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The Pests of Pests: Relationships between Termites, Parasites and Mutualists

Photo: Siddharth Kankaria/Research Matters

As summer dawns, most parts of India gear up for the rains with a moist, cool breeze that brings welcome respite from the scorching sun. And with the rains, come termites. Almost all of us would have come head to head with these tiny winged insects that resemble ants. They rapidly and unceasingly chew through any wood they can come across, but run into problems when it comes to digesting them. Of course, they have a clever work-around.

Plant cells are made of a tough fibre called cellulose, which is very difficult to break down. Rather than do the work themselves, the termites have ‘outsourced’ the job to a rather unlikely collaborator -- fungi. Termites farm a fungus called Termitomyces within their nests and let it do the hard work of digesting the cellulose, while they eat the manageable, broken-down by-products. With this arrangement, the fungi get free food and the termites are saved from much digestive trouble. This collaboration that benefits both parties is called ‘mutualism’.

Apart from mutualism, there are other kinds of collaborations that exist between species, and turns out that the termite Odontotermes obesus, is also a part of one more. This collaboration is however not a beneficial one, but is ‘parasitism’ – where the smaller partner benefits and harms the larger. Pseudoxylaria is another weedy fungus in the picture, which draws benefits from the termite farm and harms it in the process.

“Fungus growing termites are the most ancient inventors of agriculture, dating back to around 27 to 36 million years ago. This means that they have had millions of years to perfect their farming techniques and yet have to deal with weedy, parasitic fungi”, remarks Prof. Renee Borges from the Centre for Ecological Sciences, Indian Institute of Science.

Research has revealed that the mutualistic fungus, Termitomyces, reproduces ‘asexually’ -- like bacteria, where each cell splits into two. It adopts a sexual mode of reproduction when it moves from one termite colony to another. These two modes result in different genetic patterns -- asexual reproduction results in uniform genetic pattern since each cell just splits into two, while sexual reproduction involves the mix-up of different strains of the fungus, resulting in a different genetic signature.

However, there is no information about how the parasitic fungus, Pseudoxylaria, traverses from one termite nest to another. Investigating the genetic patterns in these fungi, as with Termitomyces, might provide a window into this dynamic system. This is exactly what the team of researchers led by Prof. Renee Borges decided to do.

But why is one mode of reproduction chosen over the other? Is there a ‘strategy’ behind this?  Recollecting high school biology reminds us that sexual reproduction creates diversity due to the mixing up of genes from different individuals, which in turn can create useful traits in the offspring. In case of species involved in a mutualistic replationship like Termitomyces, any change, genetic or otherwise, can potentially harm them. When the researchers analysed the DNA of Termitomyces from a single termite nest, they found that all of them had the same DNA and these DNA sequences remained unchanged two and half to five months after the first analysis; a finding not recorded by any other experiment.

In contrast, the researchers found that Pseudoxylaria, the other fungus in the termite nests, had a range of DNA sequences within the same termite nest, implying that these fungi reproduce sexually. The ‘strategy’ behind sexual reproduction was to increase the genetic diversity and produce a disharmony between mutualistic species, allowing the parasite to gain an upper hand. However, it was observed that these fungi used asexual reproduction to spread to different termite nests.

What are the implications of these findings? “This should be a wake-up call to humans who think that they can develop super crops or magic bullets to control pests. These will always have to keep changing since there will always be an arms race between crops and pests”, says Prof. Borges. Once a parasite has gained a foothold in an ecosystem, it cannot be easily removed. In fact, in a few centuries, the host might find ways to live its life around the parasite and by this time, another parasite might emerge, forcing changing in this dynamics.

As summer dawns, most parts of India gear up for the rains with a moist, cool breeze that brings welcome respite from the scorching sun. And with the rains, come termites. Almost all of us would have come head to head with these tiny winged insects that resemble ants. They rapidly and unceasingly chew through any wood they can come across, but run into problems when it comes to digesting them. Of course, they have a clever work-around.

Plant cells are made of a tough fibre called cellulose, which is very difficult to break down. Rather than do the work themselves, the termites have ‘outsourced’ the job to a rather unlikely collaborator -- fungi. Termites farm a fungus called Termitomyces within their nests and let it do the hard work of digesting the cellulose, while they eat the manageable, broken-down by-products. With this arrangement, the fungi get free food and the termites are saved from much digestive trouble. This collaboration that benefits both parties is called ‘mutualism’.

Apart from mutualism, there are other kinds of collaborations that exist between species, and turns out that the termite Odontotermes obesus, is also a part of one more. This collaboration is however not a beneficial one, but is ‘parasitism’ – where the smaller partner benefits and harms the larger. Pseudoxylaria is another weedy fungus in the picture, which draws benefits from the termite farm and harms it in the process.

“Fungus growing termites are the most ancient inventors of agriculture, dating back to around 27 to 36 million years ago. This means that they have had millions of years to perfect their farming techniques and yet have to deal with weedy, parasitic fungi”, remarks Prof. Renee Borges from the Centre for Ecological Sciences, Indian Institute of Science.

Research has revealed that the mutualistic fungus, Termitomyces, reproduces ‘asexually’ -- like bacteria, where each cell splits into two. It adopts a sexual mode of reproduction when it moves from one termite colony to another. These two modes result in different genetic patterns -- asexual reproduction results in uniform genetic pattern since each cell just splits into two, while sexual reproduction involves the mix-up of different strains of the fungus, resulting in a different genetic signature.

However, there is no information about how the parasitic fungus, Pseudoxylaria, traverses from one termite nest to another. Investigating the genetic patterns in these fungi, as with Termitomyces, might provide a window into this dynamic system. This is exactly what the team of researchers led by Prof. Renee Borges decided to do.

But why is one mode of reproduction chosen over the other? Is there a ‘strategy’ behind this?  Recollecting high school biology reminds us that sexual reproduction creates diversity due to the mixing up of genes from different individuals, which in turn can create useful traits in the offspring. In case of species involved in a mutualistic replationship like Termitomyces, any change, genetic or otherwise, can potentially harm them. When the researchers analysed the DNA of Termitomyces from a single termite nest, they found that all of them had the same DNA and these DNA sequences remained unchanged two and half to five months after the first analysis; a finding not recorded by any other experiment.

In contrast, the researchers found that Pseudoxylaria, the other fungus in the termite nests, had a range of DNA sequences within the same termite nest, implying that these fungi reproduce sexually. The ‘strategy’ behind sexual reproduction was to increase the genetic diversity and produce a disharmony between mutualistic species, allowing the parasite to gain an upper hand. However, it was observed that these fungi used asexual reproduction to spread to different termite nests.

What are the implications of these findings? “This should be a wake-up call to humans who think that they can develop super crops or magic bullets to control pests. These will always have to keep changing since there will always be an arms race between crops and pests”, says Prof. Borges. Once a parasite has gained a foothold in an ecosystem, it cannot be easily removed. In fact, in a few centuries, the host might find ways to live its life around the parasite and by this time, another parasite might emerge, forcing changing in this dynamics.