Juvenile hormone signaling and social complexity in the Hymenoptera

The seminal discovery that in adults of the highly social honey bee (Apis mellifera), juvenile hormone (JH) regulates age-related division of labor (DoL) but not adult fertility, unlike in most insects, has led to the hypothesis that the evolution of insect sociality involved modifications in JH signaling. Recent studies examining JH functions across the Hymenoptera provide two main insights: First, significant progress in studies of the bumble bee Bombus terrestris, which exhibits an intermediate level of social complexity relative to honey bees, shows that JH regulates multiple tissues involved in reproduction, but not task performance. JH also seems to function as a primary gonadotropin in bees showing solitary lifestyles or low levels of social complexity, highlighting a marked contrast with its roles in honey bees. Second, this association between JH function and social complexity in bees does not generalize to other lineages. The few studies on JH function in highly social stingless bees are not consistent with the honey bee model. In wasps and hornets, JH typically influences both fertility and age-related DoL. There is substantial variability across ant species, offering no consistent model linking JH function to social complexity. We propose that although JH signaling is commonly modified in social insects, the specific changes differ between - and sometimes within - lineages. There is no one model linking JH function to social complexity across major lineages, likely due to changes in related pathways. These modifications enable social insects to circumvent the trade-off between reproduction and maintenance.