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“It is almost unbelievable that only one gene is decisive,” says Romain Libbrecht. The result is based on a comparison of 5581 genes in seven species of ants, which come from three different subfamilies and differ in numerous characteristics. Only the expression of ILP2 was always significantly increased in reproductive animals. So queens have more of it than workers. ILP2, another finding, is only found in the brain, where it is produced in a small cluster of 12 to 15 cells.
Decoupling of reproduction and brood care as the basis for the formation of social states
At the beginning of the development towards the welfare state, it is assumed, there were wasp-like ancestors who alternated between reproduction and brood care: A female wasp laid an egg and looked after the larva until they pupated. So that eusociality could arise, these two phases had to be decoupled and the respective tasks had to be assigned to different individuals, queens and workers.
To uncover the molecular mechanisms behind this decoupling, Libbrecht and his New York colleagues used the ant species Ooceraea biroi. O. biroi is a small species 2 to 3 millimeters long that originally comes from Asia but has spread to the tropics. The animals live in underground passages, invade the nests of other species of ants and feed on the brood. What is special, however, is that O. biroi doesn't know any queens, only workers. However, all workers can reproduce via parthenogenesis. This means that females produce genetically identical females, the animals clone themselves. They follow a certain cycle: all workers lay eggs for 18 days, then they search for food for 16 days and feed the larvae. Then the cycle starts all over again.
This cyclical behavior, which is comparable to that of the solitary wasp-like ancestors, is controlled by the presence of larvae: When the first larvae hatch towards the end of the reproductive phase, they suppress the activity of the ovaries and trigger brood care behavior. And when the larvae begin to pupate towards the end of the brood care phase, the activity of the ovaries increases again and the foraging for food subsides. "We broke this cycle," explains Libbrecht about the research. The scientists synthesized the peptide ILP2 and injected it into the ants. The ants then laid eggs in the presence of larvae.
In the “brood exchange experiment”, Libbrecht investigated what happens when larvae are added to the colony during the reproductive phase and, conversely, removed during the breeding phase. “We then see that in both cases the gene expression changes and the ants adapt. However, the change happens faster if we put larvae under egg-laying ants. ”The insects then finish laying their eggs and worry about caring for the offspring. “That also makes sense. After all, it is important for survival that the larvae are taken care of quickly, ”says Libbrecht. This experiment also showed the dominance of ILP2, which reacts very early and very strongly to the changed conditions.
From asymmetry in food to asymmetry in reproduction
In a further step, the scientists turned to the role of food, which is known to be important in the training of queens or workers. A high quantity or a good quality of protein in the diet creates queens. In the case of the ant species O. biroi, a small proportion of around 5 percent of the ants in a colony consists of so-called inter-boxes. These animals are on average slightly larger, have poorly developed eye-spots and are also reproductive more strongly. This makes them a bit similar to the "normal" queens. Better feeding of larvae increases the likelihood that they will become interboxes. Fluorescence images show that these intercastes have more ILP2 in the brain than normal workers.
“Something similar may have happened with the ancestors of the eusocial insects,” says Libbrecht. "A small asymmetry in the diet resulted in an asymmetry in reproduction in adulthood." The division into queens and workers would thus be due to a first single difference - an assumption that is supported by the studies on a total of seven very different species of ants.
Further research should now show whether the results can also be transferred to other social insects or how the “superorganism ant colony” controls the overall food supply.
University of Mainz
Vikram Chandra et al .: Social regulation of insulin signaling and the evolution of eusociality in ants
Science, July 27, 2018, DOI: 10.1126 / science.aar5723
Romain Libbrecht, Peter R. Oxley, Daniel J. C. Kronauer: Clonal raider ant brain transcriptomics identifies candidate molecular mechanisms for reproductive division of labor, BMC Biology, August 13, 2018, DOI: 10.1186 / s12915-018-0558-8
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