Sensory response system of social behavior tied to female reproductive traits

Abstract

Background: Honey bees display a complex set of anatomical, physiological, and behavioral traits that correlate with the colony storage of surplus pollen (pollen hoarding). We hypothesize that the association of these traits is a result of pleiotropy in a gene signaling network that was co-opted by natural selection to function in worker division of labor and foraging specialization. By acting on the gene network, selection can change a suite of traits, including stimulus/response relationships that affect individual foraging behavior and alter the colony level trait of pollen hoarding. The 'pollen-hoarding syndrome' of honey bees is the best documented syndrome of insect social organization. It can be exemplified as a link between reproductive anatomy (ovary size), physiology (yolk protein level), and foraging behavior in honey bee strains selected for pollen hoarding, a colony level trait. The syndrome gave rise to the forager-Reproductive Ground Plan Hypothesis (RGPH), which proposes that the regulatory control of foraging onset and foraging preference toward nectar or pollen was derived from a reproductive signaling network. This view was recently challenged. To resolve the controversy, we tested the associations between reproductive anatomy, physiology, and stimulus/response relationships of behavior in wild-type honey bees.

Methodology/principal findings: Central to the stimulus/response relationships of honey bee foraging behavior and pollen hoarding is the behavioral trait of sensory sensitivity to sucrose (an important sugar in nectar). To test the linkage of reproductive traits and sensory response systems of social behavior, we measured sucrose responsiveness with the proboscis extension response (PER) assay and quantified ovary size and vitellogenin (yolk precursor) gene expression in 6-7-day-old bees by counting ovarioles (ovary filaments) and by using semiquantitative real time RT-PCR. We show that bees with larger ovaries (more ovarioles) are characterized by higher levels of vitellogenin mRNA expression and are more responsive to sucrose solutions, a trait that is central to division of labor and foraging specialization.

Conclusions/significance: Our results establish that in wild-type honey bees, ovary size and vitellogenin mRNA level covary with the sucrose sensory response system, an important component of foraging behavior. This finding validates links between reproductive physiology and behavioral-trait associations of the pollen-hoarding syndrome of honey bees, and supports the forager-RGPH. Our data address a current evolutionary debate, and represent the first direct demonstration of the links between reproductive anatomy, physiology, and behavioral response systems that are central to the control of complex social behavior in insects.

Figure 1. Relationships between Sucrose Responsiveness and Ovary Size in Worker Honey Bees.

(A) Pie charts showing the distributions of ovary sizes between bees with low (GRS 0–3, n = 141) and high (GRS 4–9, n = 149) responsiveness to sucrose. Ovary sizes are given as the total number of ovarioles per bee (i.e., summing over both ovaries). (B) Comparison between the means and standard errors of ovary sizes between bees with low and high sucrose responsiveness. Bees with high GRS scores are characterized by significantly larger ovaries on average.

Figure 2. Association of Sucrose Responsiveness, Ovary Size, and Vitellogenin mRNA Level in Worker Honey Bees.

(A) Means and standard errors of the ovary sizes in the subsets of bees selected from the extreme tails of the ovariole number and GRS and distributions; LL = small ovaries (3–9 ovarioles) and low GRS (0–2), HH = large ovaries (17–29 ovarioles) and high GRS (4–7), n = 44 and 43, respectively. L and H were laboratory handling controls that were selected only on the basis of ovary size, GRS was not determined (n = 10; ovariole number was 3–9 and 12–23, respectively, H spanned lower ovary sizes than HH as there were not a sufficient number of bees to obtain in the 17–29 range). HH and LL differ significantly for ovariole number (P<0.0001, one-way ANOVA). (B) Means and standard errors of the log-transformed vitellogenin mRNA expression level given as a relative quantity (RQ). Bees with large ovaries and high GRS are characterized by significantly higher vitellogenin levels on average (P<0.005, one-way ANOVA). The controls show no significant effect of handling.