A study of ants provides information on the evolution of social insects

September 10, 2018, Universitaet Mainz

One of the great puzzles of evolutionary biology is what induced certain living creatures to abandon solitary existence in favor of living in collaborative societies, as seen in the case of ants and other social, colony-forming insects. A major characteristic of so-called eusocial species is the division of labor between queens that lay eggs and workers that take care of the brood and perform other tasks. But what is it that determines that a queen should lay eggs and that workers shouldn't reproduce? And how did this distinction come about during the course of evolution? Evolutionary biologist Dr. Romain Libbrecht has been considering these problems over the past years and in cooperation with researchers at Rockefeller University in New York City has found a completely unexpected answer: one single gene called insulin-like peptide 2 (ILP2), which is probably activated by better nutrition, stimulates the ovaries and triggers reproduction.

"It may seem almost inconceivable that just one single gene should make all the difference," Libbrecht pointed out. The researchers drew their conclusion from a comparison of 5,581 genes in seven ant species in four different subfamilies that differ from each other with regard to numerous characteristics. But in one thing are they all alike: there is always a greater expression of ILP2 in the brain of reproductive insects. Queens thus have higher levels than workers. A further finding indicates that this peptide is found only in the brain, where it is produced in a small cluster of just 12 to 15 cells.

Division of reproduction and brood care as the basis of social colony formation

It is postulated that the origins of social behavior in insects are to be found in wasp-like ancestors that alternated between reproduction and brood care phases. A female wasp would lay an egg and take care of the larva until it pupated. However, these two phases were separated and their associated duties assigned to different individuals, namely queens and workers, during the evolution of eusociality.

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Robbed of royalty: Mutilation and social determination of female Diacamma ants

Date: February 17, 2017

Source: Okinawa Institute of Science and Technology (OIST) Graduate University

Social insects, such as ants, bees and wasps, display an organizational complexity, called eusociality, where individual members of a colony act more like parts of a whole rather than independent organisms. In their colonies, each individual performs specific tasks based on which caste they belong to: either the reproductive caste or the worker caste. In many species, the reproductive role is determined in early development -- by the time they are adults, queens and workers have set roles, complete with distinct appearances and functions. Remarkably, although ants, bees and wasps all evolved eusociality separately, all of their societies display this caste distinction. This begs the question: in these different organisms, have the same, or similar, genes evolved to differentiate social castes?

One way to answer this question is to look at exceptions to the general rule. In many ant species, queen ants are much larger than their worker counterparts. However, in some species, such as the native Okinawan Diacamma species, there are few differences in appearance between the worker caste and reproductive caste. These species are called "queenless ants" as a result, though a social hierarchy that includes both reproductive and worker castes exists within their colonies.

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More social insects have weaker immune response, highlights role of hygiene

Date: March 9, 2016

Source: North Carolina State University

Research from North Carolina State University finds that among eusocial insects -- like ants, bees and termites -- the more individuals there are in a typical species colony, the weaker the species' immune response. The finding strongly suggests that hygiene behaviors, and not just immune systems, play a key role in keeping eusocial insects healthy.

Eusocial insects live in groups. And living at close quarters with many other individuals would appear to increase their risk of contracting disease. Yet eusocial insects are incredibly successful, raising the question of how they are able to thrive.

The "social group hypothesis," argues that the eusocial lifestyle has given eusocial insects stronger immune systems. A second hypothesis, the "relaxed selection hypothesis," argues that eusocial insects have evolved specific behaviors that reduce the risk of disease transmission.

"We wanted to test the social group hypothesis to see which of these hypotheses was accurate," says Margarita López-Uribe, a postdoctoral researcher at NC State and lead author of a paper on the work.

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