Making resistance futile

| Posted by Andre DP Encarnacion

More than a century ago, a scientist named A.L. Melander wrote an article in the Journal of Economic Entomology on a disturbing turn of events in his native Washington. The year was 1914, and then, as now, farmers and entomologists were locked in combat with pests like the San Jose scale—an insect similar to the cocolisap that would nearly overwhelm the Philippine coconut industry a hundred years later.

Since its introduction in the US in the 1870s, the scale had been responsible for eliminating thousands of acres of apple trees. The frontline chemical to control it back then was sulphur-lime, an insecticide made by reacting calcium hydroxide with sulphur. Melander himself described the mixture as “fast acting” in its lethal effects. After years of success, however, to his great shock, Melander found that the insects were going against script. Some of the scales just refused to die.

More specifically, in samples taken from two Washington cities, he found that while insects from Wenatchee that he sprayed all died within a week, while 90% of the scales from Washington’s Clarkston Valley that he treated remained alive. Remarkably, even after Melander increased the active ingredient by ten times, 74% of the latter still survived.

Melander’s recounting of this phenomenon would become the first article ever to document insecticide resistance. For many, it was an ecological alarm bell. Researchers would eventually confirm that insect pests like the scale could take advantage of the laws of natural selection to better withstand insecticides over generations through metabolic or behavioural adaptations.

 

Dr. Maria Anita Bautista in the laboratory. (Photo by El Bacani, UP MPRO)
Dr. Maria Anita Bautista in the laboratory. (Photo by El Bacani, UP MPRO)

 

Against such a fast-adapting problem, it is up to scientists and farmers to update their own toolkits. For years, UP entomologist Dr. Ma. Anita “Marianne” Bautista has worked hard to do just that. And now as the head of the Philippine Genome Center’s (PGC) Agriculture, Livestock, Fisheries and Forestry Program, she’s spending her time trying to help others.

 

Lucky charm

Eventually, with complementary advances in genetics and genomics, scientists began to look into the molecular basis of insecticide resistance. While Melander went into the textbooks for his work on the San Jose scale, Bautista, too, has done considerable work on an insect pest she considers her “lucky charm”—the diamondback moth (Plutella xylostela L.).

The UP Los Baños graduate of BS Agriculture, major in Entomology, first started working with the moth as part of her thesis. Even before doing the genomics work that would become her trademark, she was already deeply interested in controlling its numbers. “Because it is a pest,” she says. “Your cabbage, your kale, your pechay—it is a notorious pest of those crops.”

 

Diamondback moth (DBM) larvae grown on cabbage seedlings for insecticide resistance experiments. (Photo by Dr. Anita Bautista, UP-PGC)
Diamondback moth (DBM) larvae grown on cabbage seedlings for insecticide resistance experiments. (Photo by Dr. Anita Bautista, UP-PGC)

 

Bautista, together with other scientists, had not only noticed that the moth and its voracious larvae were costing the world upwards of $2 billion annually, but that they had easily developed resistance to insecticides used on cruciferous crops. Her initial work on the moth as an undergraduate student led her to become a Monbukagakusho scholar, earning her PhD in Agricultural Science at Nagoya University. And it was there that she undertook her most popular research project to date.

While many of her peers at the time were looking into classical management systems to try and contain the pest, she wanted to see “at the level of the genes” what mechanisms were behind the moth’s resistance.

 

Drop by drop

The first question Bautista asked was: what genes and enzymes were responsible for the moth’s resistance? Zeroing in on resistance to the insecticide permethrin with professors Toshiharu Tanaka and Tadashi Miyata, she found a promising candidate in an enzyme called cytochrome P450.

“When the moth is exposed to insecticides, the tendency of the insect is to increase the expression of these enzymes,” she says . This means that the diamondback moths produces an increasing amount of cytochrome P450, which helps detoxify them from lethal chemicals. Most of these enzymes are found in the moth’s midgut and act via hydrolysis—breaking down permethrin with water and rendering it ineffective.

 

Droplet-feeding double stranded RNA to diamondback moth (DBM) larvae to knockdown  cytochrome P450 gene. (Photo by Dr. Anita Bautista, UP-PGC)
Droplet-feeding double stranded RNA to diamondback moth (DBM) larvae to knockdown cytochrome P450 gene. (Photo by Dr. Anita Bautista, UP-PGC)

 

The next important step was to find evidence that this relationship existed. Once a set of candidate genes were determined, Marianne and her colleagues used a technique that was novel at the time—RNA interference or RNAi. This entails injecting insects with double-stranded RNA (dsRNA) corresponding to the target genes to inhibit their expression. These dsRNAs are spliced, and they target their respective messenger RNAs—molecules that convey information from DNA, thus preventing them from being translated into enzymes like cytochrome P450.

There was only one problem. At around 5 mm. long, diamondback moth larvae would likely not survive being injected with a typical syringe. So what Marianne did was droplet-feed them–a slow and laborious process. At that time, an unorthodox effort, this, however, would eventually pay dividends.

In what became a paper published in Insect Biochemistry and Molecular Biology, Bautista and her colleagues found out that knocking down cytochrome P450 transcripts did reduce the permethrin resistance of diamondback moths. Moreover, her unique droplet-feeding approach to knocking down genes started to be adopted by others dealing with smaller insects. All Bautista’s work with her favorite pest insect finally hit paydirt.

 

Becoming a mentor

Solving such a thorny problem would open several doors for Bautista. For instance, it netted her an invitation to be a postdoctoral fellow at Ohio State University for a project to possibly make insects susceptible once more. Here, she would discover the power of Next Generation Sequencing (NGS) and bioinformatics, the two pillars of contemporary genomics research.

Staying in the US, however, was out of the question. Both Bautista’s family and her willingness to pay things forward led her back to the Philippines, and to the newly established PGC in 2013.

 

Dr. Bautsita poses with staff from the PGC and the PCARI Shared Genomics Core Laboratory. (Photo by El Bacani, UP MPRO)
Dr. Bautsita poses with staff from the PGC and the PCARI Shared Genomics Core Laboratory. (Photo by El Bacani, UP MPRO)

 

Today, she is known as the resident transcriptomics expert at both the PGC and the UP National Institute of Molecular Biology and Biotechnology (UP-NIMBB). As the head of the PGC’s Agriculture, Livestock, Fisheries and Forestry Program, her focus now is on sharing the novel genomics techniques she learned with the country’s State Universities and Colleges (SUCs) and beyond.

“I will continue to empower young scientists,” Bautista said, when asked about future plans. Her formative years studying insecticide resistance appeared to have drilled into her a more fundamental truth—that the future of science depends on nurturing the creativity of young researchers. “I think that’s how science should work. It’s really important that you become a mentor.”

The heart of this mission lies in Marianne’s belief that promoting genomics knowledge can help solve social, as well as scientific problems. One current project finds her studying another pest, mosquitoes like the dengue-carrying Aedes aegypti. Using lessons from the diamondback moth, she now tries to find correlations between the diversity of these insects and the viruses they carry, as well as the land use of surrounding barangays. “The study is expected to provide benchmarks on the impact of land use change in dengue disease transmission, “ she says.

This is a problem where scientific expertise can immediately influence the policy debate. And that sounds like an irresistible next opportunity—even for an expert on resistance.