Peripheral modulation of worker bee responses to queen mandibular pheromone

Abstract

It is generally accepted that young worker bees (Apis mellifera L.) are highly attracted to queen mandibular pheromone (QMP). Our results challenge this widely held view. We have found that unless young workers are exposed to QMP early in adult life, they, like foragers, avoid contact with this pheromone. Our data indicate that responses to QMP are regulated peripherally, at the level of the antennal sensory neurons, and that a window of opportunity exists in which QMP can alter a young bee's response to this critically important pheromone. Exposing young bees to QMP from the time of adult emergence reduces expression in the antennae of the D1-like dopamine receptor gene, Amdop1. Levels of Amdop3 transcript, on the other hand, and of the octopamine receptor gene Amoa1, are significantly higher in the antennae of bees strongly attracted to QMP than in bees showing no attraction to this pheromone. A decline in QMP attraction with age is accompanied by a fall in expression in worker antennae of the D2-like dopamine receptor, AmDOP3, a receptor that is selectively activated by QMP. Taken together, our findings suggest that QMP's actions peripherally not only suppress avoidance behavior, but also enhance attraction to QMP, thereby facilitating attendance of the queen.

Fig. 1.

Responses of bees of different ages to a QMP-impregnated strip (black circles in A, black bars in B and C) and a control strip (white circles or bars) placed in the center of the arena. (A) Two-day-old bees remained close to the strip, but only when it was impregnated with QMP (Top). The behavior of 6-day-old bees was not altered by QMP (Middle), but foragers remained as far from the QMP strip as possible (Bottom). (B) For statistical purposes, the mean area occupied by bees tested under each condition was calculated. Bees showing a preference for the center of the arena occupied a smaller area than bees preferring the periphery. Two-day-old bees occupied a significantly smaller area of the arena when tested with the QMP strip than the control strip (Top, t = 5.03; P < 0.0001), whereas foragers occupied a significantly larger area when the QMP strip was presented (Bottom, t = 5.26, P < 0.0001). Six-day-old bees showed no preference (Middle). (C) Two-day-old bees contacted the strip significantly more frequently if it was impregnated with QMP (t = 2.8, P < 0.01), whereas 6-day-old bees and foragers showed no preference for either strip. Data are means ± SEM. n = 20 in each group. Asterisks highlight significant differences between groups.

Fig. 2.

Effects of QMP on locomotor activity of 2-day-old bees and foragers. (A) Trajectories of a 2-day-old bee (Top) and forager (Bottom) responding to a control strip placed in the center of the arena (Left) vs. a QMP-impregnated strip (Right). (B) Mean distance traveled tended to be lower when bees were presented with the QMP strip, particularly in 2-day-old bees (t = 2.16, P = 0.059). (C) Two-day-old bees also exhibited a significantly lower mean maximum speed when presented with the QMP-strip vs. the control strip (t = 3.02, P < 0.02). The mean maximum speed of foragers was not affected significantly by the pheromone. (D) Forager bees spent significantly less time in the center of the arena when the QMP-strip was placed there (t = 3.16, P < 0.05). This was not true of 2-day-old bees, some of which were highly attracted to the pheromone. Data are means ± SEM. n = 10 in each group. Asterisks highlight significant differences between groups.

Fig. 3.

Expression of amine-receptor genes in the antennae of 2-day-old bees. (A) Transcript levels in bees strongly attracted to QMP (bars with cross-hatching, n = 9) expressed relative to the levels observed in bees showing no attraction to the pheromone (bars without cross-hatching, n = 7). All bees used in these experiments had been exposed since adult emergence to QMP. Transcript levels for the octopamine receptor gene, Amoa1, and the dopamine receptor gene, Amdop3, are significantly higher in bees showing strong attraction to QMP (Amoa1 t = 3.417; P < 0.004; Amdop3 t = 2.108, P < 0.054). (B) Relative expression levels in bees exposed since adult emergence to QMP (black bars, n = 5) and in bees not exposed to the pheromone (white bars, n = 5). Treatment with QMP reduced significantly the levels of expression of the D1-like dopamine receptor gene, Amdop1 (t = 2.301, P < 0.05). Data are means ± SEM.

Fig. 4.

Age-related changes in Amoa1 and Amdop3 expression in the antennae. (A) Comparison of relative Amoa1 mRNA levels in the antennae of bees of different ages revealed no age-related changes in the expression of this octopamine receptor gene. (B) Transcript levels for the dopamine receptor gene Amdop3 fell significantly during the first week of adult life (H = 20.95, P = 0.0008). Groups with the same letter above the bar are not significantly different. n = 5 in each group. Data are means ± SEM.

Fig. 5.

Mean frequency of contacts with a QMP strip or a control strip. (A–C) Frequency of contacts observed in 2-, 4-, and 6-day-old bees exposed to QMP from the time of adult emergence (QMP+) compared with bees of the same age that had not been exposed previously to QMP (QMP-). Letters above the bars indicate differences between groups. Groups that share a letter are not significantly different. Two days: F = 3.04, P < 0.05; Four days F = 7.36, P < 0.001; Six days F = 1.46, NS. n = 20 in each group. (D and E) Responses of young bees reared without QMP treatment (D, data for 2- to 6-day-old bees pooled) resemble the responses exhibited by foragers (n = 20) from the same colony (E); both groups contact the strip impregnated with QMP significantly less than the control strip (young bees t = 2.71, P = 0.008; foragers t = 5.23, P < 0.0001). Data are means ± SEM. Asterisks highlight significant differences between groups.