Gene Transfer in the Corpse Flower

Although I have been aware of them for many years, I have never seen any of the Rafflesia species; those flowers that bloom once a year, and only for a few days and smell like rotten flesh. Despite, or perhaps because of, the smell, I would jump at the chance to experience a bloom.

Here is a description of the largest flower in the world from ARKive.

  • The dramatic Rafflesia flowers are the largest single flowers in the world; the

    Rafflesia cantleyi

    leathery petals can reach over 90 centimetres across.

  • Rafflesia is a parasite that depends completely upon its host; the majority of the plant’s tissues exist as thread-like strands entirely within the host’s cells.
  • These host plants are vines of Tetrastigma spp., and the Rafflesia plant is itself not visible until the reproduction stage when flowers first bud through the woody vine and then open into the magnificent spectacle that is world-renowned today.
  • The flowers can take up to ten months to develop from the first visible bud to the open bloom, which may last no more than a few days.
  • Currently 17 species of Rafflesia are recognised and these mainly differ in the morphology of their flowers.
  • In general however, the flowers consist of five leathery petals that are orange in colour and mottled with cream-coloured warts. There is a deep well in the centre of the flower containing a central raised disc raised that supports many vertical spines.
  • The sexual organs are located beneath the rim of the disk, and male and female flowers are separate

Christie Wilcox, writing at Scientific American describes more features of this very unique plant.

The corpse flower and its host have a very intimate relationship. From the start, Rafflesia burrows into the Tetrastigma‘s tissues, growing as thread-like strands in direct contact with the surrounding vine’s cells. They are so dependant on their host that the corpse flowers have even lost the ability to make chlorophyll, a requirement for photosynthesis, and thus defy the very nature of being a plant by being unable to produce food from sunlight. These parasites feed off their host vines, growing and growing until they finally erupt, dramatically if briefly, into large, rubbery flowers that stinks like rotting flesh.

In her article, she comments on a recent paper in BMC Genomics from a researchers from Harvard.

Horizontal transfer of expressed genes in a parasitic flowering plant
Zhenxiang Xi, Robert K Bradley, Kenneth J Wurdack, K.M. Wong, M. Sugumaran, Kirsten Bomblies, Joshua S Rest and Charles C Davis

Wilcox reports:

“We found that several dozen actively transcribed genes likely originated from the flower’s host,” said Zhenxiang Xi, first author and a graduate student at Harvard University. They also found that most of these genes were incorporated into the parasites own DNA, even replacing similar genes, and another third of Rafflesia‘s own genes have evolved to look more like the vine’s.

The genes that were stolen perform a wide variety of cellular functions, including roles in respiration, metabolism, mitochondrial translation, and protein turnover. Their active expression suggests that they play a key role in the parasite’s survival, but the researchers hope that future research will determine exactly how important these genes are and whether they help the parasite evade detection by the host’s immune system. “These findings might reflect a sort of genomic camouflage, or genomic mimicry for the parasite,” says Charles Davis, co-author and head of the lab at Harvard. A bacterial pathogen of citrus trees, for example, produces a hijacked protein which limits the victim’s ability to detect and remove the intruder.

What’s truly remarkable about this study is that the rate of gene transfer between the vine and its parasitic corpse flower is as high as rates of lateral gene transfer seen in bacteria. Never before have scientists thought that horizontal gene transfer could play such a pivotal role in the evolution of plants and animals, let alone in parasite-host relationships. Given that parasites make up for an astounding 40% of the species on Earth, these findings are bound to transform our understanding of evolutionary processes and how we ended up with the diversity of life we see today.

Parasites occupy as much as 75% of the links in feed chains and are critical components of  maintaining host abundance.

Recent studies of food webs suggest that ~75% of the links in food webs involve a parasitic species. These links are vital for regulation of host abundance.

As I pointed out in a recent post, the tree of life is very bush like, and gets more interesting and important every day. Evolution is a wonder thing.

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