GROUP-I INTRONS ARE INHERITED THROUGH COMMON ANCESTRY IN THE NUCLEAR-ENCODED RIBOSOMAL-RNA OF ZYGNEMATALES (CHAROPHYCEAE)

Group I introns are found in organellar genomes, in the genomes of eub acteria and phages, and in nuclear-encoded rRNAs. The origin and distr ibution of nuclear-encoded rRNA group I introns are not understood. To elucidate their evolutionary relationships, we analyzed diverse nucle ar-encoded small-subunit rRNA group I introns including nine sequences from the green-algal order Zygnematales (Charophyceae). Phylogenetic analyses of group I introns and rRNA coding regions suggest that later al transfers have occurred in the evolutionary history of group I intr ons and that, after transfer, some of these elements may form stable c omponents of the host-cell nuclear genomes. The Zygnematales introns, which share a common insertion site (position 1506 relative to the Esc herichia coli small-subunit rRNA), form one subfamily of group I intro ns that has, after its origin, been inherited through common ancestry, Since the first Zygnematales appear in the middle Devonian within the fossil record, the ''1506'' group I intron presumably has been a stab le component of the Zygnematales small-subunit rRNA coding region for 350-400 million years.