FUNGAL ORIGIN BY HORIZONTAL TRANSFER OF A PLANT MITOCHONDRIAL GROUP-IINTRON IN THE CHIMERIC COXI GENE OF PEPEROMIA

We present phylogenetic evidence that a group I intron in an angiosper m mitochondrial gene arose recently by horizontal transfer from a fung al donor species. A 1,716-bp fragment of the mitochondrial coxI gene f rom the angiosperm Peperomia polybotrya was amplified via the polymera se chain reaction and sequenced. Comparison to other coxI genes reveal ed a 966-bp group I intron, which, based on homology with the related yeast coxI intron aI4, potentially encodes a 279-amino-acid site-speci fic DNA endonuclease. This intron, which is believed to function as a ribozyme during its own splicing, is not present in any of 19 coxI gen es examined from other diverse vascular plant species. Phylogenetic an alysis of intron origin was carried out using three different tree-gen erating algorithms, and on a variety of nucleotide and amino acid data sets from the intron and its flanking exon sequences. These analyses show that the Peperomia coxI gene intron and exon sequences are of fun damentally different evolutionary origin, The Peperomia intron is more closely related to several fungal mitochondrial introns, two of which are located at identical positions in coxI, than to identically locat ed coxI introns from the land plant Marchantia and the green alga Prot otheca. Conversely, the exon sequence of this gene is, as expected, mo st closely related to other angiosperm coxI genes. These results, toge ther with evidence suggestive of co-conversion of exonic markers immed iately flanking the intron insertion site,lead us to conclude that the Peperomia coxI intron probably arose by horizontal transfer from a fu ngal donor, using the double-strand-break repair pathway. The donor sp ecies may have been one of the symbiotic mycorrhizal fungi that Live i n close obligate association with most plants.