Juvenile hormone is considered to be a master regulator of polyphenism in social insects. In the ant Cardiocondyla obscurior, whether a female egg develops into a queen or a worker is determined maternally and caste-specific differentiation already occurs in embryos, so that queens and workers can be distinguished in a non-invasive manner from late embryogenesis onwards. This ant also exhibits two male morphs – winged and wingless males. Using topical treatment with juvenile hormone III and its synthetic analogue methoprene, a method known to influence caste determination and differentiation in some species, we investigated whether hormone manipulation affects the development and growth of male, queen- and worker-destined late-stage embryos and larvae, as well as of male and female 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 krüppel-homolog 1, a conserved JH first-response gene. In contrast, hormone treatment resulted in the emergence of significantly larger males, though male morph fate was not affected. These results indicate that in C. obscurior, maternal caste determination leads to irreversible and highly-canalized caste-specific development and growth, and call into question the assumption that juvenile hormone is the key component underlying caste determination and differentiation in all ants.

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