Gyne production is regulated by the brood in a social bee (Bombus impatiens)

Abstract

Sexual production in social insects marks the peak of colony development, yet the mechanisms regulating it remain unclear. We investigated the role of brood in colony development, worker reproduction and sexual production in Bombus impatiens. While larvae are known to reduce worker egg laying and enhance the queen's reproductive inhibition, these effects were previously tested only in small groups. We manipulated brood size in full-sized, young colonies by doubling or removing brood and monitored development. Colonies with doubled brood produced significantly more gynes, independent of the number of workers, while reduced-brood colonies exhibited a non-significant increase in male production that was driven by colony size. Worker ovary activation was lower in double-brood colonies, with no change in aggression. A follow-up experiment directly testing the effect of colony size showed that higher worker density led to higher ovary activation in workers but did not affect sexual production. These results suggest that brood strongly influences colony development and sexual production, possibly reflecting an extended phenotype of the queen, whereas worker ovary activation appears to be a more flexible process influenced by either brood presence or colony size. Understanding brood dynamics may be key to understanding the evolution of female castes in social insects.

Keywords: brood; caste determination; density; gynes; reproduction; sociality.

Figure 1.

The average colony wet mass throughout the experiment in the three treatment groups: no brood, double brood and control. Time points 0 and 1 refer to the colony wet mass before and after brood manipulation, respectively.

Figure 2.

The number of egg batches laid per day throughout the experiment in the three treatment groups: no brood, double brood and control.

Figure 3.

The average terminal oocyte size in workers in the three treatment groups: no brood, double brood and control. Oocyte size was measured at five time points throughout the experiment using a subset of workers from each colony.

Figure 4.

The effect of treatment (no brood, double brood and control) on the initial number of workers (A), the number of workers at the end of the experiment (B), the total number of workers produced (C), the number of eggs (D), larvae (E) and pupae (F) found at the end of the experiment, and the number of males (F) and gynes (H) produced in total. Different letters denote statistical differences at α = 0.05.

Figure 5.

The average colony wet mass throughout the experiment in colonies limited in size to 30 or 90 workers.

Figure 6.

The number of egg batches laid per day throughout the experiment in colonies limited in size to 30 or 90 workers.

Figure 7.

The average terminal oocyte size in workers in colonies restricted to 30 or 90 workers. Oocyte size was measured in nine time points throughout the experiment using a subset of workers from each treatment.

Figure 8.

The effect of treatment (30/90 worker colonies) on the initial brood mass (A), the initial number of workers before the manipulation (B), the number of workers found at the end of the experiment (C), the total number of workers produced (D), the number of eggs (E), larvae (F) and pupae (G) found at the end of the experiment and the number of males (H) produced in total. The total number of gynes produced is not shown since only one colony produced gynes. Different letters denote statistical differences at α = 0.05.