All ants are thought to be eusocial insects, with most individual ants within a colony foregoing their own reproductive potential to support an egg-laying queen. According to a new study, published in the journal Science, this behavior is regulated through insulin signaling and expression of the insulin-like peptide 2 (ilp2) gene.

The strict reproductive roles common to eusocial ants are thought to mirror an ancestral subsocial lifecycle when common ancestors of ants alternated between reproductive and brood care phases.

Eventually, this cyclic reproductive behavior adapted to reflect the fixed roles of egg-bearing queens and brood-caring workers of eusocial colonies seen today.

However, the origin of eusociality and its genetic underpinnings remain unclear, leaving open questions about what allows queens to lay eggs but prevents workers from doing so, among others.

“We wanted to know: what makes the queens lay eggs and the workers sterile?” said Rockefeller University’s Professor Daniel Kronauer, co-lead author of the study.

Professor Kronauer and colleagues first searched for differences in gene expression between reproducing and non-reproducing ants from a variety of species.

They discovered that the ilp2 gene — which codes for the peptide ILP2, the ant version of insulin — is consistently upregulated in reproducers.

“There is a direct link between reproduction and food intake: if the nutritional state is really low, you can’t afford to produce offspring,” Professor Kronauer said.

Next, the team studied the role of insulin in the clonal raider ant Ooceraea biroi.

This species lacks distinct queens and workers; all ants simultaneously enter a reproductive phase, followed by a brood care phase in which the insects nurture their young.

Transitions between phases are regulated by the presence of larvae: when newborns are around, the ants stop reproducing and shift into caretaking mode.

When the study authors removed larvae during the brood care phase, adult insulin production increased substantially; and when they introduced larvae during the reproductive phase, insulin production decreased.

These results indicate that the presence of larvae suppresses the production of insulin; and without sufficient levels of this peptide, the ants cannot reproduce.

In another experiment, the scientists injected ants with synthetic insulin during brood care, which resulted in ovary activation — even when larvae were nearby.

This outcome suggests that ants with heightened insulin can override larval cues and reproduce at any time.

“In the brood care phase, the presence of larvae typically reduces insulin in adults — so their ovaries switch off and they go care for the larvae. But if you experimentally inject insulin, you can break this cycle,” said co-lead author Vikram Chandra, a graduate student at the Rockefeller University.

The study offers clues about how ants evolved from solitary organisms to social species with specialized castes.

“First, insulin signaling became responsive to the presence of larvae, resulting in reproductive cycles reminiscent of those observed in Ooceraea biroi,” Professor Kronauer said.

“Such an adaption makes good sense, as ants caring for offspring must prioritize food finding over egg laying — and both behaviors are known to be regulated by insulin.”

“Following this evolutionary stage the question becomes: how do ants break out of this cycle?”

“Due to individual variation, some ants would have naturally high levels of insulin, and others would have low levels,” he said.

Notably, the researchers observed exactly this kind of variation in Ooceraea biroi.

“High-insulin individuals, like insulin-injected ants, would be able to override larval cues and reproduce continuously, whereas low-insulin individuals would be very sensitive to the presence of larvae and thus more likely to focus on brood care,” Professor Kronauer said.

“Once you have that kind of asymmetry in a colony, and the colony performs well, selection will drive insulin levels further apart.”

“The eventual result would be two castes of ants — workers and queens.”

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Vikram Chandra et al. 2018. Social regulation of insulin signaling and the evolution of eusociality in ants. Science 361 (6400): 398-402; doi: 10.1126/science.aar5723