Seen here close-up under a dissecting microscope, the flowering plant Amborella trichopoda is the oldest known existing species of petal-bearing plants on Earth. Now in a series of reports published today in the journal Science, molecular geneticists have unlocked the genomic secrets of Amborella, and with it clues as to why flowers display such successful genetic diversity.
Many of Amborella's genes were distinct from those of non-flowering plants. In its mitochondrial DNA, which tends to change less than nuclear DNA, Amborella showed a shared affiliation with mosses and green algae. Biologist Danny W. Rice of Indiana University and his team hypothesize that wounded Amborella plants obtained the shared mitochondrial genomes as a result of horizontal gene transfer between these other organisms it was living in close proximity with millions of years ago.
Here, Amborella genomic DNA is shown in blue, chloroplast DNA in green, and mitochondrial DNA in red.
Indeed, the team discovered that Amborella's mitochondrial genome provides the largest example of horizontal gene transfer – the acquisition of foreign genes from other species – in any organism. Shown here are male flowers of Amborella.
The Indiana University team -- working with biologists from the U.S. Department of Energy, Penn State University, and the Institute of Research for Development in New Caledonia -- showed for the first time that an organelle genome has captured an entire foreign genome, in this case, four of them: three green algae and one moss. It is also the first description of a land plant acquiring genes from green algae. Shown here are female flowers of Amborella.
"The Amborella mitochondrial genome is like the old lady in the song who swallows a fly, and then a spider, a bird, a cat, and so on, all the way to a horse, at which point, finally, "she's dead of course," said co-author of the study Jeff Palmer, a Distinguished Professor in the Indiana University Bloomington College of Arts and Sciences' Department of Biology. Shown here are Amborella female flowers and fruits.
"Likewise, the Amborella genome has swallowed whole mitochondrial genomes, of varying sizes, from a broad range of land plants and green algae. But instead of bursting from all this extra, mostly useless DNA, or purging the DNA, it's held on to it for tens of millions of years. So you can think of this genome as a constipated glutton, that is, a glutton that has swallowed whole genomes from other plants and algae and also retained them in remarkably intact form for eons," said Palmer in a press release. Shown here are Amborella in fruit.
View from the summit of Mt. Aoupine, New Caledonia. The flowering Amborella, whose mitochondrial genome is amazingly rich in foreign genes and even genomes, is endemic to the island of New Caledonia. The research on Amborella shows "compelling evidence that mitochondrial fusion is the driving force for mitochondrial gene transfer and that incompatibility in the mechanism of mitochondrial fusion between different phyla – plants versus animals or fungi – provides the major barrier to unconstrained mitochondrial 'sex' across the evolutionary tree of life," said Palmer.
The southwest-Australian Christmas tree, Nuytsia floribunda, which parasitizes the roots of grasses to obtain water and minerals. This parasite belongs to the group of parasitic plants (Santalales) from which the Amborella mitochondrial genome has captured many foreign genes by horizontal gene transfer.
A parasitic flowering plant (Amyema scandens) blooming in New Caledonia from its epicortical roots, which, like mistletoe, grow along the branch of its host tree. This parasite belongs to the same group of parasitic plants (Santalales) from which the Amborella mitochondrial genome has captured many foreign genes.
A parasitic flowering plant (Hachettea austro-caledonica) emerging from the ground to flower. This plant parasitizes the roots of other flowering plants and belongs to the group of parasitic plants (Santalales) from which the Amborella mitochondrial genome has captured many foreign genes. Picture taken in New Caledonia, the South Pacific island on which both Amborella and Hachettea are endemic.