For many ant species, caste determines destiny. Queens grow large, sprout wings, and lay eggs; workers stay small, wingless, and industrious. But how these castes develop—and how a young ant's future is determined by genetics and the environment, was unclear.
Now, a new study suggests that body size and caste go hand in hand. Bigger ants generally become queens, while smaller ants become workers, and both genes and the environment influence how big an ant grows. But the findings, published in PNAS, also suggest that genetics alone defines the threshold for becoming a queen. Across colonies, genetically different ants of the same size, reared in the same environment, can differ in caste morphology. These results demonstrate that genes not only influence size—they change what a given size means for the colony. Two relatively small ants could have vastly different probabilities of becoming queens.
"One of our goals is to understand how an insect society functions," says Daniel Kronauer, the Stanley S. and Sydney R. Shuman Professor at Rockefeller. "Studying how individuals differentiate in a colony can inform our understanding of the types of caste systems, from queens to workers to soldiers, that can evolve in the thousands of ant species out there."
Nature or nurture
A queen is not just a large worker ant. In addition to their greater body size, queens typically grow wings, develop a more elaborate visual system, and large ovaries for laying eggs, while workers remain wingless, small, and usually refrain from reproducing. Such a striking difference between genetically identical individuals is a classic example of developmental plasticity, and female ants are therefore an excellent model for studying how a single genotype can produce dramatically different phenotypes.
Some scientists suspected that body size can be uncoupled from whether an ant becomes a queen. This theory made sense—in fruit flies, as well as other insects, physical traits are generally to some degree regulated by environmental factors independently of body size. But recent work has suggested that ants, unlike other insects, always keep caste traits in lockstep with body size.
"It's a hotly debated topic, at least among ant biologists," says Patrick Piekarski, now a postdoctoral researcher in the Kronauer lab. "We wanted to resolve this question, by determining whether it is possible to uncouple size from something like ovary development or eye development."
The team turned to the clonal raider ant, Ooceraea biroi, a powerful model thanks to its clonal reproduction and synchronized life cycle, which allows for precise control over genetic and environmental variables. While clonal raider ants lack conventional queens, individuals known as intercastes are considered developmentally homologous to queens—they grow larger, and develop larger ovaries as well as vestigial eyes and the barest suggestion of wings. Such a link between body size and a suite of correlated queen-associated traits is widespread across the ants.
"We work with this unusual ant species because, otherwise, we would not be able to control the genotype of the ant," Kronauer says. "With clonal raider ants, we can do the equivalent of a massive identical twin study to determine how the environment impacts the adult phenotype."
Large and in charge
For the study, the researchers began by holding the larval genotype constant and manipulating the rearing environment. Prior work had demonstrated that food availability, temperature, and the genotype of the ants responsible for caring for the larvae could all influence caste development. They found that each of these factors did indeed influence the outcome, but only when also altering the larva's final body size. When genetically identical larvae were raised with less food, for instance, they would become smaller and have a decreased probability of developing queen-like traits. But if those larvae managed to reach a certain size despite the lack of food, they would still develop queen-like traits. Ultimately, caste was linked to size, and size alone could almost perfectly predict developmental outcomes.
The team then compared different larval genotypes reared under similar environmental conditions to determine whether genetic differences could change the underlying relationship linking body size to caste traits. They found that, while genes can influence size, genes can also change the size at which queen-like features begin to emerge. For instance, ants from a genetic line designated "M" consistently grew to smaller average body sizes than those from line A, even when reared under identical environmental conditions. But, for any given body size, line M ants were still more likely to develop queen-like traits. Two small ants, one from line M and one from line A, could have completely different odds of becoming queens, regardless of rearing environment.
This suggests that genetic variation can influence caste morphology in two ways—by affecting how large ants grow and by modifying the size at which queen-like traits are expressed. "An interesting illustration of that is that you have certain genotypes that start to look more queen-like at smaller body sizes," Piekarski says.
For Piekarski, the main takeaway of the study is that, "If some environmental factor affects caste, it will affect size too. It can't induce change in one and not the other. As far as we can tell, no matter which environmental variable you manipulate, the relationship between ant body size and caste remains unchanged and is instead genetically encoded."
For Kronauer, the implications of the work go beyond caste determination. His lab studies ant colonies as superorganisms—complex biological systems in which genetically identical individuals take on dramatically different roles, much like cells in a tissue. With ants as his model, Kronauer couples the study of developmental biology with behavior and social organization.
"The brains of a queen and a worker are quite different, and this is correlated with striking differences in behavior," Kronauer explains. "Workers leave the nest to forage, take care of the larvae, build and expand the nest; the queen mostly just mates and lays eggs. So understanding how body size relates to caste isn't just a question of morphology. It opens the door to understanding how social roles, brain function, and colony dynamics develop and evolve together."
