Flowering plants emerged on the planet over 160 million years ago - but it has never been entirely clear how these angiosperms came from their predecessor, gymnosperm ferns. New genetic analysis of the Amborella, a shrub with deep evolutionary roots, shows that there was a genomic doubling around 200 million years ago. The results were published today in Science from the Amborella Genome Project, which is a collaboration between scientists at various international universities, the National Science Foundation, and is hosted by Penn State University.
There are over 300,000 known species of flowers alive today, but their evolutionary history hasn’t been overly obvious. They essentially exploded, leaving hardly any fossil evidence about the transition period. Charles Darwin actually described the wildly successful emergence of angiosperms an “abominable mystery.”
The Amborella is a shrub found in the biodiversity wonderland of New Caledonia. It occupies the understory of the forest and has been described as the most basal angiosperms, as it is the closest relative of the first flower and has undergone very little evolutionary change since its emergence. Because of these factors, it is very desirable for evolutionary biologists and the Amborella Genome Project (AGP) was formed to sequence and learn more about its evolutionary history. Once the flower was fully sequenced, the researchers compared the genetic code to that of more than twenty other plants.
The AGP has yielded evidence that around 200 million years ago, some extant seed plants experienced a genomic doubling, which gave them twice as many genes. The research team believes the product of this was the first angiosperm, from which all others would come. Over the course of their evolution, flowering plants gained about 1179 novel genes that are not found in any other species of plant. With a wealth of new genes and gene families, the plants were able to make unique structures, such as vessel elements. These are dead cells that form channels to draw water up through the xylem. Though some of the most ancestral flowers have 21 groups of transcription factors (known as MADS-Box genes) that encode for the actual flower, the Amborella was discovered to have 36.
Most of the results confirmed previous suspicions that the researchers had about the evolution of the flower, but there was a big surprise in the mitochondrial DNA, which is about six times larger than any other plant’s mtDNA genome, at 3.8 million base pairs long and divided into five chromosomes. Researchers found an explanation for the length when they found complete mtDNA sequences of three types of green algae, one moss, and individual genes from other flowering plants.
Even more strangely, none of that information seems to function within the plant. The researchers suspect that when the plant becomes injured, it is able to take up foreign mtDNA from other nearby plants and integrate it with its own. However, since it doesn’t seem to have much of a purpose, they aren’t sure why this happens. It is possible that many plants do this, but don’t spare the resources to keep anything that is nonfunctional. If this is the case, then Amborella is still very unique in that regard.
Because Amborella is the oldest common ancestor for all plant life on Earth today, including much of what we eat, gaining a better understanding of how its genomic function will allow us a clearer picture of the evolutionary past, and we could also apply the information to other plants whose histories are not as well understood.