Horizontal Transfer Of DNA From The Mitochondrial To The Plastid Genome And Its Subsequent Evolution In Milkweeds (Apocynaceae)
Horizontal gene transfer (HGT) of DNA from the plastid to the nuclear and mitochondrial genomes of higher plants is a common phenomenon; however, plastid genomes (plastomes) are highly conserved and have generally been regarded as impervious to HGT. We sequenced the 158 kb plastome and the 690 kb mitochondrial genome of common milkweed [Asclepias syriaca (Apocynaceae)] and found evidence of intracellular HGT for a 2.4 kb segment of DNA from the mitochondrial genome to the rps2-rpoC2 intergenic spacer region of the plastome. The region transferred to the plastome contains an rpl2 pseudogene and is flanked by plastid sequence in the mitochondrial genome, including an rpoC2 pseudogene, which likely provided the mechanism for HGT back to the plastome through a double-strand break repair pathway involving homologous recombination. We determined that the plastome insertion is restricted to tribe Asclepiadeae of subfamily Asclepiadoideae, while the mitochondrial rpoC2 plastid pseudogene is present throughout the subfamily. This result places the two HGT events in an evolutionary context and confirms that the plastid to mitochondrial HGT event preceded the HGT to the plastome. Although the plastome insertion has been maintained over a long evolutionary time period, the rpl2 pseudogene shows minimal evidence of transcription, suggesting that the region may be non-functional and decaying at a rate slowed by the presence of flanking operons. Further, we found recent gene conversion of the mitochondrial rpoC2 pseudogene in Asclepias by the plastid gene, which reflects continued interaction of these genomes and highlights their dynamic relationship.