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Researchers identify two supergenes in one ant species that controls the number and size of queens in the colony

Ants are fascinating insects. The colony is sustained through complex social dynamics, with each member — the queen, males and workers (sterile females) — contributing to the greater community. While not common to all ants, some species add complexity to this dynamic with the addition of rather small queens. 

Researchers at the University of California, Riverside tackled why these additional queens are present by focusing on complex genetic structures, called supergenes, in the community. These supergenes control the origin and duplication of the petite queens in one particular ant species. The results of their study are available in the December issue of the journal Current Biology.

“We were intrigued by the tiny queens,” said senior author Jessica Purcell, associate professor in the Department of Entomology at the University of California, Riverside. “It’s a great system (multiple queen ant colonies) to study the evolution of supergenes, which exist in many types of organisms.”

Supergenes vary in size but consist of a cluster of genes on the same chromosome that are linked. The supergene suppresses recombination, a common shuffling of genes during cell division, and transmits and preserves the architecture of the supergene from generation to generation. These mega genes may actually be rather common and are associated with specific outward traits, or phenotypes, expressed by many organisms, including migratory patterns in fish and birds and the number of queens in an ant colony. 

While not common in all ant colonies, Purcell and her colleagues focused the small queens in one ant species, Formica cinerea, which are common throughout Europe, spanning from Spain to western Siberia and from Scandinavia to the Balkans. These ants are not pests. In fact, they are quite beneficial, happily spending their days munching on spiders, mites and even sugary aphid poop.

The research team identified the presence of two supergenes that increases the colony queen count as well as control queen size. Their research reveals how supergenes can affect the dynamics of this particular ant society. 

As a refresher, the queen is perhaps the most important member of the ant society. The largest ant in the colony, the queen has wings and can fly away, but her primary role is to lay eggs to grow generation after generation of workers, males and her successor. Workers live for about six months and spend their time foraging for food and caring for the brood. Males have a shorter lifespan. They rarely leave the colony and mate once with a future queen.

Tiny queens on the other hand obtain their power from a supergene on chromosome 3, which formed about 23 million years ago and determines whether colonies have one queen or multiple queens. The supergene is composed of 500 genes that are bestowed as a unit to the ant’s offspring. 

Surprisingly, the researchers found another supergene, this one on chromosome 9, which is responsible for ant size. The passing of the two supergenes together produces additional queens that are 20 percent smaller than a typical ant queen. 

The pairing of the two supergenes can have a considerable effect on the colony.

“Ants are social insects and create huge colonies where nestmates are related to each other, and colonies works well together to raise their relatives,” said Giulia Scarparo, a post-doctoral student in the Entomology Department at the University of California, Riverside and first author of the study. “In a polygyne (multiple queen) colony, the relationship between nestmates is lower. The presence of many unrelated nestmates can lead to reproductive conflicts among workers. ”

So why is this happening? The researchers suggest the passing of the combined supergenes may provide a strategy to help the colony expand and survive. 

Establishing an independent colony requires the queen to fly to a new location and use her body reserves housed in the wing muscles and body fat to raise the first brood of workers. This strategy is very risky and can led to the death of the queen and with her the colony. 

Tiny queens lack the energy reserves to establish a new colony, but they can join existing colonies. Despite the benefit of this approach, it raises new questions to consider regarding the ongoing cooperation among groups who are not genetically related.

“The discovery of supergenes associated with microgynes (small queens) is the first discovery of this type,” said Scarparo. “We think these kind of queens can be socially parasitic — social insects that take advantage of other social insects. Microgynes could be a good starting point to understand the evolution of social parasitism, and we may have a supergene driving this lifestyle.” 

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Purcell and Scarparo were joined on this project by Alan Brelsford and Marie Palanchon at the University of California, Riverside on the article, titled “Social antagonism facilitates supergene expansion in ants.” This study received support from the National Science Foundation.