Monday, 29 March 2021

Evolution News - Evolution by Horizontal Gene Transfer

Siverleaf whitefly, Bernisia tabaci
Photo: Nigel Cattlin/Alamy
Whitefly hijacks a plant detoxification gene that neutralizes plant toxins: Cell

A couple of days ago, I reported on the case of evolution by the rare process of gene shuffling in a species of bird, to bring about a new combination, reinforced by pre-zygotic barriers because the new combination gave a new plumage display and so new assortative mating behaviour.

This time we have an example of horizontal gene transfer giving species of whitefly a significant advantage over those without the newly-acquired gene. Although this is common in protozoans, it is rare in multicellular animals and previously unknown between plants and insects.

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A whitefly feeding on a leaf.
Credit: Jixing Xia and Zhaojiang Guo
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A whitefly on a leaf.
Credit: Jixing Xia and Zhaojiang Guo
The discovery was made by a team from Switzerland, Belgium and China led by Youjun Zhang from the Institute of Vegetables and Flowers at the Chinese Academy of Agricultural Sciences. They found a gene in a species of whitefly, Bemisia tabaci, that is normally found in plants where it was assumed to give the plants protection from the toxins they produce to defend themselves from insects which eat their leaves and suck their sap - such as the agriculturally and economically significant whitefly.

This species of whitefly is part of a complex of at least 30 cryptic species that use a large number of host plants and are remarkably adaptable. They can cause serious damage to crops by feeding on the sap, transmitting plant viruses and producing honeydew. The research team were trying to understand what makes this species so adaptable and able to feed on so many species despite the presence of plant toxins. These toxins have evolved in plants over more than 400 million years of co-existence with herbivorous insects.

The hypothesis is that, millions of years ago, a plant virus incorporated the gene into its own genome, then infected a whitefly and transferred the gene to the whitefly genome where it conveyed the ability to detoxify the toxins the plants were producing to defend themselves with. This resistance then enabled the whitefly to exploit a large number of plant species that had evolved this form of toxin. The team's findings were published a few days ago in Cell, sadly, behind a paywall. However, the Cell news release is quoted in PhysOrg:
Millions of years ago, aphid-like insects called whiteflies incorporated a portion of DNA from plants into their genome. A Chinese research team, publishing March 25th in the journal Cell, reveals that whiteflies use this stolen gene to degrade common toxins plants use to defend themselves against insects, allowing the whitefly to feed on the plants safely.

This seems to be the first recorded example of the horizontal gene transfer of a functional gene from a plant into an insect. You cannot find this gene, BtPMaT1, which neutralizes toxic compounds produced by the plant, in any other insect species.

We think a virus within the plant may have taken up this BtPMaT1 gene and, after ingestion by a whitefly, the virus then must have done something inside the insect whereby that gene was integrated into the whiteflies genome. Of course, this is an extremely unlikely event, but if you think about millions of years and billions of individual insects, viruses, and plants across time, once in a while this could happen, and if the acquired gene is a benefit to the insects, then it will be evolutionarily favored and may spread.

Ted Turlings, Co-author
Chemical ecologist & entomologist University of Neuchâtel, Switzerland
Scientists believe that plants probably use BtPMaT1 within their own cells to store their noxious compounds in a harmless form, so the plant doesn't poison itself. The team, led by Youjun Zhang from the Institute of Vegetables and Flowers at the Chinese Academy of Agricultural Sciences, used a combination of genetic and phylogenetic analyses, to reveal that roughly 35 million years ago, whiteflies stole this defense gene, granting the insect the ability to detoxify these compounds for themselves.

Whiteflies have become a major agricultural pest worldwide, able to attack at least 600 different species of plants worldwide. "One of the questions we've been asking ourselves is how these insects acquired these incredible adaptations to circumvent plant defenses, and with this discovery we have revealed at least one reason as to why," Turlings says.

Using this knowledge, Turlings' Chinese colleagues created a strategy to undo the whiteflies' stolen superpower. They developed a small RNA molecule that interferes with the whiteflies' BtPMaT1 gene, making the whiteflies susceptible to the plant's toxic compounds.

"The most exciting step of this design was when our colleagues genetically manipulated tomato plants to start producing this RNA molecule" says Turlings. "Once the whiteflies fed on the tomatoes and ingested the plant-produced RNA, their BtPMaT1 gene was silenced, causing 100% mortality of the insect, but the genetic manipulation had no impact on the survival of other insects that were tested."

With focused efforts to produce genetically modified crops that are able to silence the whitefly gene, this could function as a targeted strategy for pest control to combat agricultural devastation caused by whitefly populations.

"There are definitely still some hurdles this method needs to get over, most notably the skepticism about using transgenic plants," he says "But in the future, I do see this as a very clear way of controlling whiteflies because now we know exactly the mechanism behind it, and we are equipped to deal with possible changes in the whitefly gene that may arise.
The economic significance of this discovery can be seen from the last few paragraphs above. Using this information the team were able to produce a small RNA sequence that interfered with the BtPMaT1 gene in the whitefly. When introduced into tomato plants, this RNA was taken up by the whiteflies where it blocked the BtPMaT1 gene and made the whiteflies susceptible to the natural plant toxins. It was 100% effective as a highly targetted insecticide.

So, the challenge as ever in these pieces of information about discoveries like this in biology, is for a Creationist to explain how this fits into their favoured intelligent [sic] design notion.

Firstly, why would an intelligent designer give whiteflies a gene to protect them from the toxins it designed plants to produce to defend themselves from whiteflies?

Secondly, where is the evidence in this paper that the Theory of Evolution is a theory in crisis, soon to be overthrown by the notion of intelligent [sic] design, with its reliance on magic, and not still the fundamental basis for interpreting and understanding what we observe in nature?

Answers below, please.

Xia, Jixing; Guo, Zhaojiang; Yang, Zezhong; Han, Haolin; Wang, Shaoli; Xu, Haifeng; Yang, Xin; Yang, Fengshan; Wu, Qingjun; Xie, Wen; Zhou, Xuguo; Dermauw, Wannes; Turlings, Ted C.J.; Zhang, Youjun
Whitefly hijacks a plant detoxification gene that neutralizes plant toxins
Cell; doi: 10.1016/j.cell.2021.02.014

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