Horizontal gene transfer has the Rafflesia potentially using the host’s information against it
They’re a member of a group that includes some of the world’s best thieves. They’re so good at stealing that they don’t make or produce food for themselves.
And, to top it off, like the high school students who rule the school because they are the tallest, most attractive or most athletic, they are physically stunning. Known in the scientific community as Rafflesia cantleyi, they have some of the world’s largest flowers, spanning as much as two feet across.
Found in Malaysia and named after a 19th century curator of the Singapore Botanic Gardens, the Rafflesia doesn’t have chlorophyll, the critical green molecule that allows plants to turn light, carbon dioxide and water into food. Instead, it lives deep inside the host vine, Tetrastigma rafflesia, a member of the grape family.
But that, it turns out, is not the only thing the parasitic plant pilfers. Recent research from Stony Brook assistant professor Joshua Rest, in collaboration with Harvard Professor Charles Davis, suggests Rafflesia has somehow taken something surprising: 49 genes from the Tetrastigma.
While bacteria and viruses take genes wherever they find them and attach them to their own set of life blueprints, it is much more unusual for plants to take genetic material, much less a region this large, from another plant. That means the Rafflesia is not only invading the space and food of the Tetrastigma plant, but it is also grabbing some of the plant’s hard-earned genetic identity.
When he first examined the genetic sequence of the Rafflesia, Rest was so stunned, he wondered whether he might have “just contaminated something,” by mixing the genes of the two plants.
Careful analysis, however, confirmed it was not the researchers who mixed the genes, but rather the plant that had gone through a process called horizontal gene transfer. Unlike vertical gene transfer, where individuals get their genes from their parents, in horizontal gene transfer, an individual can acquire its code from something outside its genetic tree.
“It definitely turns the way we think about things a bit on its head,” acknowledged Rest.
This discovery is new enough that scientists like Rest and Davis can only begin to guess at what advantage the Rafflesia gets from copying the genes of its host. One plausible explanation is that the parasite weakens the grape plant’s ability to defend itself against its unwelcome guest. The copied genes might send a signal to the host plant that disguises the parasite, allowing it to live like a disguised but sated wolf among sheep.
Rest cautioned that scientists don’t understand how the gene transfer affects the ongoing battle.
“We don’t know that there’s any cost” to the grape plant, Rest offered. “To whatever extent the transfer makes the parasites better at what they do, it could make the [grape] vines worse off.”
While the copied genes may protect the parasite against an immune response, they are also a part of other activities, including metabolism and respiration.
“They are involved in different cellular functions,” he explained. “We were expecting maybe we would find genes that were just involved in the immune response.”
To be sure, the notion of combining different genes to form a new organism isn’t unique, even in the world of eukaryotes. In fact, because the DNA from mitochondria and chloroplasts are different, scientists believe that, at one point, these organelles existed separate from each other, and proto-eukaryotic cells enveloped them. The parasitic gene copying is “on a much smaller scale,” Rest assured.
Given the range of parasites, it’s likely that others besides the Rafflesia have taken more than just food, sunlight or structural support at the expense of their hosts.
“Nature is a big place, so it’s unlikely that this mechanism is unique,” Rest suggested.
When he’s not looking closely at the genes of plant parasites, Rest, a native of the Chicago area, enjoys the chance to explore nature on Long Island, where he likes to run and hike in parks along the North and South Shore.
Rest lives in Bellmore with his husband Scott Stuart, a music therapist who works in a nursing home.
As for his work, Rest is fascinated by the implications of the range of what Rafflesia takes from its grape vine.
“The parasite,” he explained, “is potentially using the host information against it.”