Horizontal gene transfer considered the most 'parsimonious explanation' for a nearly identical gene in a plant and insect

The sweet potato whitefly, Bemisia tabaci, is a costly menace to agricultural crops worldwide, and now researchers have found a possible reason for the whitefly's success: It has found a way to protect itself from the usual plant toxin defense.

The discovery by an international team of scientists appears in the April 1 edition of the journal Cell.

The whitefly is a tiny, aphid-like insect which feeds on plant tissues (phloem) that conduct the nutrients produced in plant leaves. Whiteflies are polyphagous. That is, they are known to devour hundreds of different types of plants. They also transmit many deadly plant viruses, while the sticky, sweet residue that they leave on the plant can become a host for black fungus.

As a result of its behavior, the researchers note, whiteflies can seriously reduce global crop yields.

The research group set out to find out how the whitefly successfully evades phenolic glycosides, the usual defense mechanism of plants to prevent insect damage. They found that the whitefly has a gene, BtPMaT1, that enables it to detoxify phenolic glycosides.

Surprisingly, the gene in question had its closest homologs in plants. The researchers write "No ortholog of BtPMaT1 was found in the gene of the greenhouse whitefly, Trialeurodes vaporariorum, that was the most homologous to the two flanking B. tabaci serine proteases genes.” The hypothesis of the research group is that this may be an exceptional example of horizontal gene transfer — a gene moved from plant to insect. 

But the mechanism of how this occurred remains uncertain.

The researchers write: "...using bioinformatic, molecular, and biochemical approaches, combined with insect performance assays, we show that the B. tabaci genome harbors a plant-specific and horizontally transferred gene, BtPMaT1, encoding a phenolic glucoside malonyltransferase that enables the detoxification of phenolic glycosides. This discovery reveals an unexpected route by which B. tabaci has evolved its extraordinary ability to overcome the defenses of its host plants."

The researchers suggest that the whitefly's ability to detoxify the phenolic glucoside "most likely relies on a conjugation reaction similar to herbicide metabolism in certain plants, whereby malonytransferase can catalyze detoxification by conjugating the herbicide or an intermediate metabolite with malonyl-CoA."

The research also suggests that finding a specific way to counteract the whitefly's ability to cancel out the plant's phenolic glycosides could help control whitefly damage. As they write: "... we show that interfering with the functioning of this plant-derived gene in B. tabaci can be a highly effective way to control this extremely important global pest."

Horizontal gene transfer is a possible hypothesis for this research finding, but not yet confirmed. As molecular entomologist Yannick Pauchet, at the Max Planck Institute for Chemical Ecology, told Nature magazine, “According to the data they provide, horizontal gene transfer is the most parsimonious explanation.”