Rare Amborella shrub has engulfed whole genomes from other species
A shrub from the most ancient group of flowering plants alive today steals and hoards genes from other species on a staggering scale.
Botanists have been building a case for more than a decade that Amborella trichopoda is the only known survivor of a plant lineage that branches off at the very base of the genealogical tree of living flowering plants. Now researchers describe the plant’s genes in detail.
It turns out this oddity has captured not just pieces but whole genomes from three kinds of green algae and a moss, says Jeffrey Palmer of Indiana University Bloomington. These genes aren’t in Amborella’s cell nucleus but in the separate genome of the mitochondria, the little powerhouses of the cell.
The genes that Amborella swallowed now outnumber its original mitochondrial ones 6-to-1, Palmer says. Yet so far, tests find little evidence that the hoarded DNA still works, he and his colleagues report in the Dec. 20 Science. “It’s an anal-retentive genome,” as Palmer puts it.
Researchers already knew that plants and other organisms, especially bacteria, kidnap bits of DNA now and then.
For Amborella, “what is amazing about this case is the sheer scale of the transfer and the [evolutionary] distance between the species,” says Ralph Bock of the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm, Germany, who is not part of the Amborella team.
Amborella’s incorporation of whole genomes fits with a fusion scenario to explain how mitochondria manage their heists, Palmer says. Old textbook stereotypes of mitochondria as tidy, self-contained capsules have faded away as modern imaging techniques show them routinely fusing into tubes and branches. Since the plant’s mitochondria can fuse with each other, he reasons, they might also fuse with the mitochondria in algae and moss draping the shrub’s branches.
The new research also shows how uneven the results of gene stealing can be. Genes in Amborella’s nucleus don’t show such a trove of stolen goods. That discrepancy makes it tricky to judge the role of thievery in evolution, says Patrick Keeling, an evolutionary biologist at the University of British Columbia in Vancouver, who wasn’t part of the study.
A second paper in the same issue of Science describes the nuclear genes, thanks to work by more than 75 researchers. The plant contains repeated genes, which indicates that ancestral flowering plants must have duplicated their whole genomes early in their history, says Douglas Soltis of the University of Florida in Gainesville, one of the project’s 14 research leaders.
This doubling may help answer how flowering plants “came to rule the terrestrial landscape,” he says. Further genome doubling is common in the later history of plants as a way that new species form, adds Pamela Soltis, another leader of the nuclear genome work. It can help organisms take advantage of new opportunities: With extra copies around, some genes keep doing their old job while others are repurposed for some brave new world.
D.W. Rice et al. Horizontal transfer of entire genomes via mitochondrial fusion in the angiosperm Amborella. Science. Vol. 342, December 20, 2013, p. 1468. doi: 10.1126/science.1246275.
Amborella Genome Project. The Amborella genome and the evolution of flowering plants. Science. Vol. 342, December 20, 2013, p. 1467. doi: 10.1126/science.1241089.