Genetics race against world’s worst megapest
One very hungry caterpillar has earned itself the reputation as agriculture’s worst insect pest.
It’s the caterpillar which transforms itself into the Helicoverpa moth.
Unlike other butterflies and moths (or Lepidoptera) confined to a narrower diet, this caterpillar has more than 200 extra genes for taste receptor proteins and detoxification and digestive enzymes.
That means this hungry caterpillar can devour its way through a huge variety of crops.
An international team of researchers lead by CSIRO has just completed an eight-year project to sequence the genome of this voracious and devastating pest. The study has revealed the genetic secrets to its extraordinary adaptability that could help scientists stay one step ahead in the arms race of resistance.
The sequencing project looked at not one but two, closely-related Helicoverpa species: Helicoverpa armigera (or Cotton Bollworm), which is dominant in Australia, Africa and Asia, and Helicoverpa zea (or Corn Earworm), which is found across the Americas.
Armed and warlike
There are actually at least 14 different Helicoverpa species, but these two are top of the list of agricultural pests.
The bollworm and earworm have a taste for broad-acre crops from corn to cotton, from alfalfa to tobacco, causing in excess of US $5 billion in control costs and damage each year across Asia, Europe, Africa, America and Australia.
The bollworm, which is dominant in Australia, attacks more crops and develops much more resistance to pesticides than its earworm counterpart.
“It is the single most important pest of agriculture in the world, making it one of humanity’s greatest competitors for food and fibre,” says CSIRO Scientist Dr John Oakeshott.
“Its genomic arsenal has allowed it to outgun most of our known insecticides through the development of resistance, reflecting its name – armigera which means armed and warlike.”
Spotting the difference
The genome sequencing of both species uncovered some important differences, which CSIRO researcher Dr Karl Gordon says reveal much about their shared history.
“It’s a remarkable scientific story in a way, because these two species diverged about 1.5 million years ago, as a result of a very small founder population of H. armigera reaching the Americas and becoming the genesis of what became H. zea.”
Until around 15 years ago, there was no evidence of any intermingling between these two geographically-separate species. Despite this, the genomes of the two species are still remarkably similar, although H. zea appears to have slightly fewer detoxification and taste receptor genes than H. armigera.
Significantly, H. zea may have actually lost some of the genes that it once shared with H. armigera. Dr Gordon thinks it may be because it didn’t need them.
“It’s suggesting that the ecology that H. zea found in the Americas was very different to the ones that H. armigera had to remain adapted to in its home range across Africa, Australia and Asia,” Dr Gordon says.
A taste for toxins
Currently, one of the few things standing between the two Helicoverpa species and greater agricultural damage are crops genetically modified with genes from the soil bacteria Bacillus thuringiensis. These genes cause crops including cotton, corn and soybean to produce a toxin that is harmless to humans, but deadly to pests such as Helicoverpa.
There are three Bt genes in use in crops around the world, but already there are early warning signs that the moth may be developing resistance to the toxin.
The ability to adapt to toxins appears to be key to understanding the Helicoverpa’s ongoing megapest status.
Overall, the research team identified more than 17,000 protein coding genes in the genome. One of the most important findings was that the two species of Helicoverpa possess an unusually high number of genes related to digesting and detoxifying potential chemical threats in their food.
Another was that they have a large number of genes for taste receptors; up to around four times as many as their more specialized relatives among the Lepidoptera.
“We think that it’s a good part of the reason why it’s so versatile in the crops that it can attack if it’s got this massive suite of different genes that would help it detoxify the plant defences and identify the right crops in the first place,” Dr Oakeshott says.
“The versatility of its detoxification system is a double whammy in terms of both giving it a broad host range and giving it the capacity to develop resistance.”
The large number of genes controlling digestion and detoxification suggest the Helicoverpa genome has a high degree of plasticity, meaning it is able to adapt and evolve very quickly into new ecological niches to take advantage of the opportunities and respond to the threats.
Confronting a hybrid
By mapping the genome of both species, researchers also found the chromosomes are structurally very similar; the same sorts of genes are found in the same sorts of places.
This means that it will be relatively easy for the two species to interbreed and form fertile hybrids, with less risk of the sorts of genetic incompatibilities that often prevent this from happening in other living creatures; for example, the sterility that blights the offspring of a horse and a donkey.
This is important because the bollworm found its way to Brazil about 15 years ago and is now spreading rapidly across the Americas, with cases reported already of it hybridising with the earworm. This poses a real threat that the new and improved “superbug” could spread into the United States.
For that reason, Dr Oakeshott says, we need to understand the genome of both species.
“It’s very important to know what the genetic differences between the two species are, because what we suspect will happen is hybrids will form which could well have the worst features of both,” says Dr Oakeshott.
Keeping one step ahead
The good news is that now researchers are armed with a complete map of the H. armigera and H. zea genomes, they can begin to look for potential new genes to target with insecticides or other control measures such as RNA silencing, which uses small sequences of RNA to shut down particular genes.
Understanding Helicoverpa’s genetic past will help researchers around the world to predict its possible futures. The genome study, and other research, is showing just how quickly Helicoverpa is able to spread across a continent, how it responds to selection pressures such as changes in agricultural practices, and how quickly it is able to share genes across the species boundary, Dr Gordon says.
“We can now combine those sorts of analyses and insights and monitor the insects’ changing genetic profile and structure far more efficiently around the globe,” he says. “It means we’ll be able to keep up, if not stay ahead of, its adaptation to insecticides and control strategies.”