One of the great puzzles of evolutionary biology is what led certain living things to give up solitary existence in favor of cooperative life, as in the case of ants and other social, colony-forming insects. An important feature of so-called eusocial species is the division of labor between queens who lay eggs and workers who take care of the brood and perform other tasks. But what determines that a queen must lay eggs and that workers are not allowed to reproduce? And how did this distinction arise in the course of evolution? Evolutionary biologist Dr. Romain Libbrecht has considered these issues for the past few years and, in collaboration with researchers at Rockefeller University in New York City, has found a completely unexpected answer: a single gene called insulin-like peptide 2 (ILP2), which is likely activated by better nutrition. the ovaries and stimulates reproduction.
“It may seem almost unthinkable that just a single gene would make all the difference,” Libbrecht noted. The researchers concluded this from a comparison of 5,581 genes in seven ant species in four different subfamilies that differ from each other in many characteristics. But in one thing they are all the same: there is always a greater expression of ILP2 in the brains of reproductive insects. Queens thus have higher levels than workers. Another finding indicates that this peptide is only found in the brain, where it is produced in a small cluster of only 12 to 15 cells.
Distribution of reproduction and brood care as the basis of social colony formation
It is hypothesized that the origin of social behavior in insects is to be found in wasp-like ancestors that alternated between reproductive and brood care phases. A female wasp would lay an egg and care for the larva until it pupates. However, these two phases were separated and the associated tasks assigned to different individuals, namely queens and workers, during the evolution of eusociality.
Libbrecht and his colleagues in New York City investigated the ant species Ooceraea biroi to determine the molecular mechanisms underlying this division of labor. O. biroi is a small species, 2 to 3 millimeters long, native to Asia but which has spread throughout the tropics. The insects live in underground tunnels, attack the nests of other ant species and feed on their brood. The unusual thing about the O. biroi species is that there are no queens, only female workers. However, any female worker can reproduce by parthenogenesis. This means that one female produces another identical female – the insects basically clone themselves. And they always follow a specific cycle: all workers lay eggs for 18 days, after which they spend 16 days collecting food and feeding the larvae. The cycle then starts again.
This cyclical behavior is similar to that of the solitary wasp-like ancestors and is governed by the presence of larvae. When the first larvae hatch at the end of the reproductive stage, their presence suppresses ovarian activity and triggers brood care behavior. When the larvae begin to pupate at the end of the brood care phase, ovarian activity is scaled up and foraging is scaled back. “What we did was break this cycle,” explains Libbrecht. The researchers synthesized the peptide ILP2 and injected it into the ants. This caused the ants to lay eggs in the presence of larvae.
Libbrecht used a brood substitution approach to investigate what happens when larvae are introduced into the colony during the reproductive phase and, conversely, when they are removed during the brood care phase. “What we see is that gene expression in the brain changes in both phases and the ants change their behavior and physiology accordingly. However, this response happens faster when we confront egg-laying ants with larvae.” The insects then stop laying eggs and start caring for the brood. “This makes sense. After all, it’s important for survival to start feeding the larvae quickly,” he added. This experiment also revealed that the expression of ILP2 in the brain changed rapidly and significantly in response to the change in social conditions.
From asymmetry in nutrition to asymmetry in reproduction
The researchers also looked at the relevance of diet, which is known to be important when it comes to distinguishing between queens and workers. A large amount or good quality dietary protein promotes the development of female larvae into queens. In colonies of the species O. biroi, a small part of the ants are so-called intercastes. These insects are slightly larger, have eyes and are more reproductive. This allows them to be compared to normal queens to some extent. The chances of a larva becoming an intercaste increase if it is fed better. Fluorescence imaging shows that these intercastes have more ILP2 in their brains than normal workers.
“In the case of the ancestors of eusocial insects, something similar may have happened,” suggested Dr. Romain Libbrecht. “Perhaps a slight asymmetry in larvae feeding has led to asymmetry in the reproductive behavior of adults developing from those larvae.” The assumption that the division into queens and workers could therefore have started with a single difference is supported by experiments conducted on a total of seven different ant species.
Further research needs to be done to determine whether the findings also apply to other social insects and how ant colonies as superorganisms control the overall food supply.
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