Origin and evolutionary implications of introns from analysis of cellulose synthase gene

Spliceosomal introns are present in diverse eukaryotic but not prokaryotic genes. However, their evolutionary origin is still arguable. Here, we address this question through an analysis of the cellulose synthase gene, which is present only in urochordate among the animal lineages due to horizontal transfer from a bacterial ancestor in the distant past. We hypothesize that this gene in urochordate contains primordial intron positions and attempted to find them by comparisons with the plant cellulose synthase gene intron positions. However, we could not find distinct conservation of intron positions; therefore, we believe that there was only random insertion of introns in the urochordate cellulose synthase gene. In addition, the protosplice site tendency to conserved phase zero intron positions in urochordates is only 54%, compared with 76% in plants. This shows independent origins of introns in the cellulose synthase gene of urochordates and plants during eukaryotic evolution. Therefore, we concluded that intron origin is a eukaryotic invention.