The varieties of Rafflesia, found only in southeast Asia, are parasitic plants with no leaves, just stems that live inside other plants. Their flowers smell bad in order to attract carrion flies. Rafflesias Arnoldi is the world’s largest flower. But the strangest thing about them so far has just come to light:
More than a decade ago, Rafflesiaceae parasites caught the eye of Jeanmaire Molina, an evolutionary plant biologist at Long Island University in Brooklyn, who wondered if their genomes were as bizarre as their outward forms. Her initial investigations suggested they were. As she and her colleagues described it in a 2014 paper in Molecular Biology and Evolution, they successfully assembled the mitochondrial DNA from one Philippines species of Rafflesia. But they were unable to detect any functional genes from its chloroplasts. The plants seemed to have simply ditched their entire chloroplast genome. That was almost unthinkable. Chloroplasts are best known for using light to make food, but like all the food-making organelles called plastids, they contain genes that are involved in many key cellular processes. Even malaria parasites still carry a plastid genome, Molina noted, and their last photosynthetic ancestor lived hundreds of millions of years ago.
Christie Wilcox, “DNA of Giant ‘Corpse Flower’ Parasite Surprises Biologists” at Quanta
One variety had lost 44% of its genes. But that’s not all:
Davis’ team estimated that at least 1.2% of the plant’s genes came from other species, particularly its hosts, past and present. That might not sound impressive, but this kind of horizontal gene transfer is considered exceptionally rare outside of bacteria. So even a single percent of genes arising this way raises eyebrows.
Because these parasites have been stealing genes for millennia, Cai noted, their genome is like “a huge graveyard of DNA.” By carefully digging through that graveyard and comparing its contents to the genomes of 10 types of vines that seemed like potential hosts, Cai and her colleagues were able to peer back in time. “These horizontally transferred genes are serving as DNA fossils,” she said.
Christie Wilcox, “DNA of Giant ‘Corpse Flower’ Parasite Surprises Biologists” at Quanta
Researchers are still trying to figure out why the parasite has such a huge genome. Commendably, they are not claiming it’s all junk.
See also: Devolution: Getting back to the simple life