Sex- and caste-specific developmental responses to juvenile hormone in an ant with maternal caste determination

Queen-worker caste polyphenism in social insects is a prime example for developmental plasticity. Most of what we know about caste development comes from studies of the honeybee, in which female caste is determined during larval development and workers retain functional ovaries. The ant genus Cardiocondyla is one of only few genera in which complete worker sterility has evolved, so that adult workers completely lack reproductive organs. In C. obscurior, queen- and worker-destined individuals are distinct in their development by late-embryogenesis, and castes can be distinguished in a non-invasive manner from this stage onwards. This provides the opportunity to investigate the degree of flexibility in caste development in a species with early caste determination. Using topical juvenile hormone treatment, a method known to influence caste determination and differentiation in some species, we investigated whether hormone manipulation affects the development and growth of queen and worker-destined late-stage embryos and larvae, as well as of early-stage embryos which cannot yet be distinguished by caste. We found no effect of hormone treatment on female caste ratios or body sizes in any of the treated stages, even though individuals reacted to heightened hormone availability with increases in the expression of kruppel-homolog 1, a conserved JH first-response gene. In contrast, hormone treatment resulted in the emergence of significantly larger males. These results show that in C. obscurior, early, presumably maternal caste determination leads to irreversible and highly-canalized caste-specific development and growth.