Differential production of juvenile hormone and its desoxy precursor by corpora allata of honeybees during a critical period of caste development

Abstract

In social insects with pronounced caste polymorphism, the caste-inducing external signals are generally transformed into differences in juvenile hormone titer which in turn regulate the expression of polymorphic characters [1]. In the honeybee, the critical period for caste differentiation was found to coincide with the phase of differential feeding of future queens and workers by nurse bees [2]. During the feeding phase of the last (fifth) larval instar, the juvenile hormone titer in future queens is considerably higher than in worker larvae [3, 4].By means of a radiochemical in-vitro assay [5] we could show that an elevated juvenile hormone synthesis in the corpora allata of queen larvae is the prime regulating element for the caste-specific juvenile hormone titer [6]. Since release rates of juvenile hormone were found to be strictly dependent on hormone production [6], potential differences in hormone storage could be excluded as an explanation for the observed phenomenon. At the same time, the intraglandular contents of juvenile hormone III and its immediate desoxy precursor methyl 3,7,11-trimethyldodeca2(E),6(E),10-trienoate (methyl farnesenoate; often termed methyl farnesoate) were found to be correlated in queens but not in workers [6], indicating caste-specific differences in the biosynthetic pathway of juvenile hormone. Based on these results, two hypotheses for the generation of caste differences in juvenile hormone titer were formulated: an enhanced synthetic capacity resulting from an enlargement of the corpora allata in queen larvae, or rate limitations in the terminal steps of juvenile hormone biosynthesis in worker larvae. The present study attempts to answer these questions.