A reeking, parasitic plant lost its body and much of its genetic code


For most of their lives, crops within the Sapria genus are barely something — skinny ribbons of parasitic cells winding inside vines in Southeast Asian rainforests. They change into seen solely once they reproduce, bursting from their host as a dinner plate–sized flower that smells like rotting flesh.

Now, new analysis on the genetic code of this uncommon plant reveals the lengths to which it has gone to change into a specialised parasite. The findings, revealed January 22 in Current Biology, counsel that at the least one species of Sapria has lost nearly half of the genes generally present in different flowering crops and stolen many others immediately from its hosts. 

The plant’s rewired genetics echo its weird biology. Sapria and its kinfolk within the household Rafflesiaceae have discarded their stems, roots and any photosynthetic tissue.

“If you’re out in the forest in Borneo and these [plants] aren’t producing flowers, you’re never even going to know they’re there,” says Charles Davis, an evolutionary biologist at Harvard University. 

For years, Davis has been learning the evolution of this group of otherworldly parasites, which incorporates the biggest flower on the planet, Rafflesia arnoldii (SN: 1/10/07). When some genetic knowledge confirmed a detailed relationship between these parasites and their vine hosts, Davis suspected horizontal gene switch. That’s the place genes transfer immediately from one species to a different — on this case, from host to parasite. But nobody had but deciphered the genome — the total genetic instruction e-book — for these crops.

So Davis and his group sequenced many tens of millions of items of Sapria himalayana’s genetic code, assembling them right into a cohesive image of that species’ genome. When the group analyzed the genome, they discovered an abundance of oddities. 

About 44 p.c of the genes present in most flowering crops had been lacking in S. himalayana. Yet, on the identical time, the genome is about 55,000 genes lengthy, greater than that of another non-parasitic crops. The rely is inflated by many repeating segments of DNA, the group discovered.

Loss of the chlorophyll pigments accountable for photosynthesis is widespread in parasitic crops that depend on their hosts for sustenance. But S. himalayana seems to have even scrapped all genetic remnants of its chloroplasts, the mobile constructions the place photosynthesis happens. 

Chloroplasts have their very own genome, distinct from the nuclear genome that runs a plant’s cells and the mitochondria that produce power for the cells. S. himalayana appears to have lost this genome altogether, suggesting that the plant has purged the final remnants of its ancestral life that allowed it to make its personal meals.

“There is no other case” of an deserted chloroplast genome amongst crops, says Davis. Earlier work by different researchers had advised that the genome may be missing. “Our work clearly verifies that indeed it’s totally gone,” he says, noting that even genes in S. himalayana’s nuclear genome that will regulate elements of the chloroplast genome have vanished. 

It could also be too early to declare the chloroplast genome utterly lacking in motion, cautions Alex Twyford, an evolutionary biologist on the University of Edinburgh who was not concerned with this analysis. It could also be troublesome to definitively show the genome is gone, he says, particularly if the chloroplast is “unusual in its structure or abundance” and subsequently troublesome to determine.

Among the remaining components of the nuclear genome, the group additionally discovered that greater than 1 p.c of S. himalayana’s genome comes from genes stolen from different crops, probably its present and ancestral hosts.

The potential scale of the vanished genome and the amount of repeating bits of DNA are “insane,” says Arjan Banerjee, a biologist on the University of Toronto Mississauga additionally not concerned with this examine. The “industrial scale” of the plant’s gene theft can also be spectacular, he says.

There are nonetheless lots of bizarre components left in S. himalayan’s genome to discover, says examine coauthor Tim Sackton, an evolutionary biologist additionally at Harvard. For instance, the plant has bloated its genome with extraneous DNA, whereas most parasites streamline their genomes. “There’s something weird and different going on in this species,” he says, including that many of the DNA fragments the parasitic plant is stealing from its host don’t seem to encode any genes, and probably don’t do something vital.

The new discovery illustrates the extent of dedication S. himalayana and its kinfolk have given to evolving a parasitic life-style, and present a comparability to different excessive plant parasites (SN: 7/31/20). And for Davis, crops like S. himalayana might help researchers decide some of biology’s limits. These crops have lost half their genes, but they nonetheless survive, he notes. “Maybe these organisms that stretch the boundaries of existence tell us something about how far the rules can be bent before they can be broken.”




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