Beating the bugs
Wednesday, 04 December, 2002
There is a complex and ongoing battle between humans and insect pests, according to Dr Phil Batterham, a researcher in the University of Melbourne's Department of Genetics and the deputy director and program leader at the ARC-funded Centre for Environmental Stress and Adaptation Research (CESAR). Many chemical and biological weapons have been devised to control and prevent insects from attacking crops, but insects are highly adaptive and sooner or later they evolve their way around every weapon that is used against them.
According to Batterham, essential techniques of modern biology, namely genetics and genomic approaches, may provide the tools to create new weapons. "Insect resistance work tells us that there is a crying need for more insect genomics to be done," he says.
Batterham describes three parts of this battle that can be significantly aided by the use of genetics and genomics -- surveillance, defence and offence tactics. Constant surveillance is required to determine where the insects come from and go to, he explains. Some insects, such as the cotton bollworm Helicoverpa armigera, are highly migratory, and other insect species appear and disappear with little or no indication of where they have come from and where they are going.
The use of molecular markers can assist researchers in tracking insect movements in the field. In addition, suitable molecular markers could allow early identification of newly emerging resistance, says Batterham. Defence tactics for insect control include using genetics, genomics and biochemical techniques to investigate the insect's resistance mechanisms at a fundamental level. "We know precious little about detoxification systems used by insects to degrade toxins," says Batterham. One such system was recently described by Batterham and his research team, a cytochrome P450-related gene called Cyp6g1 discovered in fruit fly and lab model Drosophila melanogaster that confers broad spectrum resistance to organochlorides and organophosphates including DDT.
Other enzymatic mechanisms utilise carboxylesterases or glutathione-S-transeferases to break down insecticides. Batterham says it is vitally important to work out how the introduction of a pesticide affects genes like this. Another defence tactic that needs more work, he believes, is determination of the molecular targets for the pesticides themselves. "People usually don't know what the insecticide target is before it is introduced -- this applies to both conventional and genetically modified insecticides," says Batterham.
A case in point is the discovery of the target protein for the Bt insecticidal toxin used both as a spray for crops and in genetically modified crop plants like cotton and canola. Bt is a protein from Bacillus thuringiensis that binds to surface proteins on insect cells and creates pores in the cells that kill the cells and eventually the insect. But although Bt toxins have been used for years, the first known target protein on insect cells, cadherin, was only identified last year in the tobacco budworm Heliothis virescens in the lab of Dr David Heckel, another University of Melbourne and CESAR geneticist.
Batterham notes there is a major point of difference between the agrochemical industry and the pharmaceutical industry here. While regulatory bodies including the US Food and Drug Administration and Australia's Therapeutic Goods Administration require detailed understanding of the mechanisms of action and modes of inactivation before they will consider approving a new drug, there is not the same oversight for agrochemicals. "There's no way that the FDA would allow drugs on market if the degradation profile was not known. The chemical industry takes more of a shotgun approach, compared to rational drug design," Batterham claims. He believes that a more rational approach to pesticide design would allow science to keep pace with the ever-evolving resistance genes.
CSIRO entomologist Dr Peter East is also a proponent of rational insecticide design and in vitro screening. He believes the agrochemical industry is starting to embrace the concept of developing novel targets. For example, he says, Bayer CropScience and Exelixis have set up joint venture company Genoptera to discover targets for new pesticides, using a genomic approach. "If we went to companies with validated insect targets they'd probably be willing to pay a lot of money for them," he says. The gradual shift to a more highly regulated agrochemical industry is one incentive for the change in approach, East notes.
At CSIRO, East uses genomics and proteomics approaches to investigate insect and plant host interactions, reasoning that the complex co-evolutionary processes in play might provide some useful targets for new pesticides. The project received a major boost from the Grains Research and Development Corporation (GRDC)this year, in the form of a $20 million collaboration between the two organisations.
The idea of rational pesticide design is also part of the third tactic, offence. Batterham believes that identifying targets using genomics and genetics approaches, would allow new pesticides with alternative modes of action to be developed and deployed.
"It's really only now that a fair amount of effort is going into identifying suitable targets in insects," he says. But in order for the chemical and biological warfare programs envisaged by these researchers to be feasible, a lot more knowledge of insect genetics and genomics is required. At present, there is one insect, which has been the main focus of genomic analysis -- D. melanogaster -- which is a favourite model organism of developmental biologists. While it has been sequenced, Batterham says that high resolution maps and comprehensive "expressed sequence tag" (EST) databases are necessary to allow targets to be identified.
And Batterham believes that at a minimum, ESTs and high-resolution maps are a requirement for any insect pest. But there is not a lot of support worldwide for insect genomics projects. In addition to Drosophila and the malaria mosquito species Anopheles, a genomics project on the honey bee (Apis mellifera) is in the works and a Lepidopteran (moth and butterfly) genome project is also being planned.
Other researchers agree with him. "We don't have these resources for pest insects, we need to generate them ourselves, and there are not large dollars in this," says East. "It's almost an insurmountable divide really, there is very little money for insect genomics in Australia, or even globally." The result is that Australian researchers end up having to invest significantly in genomics research from their own grants, or find commercial partners who are willing to fund these activities, despite the long term nature of the research. Industry groups like the Cotton Research and Development Corporation and the GRDC provide some funding in these areas, as do some of the larger agrochemical companies.
But East says the research ends up as "bits and pieces" of smaller, targeted activities, instead of the large organised genome projects.
Heckel is part of the Lepidopteran project, which aims to sequence the genome of the silkworm Bombyx mori and undertake comparative genomics of other Lepidoptera, including Helicoverpa species. He says that while some of the EST work will be performed by Australian researchers, including scientists in his lab, the bulk of the sequencing effort will take place in Japan and the USA. Most of the funding will also come from offshore, although the Australian Genome Research Facility has contributed to the study by helping out with a pilot EST study. Heckel and Batterham believe that part of the problem is that most insect control programs are reactive, rather than proactive.
"There is a contrast between plant programs and insect control programs -- crop improvement programs are long term, whereas insect control programs respond to the latest emergency. We need to realize certain problems need to be looked at in the long term. Entomology is struggling as it does not have the critical mass to deal with the long-term view," explains Heckel.
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