Scientists have uncovered how a particular hormone influences the differentiation of ant castes by analyzing organ-level developmental changes and performing transcriptome analyses.
Most ants are characterized by two distinct adult castes: queens and workers. Each caste has a unique and irreversible specialization; queens are focused on reproduction, while workers contribute to non-reproductive tasks such as foraging, defense, and caring for the young. Typically, adult gynes (virgin queens) possess a larger body size, wings, and prominent frontal eyes, along with enlarged ovaries and a sperm storage organ. In contrast, worker ants are wingless, smaller, and have reduced reproductive systems, generally lacking a sperm storage organ. In 1910, American entomologist William Morton Wheeler pointed out in his work, Ants, that ant colonies function similarly to complex organisms, with queens and workers representing the germline and somatic cells at a higher level of organization— the colony.
It is widely understood that germline cells in animals are designated for reproductive duties soon after fertilization, as an embryo begins to form. In many social insects, where there are different castes for queens (serving as the colony germline) and workers (acting as the colony soma), this differentiation occurs during the larval phase and can be altered hormonally. However, in certain ant species, including some, this caste distinction begins even earlier, in the embryonic (egg) phase, raising the question of whether the transition of workers into their roles becomes as irreversible as the differentiation seen with animal body cells as they develop.
A recent study from the Department of Biology at the University of Copenhagen, funded by a Villum Investigator Grant, has found that caste development in pharaoh ants, Monomorium pharaonis, is considerably reversible, even when caste determination takes place during the egg stage. “We administered controlled doses of juvenile hormone (JH) analog to 237 pharaoh ant worker larvae during their third larval instar, which is the last stage of the larval development,” explains Ruyan Li, the study’s lead author. “Our research revealed that the hormone-treated worker ants exhibited many characteristics similar to gynes, such as increased body length, three additional frontal eyes, the presence of wings and flight muscles, and even gyne-like brains. They also developed a sperm storage organ, which workers of this species typically lack.”
However, despite these similarities with naturally developed gynes, the JH-treated workers did not develop ovaries, the reproductive organs that fundamentally distinguish gynes from workers. This suggests that the specific sensitivity to juvenile hormones for caste development does not coincide with the egg stage. The research also offers new insights into how variations in growth hormone sensitivity may have influenced the development of new castes in other ant species, like soldiers (modified workers) or permanently wingless gynes. “Such new castes often emerge as mosaic phenotypes that exhibit a mix of gyne and worker traits, which may become permanent if natural selection favors a subsequent change in JH sensitivity,” states Guojie Zhang, the study’s corresponding author and a professor at Zhejiang University in Hangzhou, who also holds an adjunct position at the University of Copenhagen.
– “The findings support the analogy between cell differentiation in animal organisms and caste differentiation in ant colonies. Just as animal breeders have yet to use somatic cells to create new animals, this study reveals similar constraints at the colonial level in ants,” concludes Professor Koos Boomsma from the University of Copenhagen, who is a senior co-author of the research.
