Juvenile hormone pathway in honey bee larvae: A source of possible signal molecules for the reproductive behavior of Varroa destructor

Abstract

The parasitic mite Varroa destructor devastates honey bee (Apis mellifera) colonies around the world. Entering a brood cell shortly before capping, the Varroa mother feeds on the honey bee larvae. The hormones 20-hydroxyecdysone (20E) and juvenile hormone (JH), acquired from the host, have been considered to play a key role in initiating Varroa's reproductive cycle. This study focuses on differential expression of the genes involved in the biosynthesis of JH and ecdysone at six time points during the first 30 hr after cell capping in both drone and worker larvae of A. mellifera. This time frame, covering the conclusion of the honey bee brood cell invasion and the start of Varroa's ovogenesis, is critical to the successful initiation of a reproductive cycle. Our findings support a later activation of the ecdysteroid cascade in honey bee drones compared to worker larvae, which could account for the increased egg production of Varroa in A. mellifera drone cells. The JH pathway was generally downregulated confirming its activity is antagonistic to the ecdysteroid pathway during the larva development. Nevertheless, the genes involved in JH synthesis revealed an increased expression in drones. The upregulation of jhamt gene involved in methyl farnesoate (MF) synthesis came into attention since the MF is not only a precursor of JH but it is also an insect pheromone in its own right as well as JH-like hormone in Acari. This could indicate a possible kairomone effect of MF for attracting the mites into the drone brood cells, along with its potential involvement in ovogenesis after the cell capping, stimulating Varroa's initiation of egg laying.

Keywords: drone; ecdysteroid; jhamt; kairomone; larvae; methyl farnesoate.

FIGURE 1

Brood comb containing worker progeny in different stages of development before and after capping together with nurse bees

FIGURE 2

Differential gene expressions in molting hormone pathway. Differential expressions at Tp 8, 12, 16, 24, and 30 relative to Tp 0 for drone (indigo) and worker (yellow) larvae; drone versus worker differential expressions at Tp 0, 8, 12, 16, 24, and 30, respectively, are shown in green. Statistically significant logFC values for adjusted p: *p < 0.05, **p < 0.01, ***p < 0.001 (available in supporting information file, Sheets 1–3)

FIGURE 3

Differential gene expression in Juvenile hormone pathway. Differential expressions at Tp 8, 12, 16, 24, and 30 relative to Tp 0 for drone (indigo) and worker (yellow) larvae; drone versus worker differential expressions at Tp 0, 8, 12, 16, 24, and 30, respectively, are shown in green. Statistically significant logFC values for adjusted p: *p < 0.05, **p < 0.01, ***p < 0.001 (available in supporting information file, Sheets 4–6)

FIGURE 4

Relative gene expression for jhamt gene. Expression of jhamt at Tp 8, 12, 16, 24, and 30 relative to Tp 0 for drone (indigo) and worker (yellow) larvae; drone versus worker relative expression at Tp 0, 8, 12, 16, 24, and 30, respectively, is shown in green, ***p < 0.001 (supporting information file, Sheet 10)

FIGURE 5

Differential gene expression of fibroins. Differential expressions at Tp 8, 12, 16, 24, and 30 relative to Tp 0 for drone (indigo) and worker (yellow) larvae; drone versus worker differential expressions at Tp 0, 8, 12, 16, 24, and 30, respectively, are shown in green. Statistically significant logFC values for adjusted p: *p < 0.05, ***p < 0.001 (available in supporting information file, Sheets 4–6)

FIGURE 6

The prevalence of molting and juvenile hormone metabolites in honey bee larvae in the first 30 hr postcapping. The male (♂) and female (☿) symbols are indicating the genes for the enzymes that are upregulated in drones and worker larvae, in time and/or sex‐related analysis. The key enzymes in molting and juvenile hormone pathway that could have implication in Varroa's reproduction are highlighted in yellow and blue, respectively; the metabolites that are more prevalent or not restrictive (neverland) in drones are written in red. The hormone pathways are modified from KEEG (Kanehisa et al., 2016)