Juvenile hormone induces queen development in late-stage larvae of the ant Harpegnathos saltator.

A link between hormones and developmental plasticity has long been established, but understanding how evolution has shaped the physiological systems underlying plasticity remains a major question. Within the eusocial insects, developmental plasticity helps define a reproductive division of labor through the production of distinct queen and worker castes. Caste determination may be triggered via changes in juvenile hormone (JH) levels during specific JH-sensitive periods in development. The timing of these periods, however, can vary and may relate to phenotypic differences observed among species. In order to gain insight into the evolution of caste determining systems in eusocial insects, we investigated the presence of a JH-sensitive period for queen determination in the ant Harpegnathos saltator. This species displays a number of ancestral characteristics, including low queen-worker dimorphism, and should allow insight into the early evolution of caste determining systems in ants. We identified four larval instars in H. saltator, and we found that the application of a JH analog (JHA) to third and fourth instar larvae induced queen development while treatment of early instars did not. This indicated the presence of a JH-sensitive period for queen determination at the end of the larval stage. These results contrast with what has been found in other ant species, where queen determination occurs much earlier in development. Therefore, our results suggest that caste determination originally occurred late in the larval stage in the ancestral condition but has shifted earlier in development in species that began to acquire advanced characteristics. This shift may have facilitated the development of greater queen-worker dimorphism as well as multiple worker castes.