Abstract
Juvenile hormone (JH) controls key events in the honey bee life cycle, viz. caste development and age polyethism. We quantified transcript abundance of 24 genes involved in the JH biosynthetic pathway in the corpora allata-corpora cardiaca (CA-CC) complex. The expression of six of these genes showing relatively high transcript abundance was contrasted with CA size, hemolymph JH titer, as well as JH degradation rates and JH esterase (jhe) transcript levels. Gene expression did not match the contrasting JH titers in queen and worker fourth instar larvae, but jhe transcript abundance and JH degradation rates were significantly lower in queen larvae. Consequently, transcriptional control of JHE is of importance in regulating larval JH titers and caste development. In contrast, the same analyses applied to adult worker bees allowed us inferring that the high JH levels in foragers are due to increased JH synthesis. Upon RNAi-mediated silencing of the methyl farnesoate epoxidase gene (mfe) encoding the enzyme that catalyzes methyl farnesoate-to-JH conversion, the JH titer was decreased, thus corroborating that JH titer regulation in adult honey bees depends on this final JH biosynthesis step. The molecular pathway differences underlying JH titer regulation in larval caste development versus adult age polyethism lead us to propose that mfe and jhe genes be assayed when addressing questions on the role(s) of JH in social evolution.
Highlights
Juvenile Hormone (JH), synthesized by the corpora allata (CA), a small pair of glands located in the retrocerebral complex of the insect brain, is best known for its pleiotropic functions in insect metamorphosis and reproduction [1]
In silico analyses of JH biosynthetic pathway genes Juvenile hormone biosynthesis involves the production of farnesyl pyrophosphate from acetyl-CoA via the mevalonic acid pathway, followed by converting farnesyl-PP into JH-precursors (Figure S1)
We annotated 25 genes of the JH biosynthetic pathway in the honey bee genome and assayed the expression levels of 24, showing that several of these are prominently expressed in corpora cardiaca (CA-CC) complexes, when compared to other tissues
Summary
Juvenile Hormone (JH), synthesized by the corpora allata (CA), a small pair of glands located in the retrocerebral complex of the insect brain, is best known for its pleiotropic functions in insect metamorphosis and reproduction [1] Building on these basic functions, different classes of insects have apparently co-opted this hormone and its downstream signaling pathways and regulatory modules into functions that permit specific adaptations in their life cycles (e.g., diapause, [2]) or complex life histories (e.g., seasonal or caste polyphenisms, [3]). After a small peak of JH that initiates vitellogenin synthesis in the late pharate adult stage of queens [9], the JH titer stays at low levels throughout a queen’s adult life cycle It has taken on a role in pleiotropically setting the physiological conditions for age-specific tasks in adult workers. As the bees grow older and switch to performing more hazardous extranidal task, viz. foraging for nectar, pollen and water, their JH titers are typically increased [4]
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