Scientists Discover Driving Force Behind Mitochondrial 'Sex' in Ancient Flowering Plant

How do plants acquire genetic material? It turns out that at least once ancient flowering plant acquired foreign DNA from different unrelated species. Scientists have uncovered an unprecedented example of horizontal gene transfer in a South Pacific shrub that is considered to be the sole survivor of one of the two oldest lineages of flowering plants.

The shrub is called Amborella trichopoda and can be found on an archipelago located 750 miles east of Australia. This plant is the "last man standing" in one of the two oldest flowering plant lineages. The other lineage comprises the remaining 300,000 species of flowering plants.

In order to learn a bit more about this plant, the researchers performed computational analysis of the plant's DNA. The plant's mitochondrial genome is enormous, consisting of nearly four million nucleotides, which is about 240 times larger than a human mitochondrial genome. In the end, they were able to identify many donors of the foreign DNA located in the plant; these included at least three green algae and one moss.

"The Amborella mitochondrial genome is huge, and most of its DNA is foreign, acquired from the mitochondrial genomes of other plant species," said Alverson, one of the researchers, in a news release. "We've never seen horizontal gene transfer at this scale. It's not acquiring genes or bits of genes in a piecemeal way. It's been swallowing up whole genomes. One of our main tasks was to determine the ancestry of its several hundred 'extra' genes."

This mitochondrial genome is unusual for at least three reasons. Ecologically, it's had greater exposure to the foreign mitochondria of epiphytes and parasites than most plants. Developmentally, it's had a greater chance of incorporating this DNA in a new germline and molecularly and there is a greater chance of keeping this DNA due to its exceptionally low rate of DNA loss.

"One of the really interesting things about Amborella is that although it is loaded with all of this extra DNA, most of it is junk," said Alverson in a news release. "The genes are degenerated and nonfunctional. Amborella is a hoarder. Its genome is a museum of dead DNA."

The findings reveal a bit more about this strange plant. In addition, it provides evidence that mitochondrial fusion in plants is incompatible with the way mitochondrial fusion occurs in animals or fungi.

The findings are published in the journal Science.