An international team of researchers has sequenced and analyzed the genome of Sapria himalayana, a rare holoparasitic flowering plant in the family Rafflesiaceae. Their findings, published in the journal Current Biology, offer a unique perspective into how flowering plant genomes can be altered to fit an extreme form of plant parasitism.
Sapria himalayana represents the extreme manifestation of the parasitic lifestyle, being totally dependent on its host for water nutrients and photosynthates.
Its flowers are about 20 cm (8 inches) across, bright red in color covered with sulfur-yellow spots. They appear above the ground and have a putrid odor.
Sapria himalayana’s more famous cousin, Rafflesia arnoldii, produces flowers up to 1 m (3.3 feet) in diameter, the largest in the world.
“In many ways, it’s a miracle that these plants exist today, let alone that they seem to have persisted for tens of millions of years,” said Professor Charles Davis, a researcher in the Department of Organismic and Evolutionary Biology at Harvard University and curator of vascular plants in the Harvard University Herbaria.
“They’ve really jettisoned many things we identify as a typical plant yet they are deeply embedded within the plant tree of life.”
Professor Davis and his colleagues from the United States, China, Malaysia and Thailand sequenced and analyzed the highly modified genome of Sapria himalayana.
Their analysis revealed an astonishing degree of gene loss and surprising amounts of gene theft from its ancient and modern hosts.
These findings bring unique perspectives into the number and kind of genes it takes to be an endoparasite, along with offering new insights into how far the genomes of flowering plants can be altered and still remain functional.
What struck the researchers immediately was the degree of gene loss Sapria himalayana experienced as they abandoned their bodies and adapted to become endoparasites.
About 44% of all genes found in most flowering plants are absent in the genome of Sapria himalayana. That extent of gene loss is more than four times the degree of loss in other plant parasites.
Many of the genes lost include what are considered the key genes responsible for photosynthesis, which converts light into energy.
At the same time, the data demonstrated an underlying evolutionary convergence to becoming a parasite because Sapria himalayana and the parasitic plants the scientists compared them to lost many of the same types of genes despite evolving separately.
They also identified dozens of genes that came into the Sapria himalayana genome through a process called horizontal (or lateral) gene transfer instead of the traditional parent-to-offspring transmission.
Basically, it means Sapria himalayana stole DNA from their host instead of getting it passed down to them.
The authors then reconstructed the lateral gene transfers they detected to put together a hidden history of former hosts going back millions of years.
“We concluded that there is a common genomic or genetic roadmap to how plant parasites evolve,” said Dr. Cai Liming, a researcher at the University of California, Riverside.
Liming Cai et al. Deeply Altered Genome Architecture in the Endoparasitic Flowering Plant Sapria himalayana Griff. (Rafflesiaceae). Current Biology, published online January 23, 2021; doi: 10.1016/j.cub.2020.12.045
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