Protecting farmers and their agricultural output around the world is a daunting task. Each region has its own trials and tribulations related to their temperature, rainfall, soil consistency, and of course, their pathogens. An unfortunate side effect of the otherwise hugely beneficial impact of international trade is that the pathogens faced in one corner of the world are rapidly becoming a problem for everyone else as well. Even before shipping and airplanes were a thing, there were many pathogens that loved to spread across connected continents, following the similar spread of the crops they feasted upon to new areas.
Scientists and farmers used to think that gene flow, even between related species, was limited in scope. Sure, a gene here or there may get transferred from some event, but it was always a limited and rare occurrence. These assumptions have largely fallen apart in recent decades, especially among pathogen researchers, as hybridization between separate species appears to be a more common happening than anyone realized.
The Worst of the Pests
When speaking of pathogens of crops, some form of bollworm likely comes up pretty quickly in the list. The moth larva that make up this group of insects are feared due to their ability to ruin fruiting crops from the inside out and it is difficult to combat them due to how they use the body of the very plant itself as protection. Add in that they seem to have an uncanny capability to pick up new resistance genes in the blink of an eye and you end up with a real problem.
Out of all of the species in this group, it is the cotton bollworm, Helicoverpa armigera, that is likely brought up as one of the most often despised pathogens, especially in the developing world that relies on cotton as a prime export source. It doesn’t help that this species and all the sub-species varieties of it are well-traveled and can grow in multiple climates. Thanks to this, it can be found throughout Europe, Asia, and Africa, along with the Australia and Oceania regions.
Such outbreaks have, for now, been controlled through the use of targeted chemical spraying and the production of genetically modified cotton that kills any larva that try to feast upon it. But the pests require constant vigilance and versatile approaches. And it appears that this moth has still managed to get around our precautions.
A Spreading Disease
It was discovered in 2013 that the cotton bollworm had managed to somehow spread to Brazil in South America. The exact manner of how it accomplished this remains a mystery, but it has been devastating for the agricultural sector of the country. Now, it appears things might get even worse.
The evolutionary ancestry of the bollworm is just as adventurous and worldly as its modern descendant has proven to be. Around 1.5 million years ago, this ancestor managed to reach the Americas as well and has evolved since then into the species known as the corn earworm, Helicoverpa zea. For the western hemisphere, it is this pest that has proven one of the most controversial and damaging. Corn being a primary product of North and South America, that means a pathogen specifically aimed to consume it is an issue that can’t be ignored.
Researchers at Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) have been following this development and spread of the cotton bollworm for more than a decade. They recently released their findings after taking samples of both the bollworm and the earworm in Brazil after the former began populating the region. And what they found is not good news.
The Hybrid Mega-Pest
There appears to have been significant hybridization occurring between the two moths. The scientists ran a whole genome resequencing on all of the eight specimens they had collected in Brazil and compared their genomes to the already sequenced genomes of the several dozen other samples from the two species they had gathered over the years. After that, they also ran a phylogenetic analysis of the single nucleotide differences in the observed genomes, along with matching them to the population structure of the species.
The first thing they revealed is that the cotton bollworm as a whole appears to have a population with incredibly high genetic diversity among its members. This might explain why it is so adaptive across multiple climes and can develop resistances so quickly so that it can invade new territory. But the big news is that it appears that the cotton bollworm is part of an ongoing, multigenerational hybridization with the corn earworm and it isn’t the sort of joining that just makes it easier to deal with both of them at the same time. Oh no, it’s much more complicated than that.
Every single one of the hybrids among the 8 samples were unique, with each of them having apparently hybridized on their own and forming a separate combination of genes from the two species. This sort of event is referred to scientifically as a “hybrid swarm”, where multiple generations past the initial event have happened, allowing the hybrids to genetically differentiate themselves from each other.
The worst part is that the hybrids seem to share resistance genes from both species, giving them a weapon against any chemical method agricultural researchers and farmers try to employ. The CSIRO scientists were able to estimate that around 65% of the US’ agricultural output is at risk if this hybrid spreads northward and things will likely be worse for other countries in the Americas that have even more reliance on crops like corn and cotton.
A Limited Amount of Options
Clearly, some sort of response has to be taken in order to prevent the growth of these hybridization events and to stop the hybrid offspring from extending beyond Brazil. Especially to stop them from returning back to Europe or other parts of the interconnected continents where they could bring back corn earworm traits and resistances to the rest of the cotton bollworm species.
Whatever actions do end up being taken, it may likely turn out to not be good enough. With that significant possibility taken into consideration, the best option may be to focus on new and improved methods of dealing with these new and improved pathogens. Either that or just bite the bullet on the crop losses, but it is doubtful anyone will pick that options.
For how things will eventually turn out, we’ll just have to wait and see.
Photo CCs: Corn earworm from Wikimedia Commons