EVIDENCE FOR THE LATE ORIGIN OF INTRONS IN CHLOROPLAST GENES FROM AN EVOLUTIONARY ANALYSIS OF THE GENUS EUGLENA

The origin of present day introns is a subject of spirited debate. Any intron evolution theory must account for not only nuclear spliceosoma l introns but also their antecedents. The evolution of group II intron s is fundamental to this debate, since group II introns are the propos ed progenitors of nuclear spliceosomal introns and are found in ancien t genes from modern organisms. We have studied the evolution of chloro plast introns and twintrons (introns within introns) in the genus Eugl ena. Our hypothesis is that Euglena chloroplast introns arose late in the evolution of this lineage and that twintrons were formed by the in sertion of one or more introns into existing introns. In the present s tudy we find that 22 out of 26 introns surveyed in six different photo synthesis-related genes from the plastid DNA of Euglena gracilis are n ot present in one or more basally branching Euglena spp. These results are supportive of a late origin for Euglena chloroplast group II intr ons. The psbT gene in Euglena viridis, a basally branching Euglena spe cies, contains a single intron in the identical position to a psbT twi ntron from E.gracilis, a derived species. The E.viridis intron, when c ompared with 99 other Euglena group II introns, is most similar to the external intron of the E.gracilis psbT twintron. Based on these data, the addition of introns to the ancestral psbT intron in the common an cester of E.viridis and E.gracilis gave rise to the psbT twintron in E .gracilis.