Effects of juvenile hormone in fertility and fertility-signaling in workers of the common wasp Vespula vulgaris

Abstract

In the highly eusocial wasp, Vespula vulgaris, queens produce honest signals to alert their subordinate workers of their fertility status, and therefore they are reproductively suppressed and help in the colony. The honesty of the queen signals is likely maintained due to hormonal regulation, which affects fertility and fertility cue expression. Here, we tested if hormonal pleiotropy could support the hypothesis that juvenile hormone controls fertility and fertility signaling in workers. In addition, we aimed to check oocyte size as a proxy of fertility. To do that, we treated V. vulgaris workers with synthetic versions of juvenile hormone (JH) analogue and a JH inhibitor, methoprene and precocene, respectively. We dissected the treated females to check ovary activation and analyzed their chemical profile. Our results showed that juvenile hormone has an influence on the abundance of fertility linked compounds produced by workers, and it also showed to increase oocyte size in workers. Our results corroborate the hypothesis that juvenile hormone controls fertility and fertility signaling in workers, whereby workers are unable to reproduce without alerting other colony members of their fertility. This provides supports the hypothesis that hormonal pleiotropy contributes to keeping the queen fertility signals honest.

Fig 1. Measurement of oocyte size in Vespula vulgaris workers.

The yellow bar indicates the oocyte length measure with ImageJ.

Fig 2

(A) Oocyte size is largest in methoprene treatment, followed by acetone and precocene. 1-tailed pairwise contrasts shows a significant difference between all treatments size. The control treatment acetone did not differ from both the methoprene (t ratio = -1.76, p = 0.081) and precocene (t ratio = 1.68, p = 0.097) treatments, but between methoprene and precocene treatments the biggest oocyte size differs significantly (t ratio = 2.64, p = 0.013). (B) Effect plot of model fitted based on the measurements of the head width of workers.

Fig 3. On the left, a heatmap based on the differences in cuticular hydrocarbon profiles on treated individuals, on the right showing all the identified compounds with their significance contrasts based on p-values.

Queens are pooled (Q) and different worker treatments (Wm, Wa and Wp) represent methoprene, acetone and precocene, respectively. Contrasts were not significant between Wa-Wp, therefore are not shown. The numbers in the heatmap shows the average relative peak area per compound for each treatment, the background color represents the z-scores. Clusters were obtained by UPGMA clustering and 1-Pearson correlation for the distance matrix. Tukey posthoc contrasts between treatments are obtained from a linear mixed model with Bonferonni correction: *p < 0.05, **p < 0.01, ***p < 0.001.