Comparative proteome analysis of honey bee workers between overwintering and brood-rearing seasons

Highlights

• Proteomes of honey bee between overwintering and brood-rearing seasons were compared.

• Most proteins were more abundantly expressed in overwintering season.

• These proteins involved those associated with energy storage and metabolism.

• Expression level of stress-responsive proteins was higher in brood-rearing season.

Abstract

In winter, honey bees thermoregulate their hives to survive cold temperatures and maintain their physiological activity, without becoming completely dormant. At this time, nurses and foragers are not distinguishable. In late winter or early spring, as the brood rearing re-initiates, the division of labor resumes among the workers born in the fall. To understand the overall physiological changes of honey bee workers from late winter (end of overwintering) to early spring (beginning of brood rearing), we collected honey bees in January and February and compared their protein expression profiles. Among the 50 and 85 proteins showing greater than two-fold differences in expression levels in the head and abdomen, respectively, 20 proteins with relatively large differences in expression level between the months were selected and identified. Most proteins were more abundantly expressed in January than February and were mainly involved in nutrient storage, energy metabolism, and biosynthesis pathways in both the head and abdomen. This finding suggested that overwintering honey bees require large energy storage and metabolize stored nutrition to generate high cellular energy for thermoregulation of their hive without diapause and/or to prepare for the initiation of brood rearing in January.

Introduction

In addition to environmental and agricultural importance, the honey bee is a remarkable insect species with which to study the evolution of sociality because of its specialized division of labor and the flexibility of colony management. After emergence from the pupal stage, the adult bee passes through various behavioral tasks (Winston, 1987). Worker bee behavior changes dramatically with age and they perform different tasks associated with colony growth and development, a phenomenon known as age polyethism (Free, 1965, Seeley, 1985). Young “nurse” bees (ca. 1–12 days old) primarily feed and care for larvae and the queen in the central region of the nest (the brood nest), middle-age bees (ca. 13–20 days old) maintain the hive and store food in the peripheral food storage region, and the oldest “forager” bees (20 + days old) forage for nectar and pollen outside the nest and defend the hive (Seeley, 1982, Shapira et al., 2001). However, young bees raised in the absence of older foragers begin to forage precociously (Huang and Robinson, 1992), and foragers also revert to nursing tasks when young nurse bees are artificially removed from bee hives (Robinson et al., 1992), indicating the flexibility of labor division and the ability of bees to respond to environmental changes.

Even in natural conditions, active and flexible colony management is achieved. In temperate climate regions, colony activity is highly accelerated, and adult workers exhibit an age-based division of labor during the foraging season, as described above, and the population size of bee colonies is reduced from late fall to early spring. In winter, nurses and foragers are not distinctly distinguished (Johnson, 2010), and they thermoregulate the bee hive to survive winter conditions without becoming completely dormant through the formation of thermoregulating clusters (Doke et al., 2015, Huang and Robinson, 1995). In addition to thermoregulation, the overwintering honey bee colonies enter a distinct physiological and behavioral state, including reduced individual activity, changes in endocrine profiles, increased nutrient stores, increased longevity, and cessation of brood rearing (Doke et al., 2015). In late winter/early spring, as brood rearing re-initiates, the division of labor resumes among a group of workers born in the fall that survived the winter (Seeley and Visscher, 1985).

Regarding the physiology of overwintering workers, the juvenile hormone (JH) biosynthesis rate decreases from fall and reaches its lowest level in winter (Huang and Robinson, 1995), whereas the level of vitellogenin (Vg) and hemolymph proteins are higher and the size of the hypopharyngeal gland (HPG) is significantly larger than those in the foraging season (Fluri et al., 1982). The JH titers begin to rise in early spring, which correlates with a decrease in Vg levels, hemolymph protein levels, and HPG size. In comparison with the foragers, nurses exhibit a lower JH titer, higher Vg and hemolymph proteins levels, and larger HPG size, indicating that the physiological condition of the overwintering bees is similar to that of nurses in winter, whereas overwintering bees return to the physiological state of foragers in spring (Doke et al., 2015, Fluri et al., 1982). Nevertheless, no information on the physiological changes in worker bees during the narrow transition period from overwintering to the re-initiation of brood rearing is available.

In the present study, we collected worker bees in January and February, and compared protein expression profiles to better understand the overall physiological changes of workers over the period between the end of overwintering and the beginning of brood rearing.