Females do all the work of the hive, and their reproductive organs normally are not fully developed. This state is known as reproductive self-restraint, and it occurs because the cues the workers receive tell them that the queen is laying an adequate number of eggs. The cues come from the queen in the form of pheromones, including odors that mark the eggs as belonging to the queen. Even the larvae participate in this process, giving off chemical signals, specifically, aliphatic acids, that influence the suppression of the worker's ovaries, helping to keep the workers focused on their tasks and not distracted by instincts to reproduce.
That being said, Madeleine Beekman and Ben Oldroyd found that approximately 1 percent of the workers in the European honey bee colonies that they studied had active ovaries and were able to lay eggs. Adult bees do not increase in size as they grow older, but their role in the hive changes as they age, and this process creates the division of labor in the colony. Their age-related transitions to different roles occur not because of hereditary differences, but rather due to the degree to which groups of genes in the bee brain are activated.
This activation results in behavioral change and is known as genomic plasticity. As a result of mapping the bee genome as isolated through bee brains, Charles Whitfield and his colleagues were able to predict the behavior of fiftyseven out of sixty bees by measuring the levels of activation of a large group of genes. When a worker first emerges from her pupal case, she spends the first day or two letting her wings and exoskeleton harden.
Then she begins to work at tasks associated with the interior of the nest, including bringing food to the larvae in their cells and grooming the queen and the other workers. Until she is seven to ten days old, she consumes protein-rich pollen so that the hypopharyngeal glands in her head fully develop. These glands produce a fluid that she will feed to the larvae as her next task. As the bee ages, she moves on to keep the hive clean, make wax, and participate in building the honeycomb. When she is mature, which means about three weeks old, the typical worker shifts her attention to tasks that bring her in contact with the world outside the nest. These tasks include receiving nectar from foragers and placing it in storage cells, guarding the entrance to the nest, and scouting for food.
The transition from working inside the nest to working outside the nest is a big change in the life of a worker bee. Flying back and forth to collect pollen and nectar is done only by the most experienced bees in the colony during the last week or two of their lives. Several factors contribute to these behavioral changes, including hormones, lipid stores, neurochemicals, and environmental cues. Mark Drapeau and others have identified proteins, as a result of analysis of the bee genome, that play a role in reproductive maturation and stage-specific development.
There are observable differences in gene expression in young nurse bees compared to mature foragers, and an increased expression of foraging genes was observed by Seth Ament and colleagues in the older bees. It can be difficult to tell if the increased activity of foraging genes in older bees causes foraging, or if changes in foraging gene activity are the result of foraging, but there is clearly a connection between the genetic activity and the behavior. Research by Charles Whitfield and his colleagues found that a brood pheromone that inhibits behavioral development is present in the crops (nectar storage pouches) of foragers at levels thirty times higher than it is in the crops of nurses.
The foragers pass along the pheromone to the younger workers when they exchange food with them, and this inhibits the young bees' development by depressing the amount of juvenile hormone in their hemolymph (body fluid). When it is time for the workers to move on to tasks requiring more maturity, the amount of brood pheromone received by them is probably reduced and the foraging gene activity increases.
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08152010
1. What is special about bees living in colonies
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