CRISPR MutAnts Lose Interest in Socialising

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New gene editing technologies have revolutionised genetic science, but social insects like ants have proved difficult to genetically modify because of their complex lifecycle and social structure. Now, two separate labs have succeeded in using the CRISPR-CAS9 system to genetically modify two unusual ant species, switching off genes and disrupting their social behaviour in the process.

CRISPR-CAS9 is a genetic tool developed over the last 12 years as a promising new method for editing genes. It has also been used to study basic biology and even behaviour in animals (ref), but it’s proved difficult to apply to ants because of their colonial way of life. In two studies published last week in the Journal Cell, separate teams of researchers have successfully edited ant genes to create entire mutant colonies. These mut-ANTS* showed altered social behaviour, after scientists mutated a gene involved in their sense of smell.

The gene both teams chose to edit was orco – which is an essential part of the production process for all odorant receptors – including those for detecting pheromones. Pheromones are vital for ants to communicate, forage, identify one another and organise their colonies. Ants have over 350 genes for odorant receptors, which are expressed on the surface of specialised odorant receptor neurons in their antennae, and are sensitive to particular smells. Each of these neurons connected to a blob-like brain structure known as a ‘glomerus’, where odors are processed by the brain.

Without their sense of smell, these studies show, the ants wander aimlessly, don’t cluster together with other ants, refuse to build a nest, and steal food from one-another – much like non-social insects. This shows just how crucial smell is to keep social insect colonies running harmoniously.

The reason ants are particularly challenging to edit genetically is because they live in colonies that posses many of the traits of an organism in it’s own right. In fact, ant colonies are often described as ‘superorganisms’ because the individual ants within a colony in some ways resemble the cells within a larger organism, like you or me. Queens and males can be thought of as the eggs and sperm of the colony, while the workers are the ‘somatic cells’ – the cells that make of the rest of the body, perform all the other necessary functions of life, but don’t get to contribute directly to the next generation. This aspect of their biology makes ants fascinating for studies of complex systems.

Whereas we can now edit many creatures with relative ease by altering the genes in the early embryo, ants and other social insects that live in colonies have proved more challenging because we need the whole colony to carry the same genetic edits for the results to be meaningful. That’s just what two teams of researchers in New York have finally managed to do, albeit for unusual species of ant whose biology offers a clever solution to the superorganism problem.

Mutant raider ants

Ooceraea biroi workers tagged with color dots for individual behavioral tracking. Credit: Daniel Kronauer, The Rockefeller University.

Daniel Kronauer (Rockefeller University) and a team of researchers chose the clonal raider ant, Ooceraea biroi for their study, because it allowed them to exploit a unique aspect of this species’ biology. These unusual ants reproduce clonaly (as the name suggests), with adults developing from unfertilised eggs. They have no queens in their colonies, no males, and a very unusual way of . This makes it much easier to spread mutations through an entire colony – all you need to do is edit a single egg and within a short space of time you’ve got a colony of identical mutant ants.

Like other ants, raider ants are thought to orchestrate their colony life using pheromones, and ants have massively expanded on the basic set of odorant receptors found in other insects. In fact, raider ants are particularly dependent on smell, being blind. By editing the single gene orco, Kronauer were able to disrupt every single odorant binding receptor the ants produce, nearly eliminating their sense of smell.

In the study they found that mutant ants were no longer repulsed by the smell of a marker pen, which usually repels them. They wouldn’t follow foraging trails, nor create nests like wild raiders ants do. The authors concluded that the loss of the odorant receptors had interfered with their ability to communicate with other ants, shutting off the social-side of their lives.

Jumping Mutants

Inspired by Kronauer’s success, Claude Desplan (New York University) and his colleagues tried a similar trick with the Indian Jumping Ant, Harpegnathos saltator. This species doesn’t reproduce clonally like the raider ant, but it does have workers capable of turning into ‘‘, reproducing, and taking over the colony. Normal colonies are headed by a queen, but when she dies her workers can activate their ovaries and take over the dominant role.

Desplan and his team injected CRISPR-Cas9 into worker embryos and then allowed the adult to switch into a pseudoqueen, who would then found a new colony, containing workers carrying this new mutation. These ants, like the mutant raider ants, couldn’t communicate, forage or compete with other ants to become a pseudoqueen, and generally wandered around aimlessly, leaving the colony at random (a behaviour that the authors called, “space cadets”).

Two H. saltator ants fight. Public domain image from Wikimedia commons.

As well as showing altered social behaviour and anosmia (the loss of the sense of smell), mutant ants in both studies also showed unusual brain architecture. The associated with smells sensed by their antenna showed major reductions in size compared to normal ants. This suggests that receptor function in needed for these brain regions to develop in the ant brain. However, the result could also be explained if these regions initially develop but then wither because of lack of use.

The mutant ants showed changes that haven’t been seen in similar studies knocking out the orco gene in insects such as mosquitoes, fruit flies and moths. In these insects, the connections between odorant receptors and glomeruli are hard-wired in development – even without orco, flies, moths and other insects still develop the antennal glomeruli in their brain. Ants, it seems, are more like mammals in this regard, where connections between neurones and glomeruli develop in a much more flexible way during development, based on the presence of specific odorant receptors.
Greater flexibility in odorant perception may have been key to help ants evolve complex social behaviour.

A better understanding of how ants’ brains process social information and communicate within their colonies could hold valuable insights into our own brains.

“While ant behavior does not directly extend to humans, we believe that this work promises to advance our understanding of social communication, with the potential to shape the design of future research into disorders like schizophrenia, depression or autism that interfere with it,” said corresponding author Claude Desplan, Professor at New York University’s Department of Biology.

* Sorry for the shameless pun.

Want to Know More?

1. Trible et al (2017) “orco mutagenesis causes loss of antennal lobe glomeruli and impaired social behavior in ants”, Cell DOI: 10.1016/j.cell.2017.07.001
2. Yan et al (2017) “An engineered orco mutation produces aberrant social behavior and defective neural development in ants.” Cell DOI: 10.1016/j.cell.2017.06.051
3. Ravary and Jaisson (2002) “The reproductive cycle of thelytokous colonies of Cerapachys biroi Forel (Formicidae, Cerapachyinae)“, Insectes Socia 49(2), DOI: 10.1007/s00040-002-8288-9

Clonal raider ants live in cyclical colonies that alternate between two phases. In the reproductive phase, adult ants all activate their ovaries at once. These unmated females lay unfertilised (clonal) eggs for a few days and then care for them as they develop into larvae and eventually pupate. As their young are developing, they enter the second phase – the foraging phase. During this time, adults forage for food by raiding other ants’ nests and stealing their food and young, which they feed to the developing larvae. When the next generation emerges they enter the reproductive phase once more, and the cycle continues.
The jumping ants are quite closely related to the species I studied for my PhD, Dinoponera quadriceps. While in H. saltator colonies have a queen who is eventually replaced by pseudoqueens, also known as gamergates, in D. quadriceps they’ve done away with the queen caste entirely and all colonies are headed by these reproductively active workers. In both cases, the switch to becoming a pseudoqueen or gamergate is triggered by pheromones – the current reproductive female secretes signals telling the other ants she’s alive, but once these disappear, workers will start to fight it out to decide who gets to activate their ovaries and take over the colony.
Around 500 antennal lobe glomeruli in O. biroi, and 275 in H. saltator, incase you’re interested

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