By GEOFFREY MOHAN
Los Angeles Times
LOS ANGELES —
Never mind the selfish gene — the cellular family history of the oldest living species of flowering plants is marked by enough sex and gluttony to earn a place in Shakespeare’s folio.
The powerhouse organelles inside cells of Amborella trichopoda, a woody shrub that grows only in the humid jungles of New Caledonia in the South Pacific, gobbled up and retained the entire genome from the equivalent organelles of four different species, three of moss and one of algae, according to a study of the plant’s mitochondrial DNA published this week in the journal Nature.
The results are the product of a years-long effort to sequence the full genome of the plant, a crucial step in solving what Charles Darwin once called “the abominable mystery” — the sudden flourishing long ago of several hundred thousand species of flowering plants.
An analysis of the nuclear DNA of the species, published in the same edition of Nature, revealed that the plant is the equivalent of the animal kingdom’s duck-billed platypus — a solitary sister left behind more than 100 million years ago by what became a panoply of flowering, or fruiting, plants.
The genome map, undertaken by the Amborella Genome Project, an international consortium of universities, enabled researches to infer the genetic makeup of the most recent common ancestor of A. trichopoda. It found that this ancestor likely evolved after an event that doubled its genome roughly 200 million years ago. Such an event could help explain what so baffled the father of evolutionary theory, the researchers said.
The map of the genome also serves as a reference point for explaining the evolution of many of the subsequent species that form the backbone of the human food chain.
But enough about nuclear DNA — let’s get to the sex and gluttony.
Researchers who mapped the genome of the plant’s mitochondria, the organelles that are the energy factories inside cells, were stunned at its size and diversity and by evidence of some intracellular shenanigans unsuitable for prime time.
“We think this paper will help bring mitochondrial sex out of the closet, if you will,” said Jeffrey D. Palmer, an Indiana University evolutionary biologist and author of the mitochondrial DNA paper.
“There’s not another genome in any organism of any type like this,” added Palmer. “No one has found this scale of horizontal DNA transfer even in a bacterium, where it’s really common. No one has found whole genomes, much less four of them, taken up by horizontal transfer.”
A somewhat downtrodden bottom-dweller of the South Pacific tropics, A. trichopoda hosts a bevy of algae, moss and other hitchhiking plants, some of them parasitic. In that rough-and-tumble world, tears and wounds likely exposed the plant to foreign mitochondria, which then fused with its own mitochondria - organelle sex.
“In some real sense we should regard mitochondria to be engaging in a kind of sex, just like nuclear genomes do,” he said.
The plant’s mitochondria thus retain a kind of fossil record largely absent in other plant species. The foreign DNA amounts to about six genomes, the researchers believe — at least four of them acquired in whole.