexities [8].
