PDK1 and HR46 gene homologs tie social behavior to ovary signals

Abstract

The genetic basis of division of labor in social insects is a central question in evolutionary and behavioral biology. The honey bee is a model for studying evolutionary behavioral genetics because of its well characterized age-correlated division of labor. After an initial period of within-nest tasks, 2-3 week-old worker bees begin foraging outside the nest. Individuals often specialize by biasing their foraging efforts toward collecting pollen or nectar. Efforts to explain the origins of foraging specialization suggest that division of labor between nectar and pollen foraging specialists is influenced by genes with effects on reproductive physiology. Quantitative trait loci (QTL) mapping of foraging behavior also reveals candidate genes for reproductive traits. Here, we address the linkage of reproductive anatomy to behavior, using backcross QTL analysis, behavioral and anatomical phenotyping, candidate gene expression studies, and backcross confirmation of gene-to-anatomical trait associations. Our data show for the first time that the activity of two positional candidate genes for behavior, PDK1 and HR46, have direct genetic relationships to ovary size, a central reproductive trait that correlates with the nectar and pollen foraging bias of workers. These findings implicate two genes that were not known previously to influence complex social behavior. Also, they outline how selection may have acted on gene networks that affect reproductive resource allocation and behavior to facilitate the evolution of social foraging in honey bees.

Figure 1. Effects of pln-QTL on ovary size.

Behavioral pln2 and pln3 QTL showed direct, additive genetic effects on the total number of worker ovarioles (mean±s.e.m.) in a high strain backcross between the selected pollen-hoarding strains.

Figure 2. Associations between ovary size and behavior.

A: Total ovariole number (mean±s.e.m) in different foraging behavior groups derived from a backcross of high and low pollen-hoarding strains (BOTH = pollen and nectar; EMPTY = no measurable pollen and nectar; POLLEN = pollen and NECTAR = nectar). B: The capture age (first foraging age) (mean±s.e.m) in different behavior groups. C, D: The pollen load proportion and nectar load concentration of the same bees divided by different capture age groups. *, <0.05; **, <0.01; ***, <0.001.

Figure 3. Comparison of pln candidate gene expression in pollen-hoarding strain bees.

Log transformed mRNA levels (mean±s.e.m., relative quantities, n = 12) in the abdominal fat body of high (blue bars) and low strain bees (red bars). A: PDK1 is expressed at higher levels in high strain foragers than in low strain foragers. B: HR46 is expressed at higher levels in larvae, newly emerged workers and foragers of the low strain in comparison to the low strain. *, <0.05; **, <0.01; ***, <0.001.

Figure 4. Associations between gene expression, ovary size, and behavior.

Log transformed mRNA levels (mean±s.e.m., relative quantities, n = 24) of candidate genes in the abdominal fat body of newly emerged workers and foragers derived from a backcross of high and low pollen-hoarding strain bees. Workers are divided into groups with extremely low (LO) or high (HO) ovariole numbers. A: PDK1 shows no significant difference between groups of newly emerged bees with different ovariole number. B: HR46 mRNA levels are significantly higher in newly emerged bees with low ovariole number compared to the group with high ovariole number. C: PDK1 gene expression is significantly higher in foragers with high ovariole number compared to the group with low ovariole number. D: HR46 shows no significant difference in the foragers. *, <0.05; **, <0.01; ***, <0.001.

Figure 5. Genetic architecture of honey bee foraging behavior.

HR46 and PDK1 can influence foraging behavioral decisions by acting during different life-stages and in different ways. As a putative affector of βFTZ-F1 activity, HR46 may be part of the apoptotic signaling system that determines worker ovary size. Bees with large ovaries tend to have higher levels of Vg mRNA as young adults. Vg is a key behavioral affector gene that influences foraging onset and foraging bias: increased levels are associated with a bias toward early foraging onset and pollen bias. In foragers, ovary size is linked to PDK1 gene activity, so that ovary size and PDK1 mRNA levels both are higher in workers with a foraging bias toward pollen. In short, we propose that HR46 acts in larvae to determine ovary size, which influences ovarian signaling in adult workers quantitatively and/or qualitatively, with effects on Vg, PDK1/IIS and foraging behavior.