Tuesday, March 6, 2007
This post is devoted to my favorite invertebrate, Apis mellifera, the Western honeybee. The honeybee genome is the most recent genome to be fully sequenced (published October of 2006), and I have heard many demean this development as pointless and superfluous. I feel otherwise; as eusocial insects, these little critters lead fascinating lives, which may help us understand the evolution of social behavior. (They're not bad dancers, either). I've been meaning to post on honeybees for a while, and was finally inspired to do so by an article published today in PLOS Biology.
Eusocial insects, including termites, ants, and many bees and wasps, exist in groups numbering up to hundreds of thousands of individuals, yet these colonies seem to function as single organisms. Indeed, no individual is capable of nourishing, protecting, and reproducing itself. As a collection of thousands of specialized individuals, each dedicated to one task, however, such a “superorganism” can perform every task simultaneously.
Although considerably specialized, these divisions are dynamic; insect colonies respond to changing environmental conditions by adjusting the number of workers performing a given task. The western honeybee offers a striking example of this adaptability: a young bee times her maturation into a forager in response to the colony’s needs. For the first 2-3 weeks of her life (yes, "her": the females do all of the work in the hive, while males are haploid drones with have no role beyond insemination), a worker bee will work solely in the hive, performing activities such as caring for larvae as a nurse. At approximately 3 weeks, she will mature into a forager, leaving the hive to collect pollen and nectar for the remainder of her 4-6 week lifespan.
This stereotyped repertoire of behavioral maturation is highly dependent on the social context; the colony rations its 40,000 to 80,000 workers to different tasks depending on its needs. If a large number of foragers fall victim to predation, the remaining workers in the colony react by expediting their own development, and may become foragers at as early as 5 days. Correspondingly, if a brood disease kills many nurses in the hive, behavioral maturation is suppressed and the age at which a bee will become a forager is delayed; some bees already occupied as foragers may even revert back to nursing to make up for the deficiency. These considerable lifestyle changes require entirely new behaviors, and is accompanied by dramatic physiological changes, including alterations in exocrine gland activity, hormone and neurotransmitter levels, brain structure, responsiveness to certain stimuli, and gene expression levels.
A fascinating question is how the bee can sense when the colony requires more foragers, and regulate her development accordingly. This study, coming out of Gro Amdam's lab at the University of Arizona, looked at one particular protein, vitellogenin, that may be instrumental in regulating this behavioral maturation.
To determine the role for vitellogenin in coordinating social development, the authors used a technique called RNA interference (RNAi), which basically prevents specific proteins (vitellogenin, in this case) from being synthesized; thus, it "silences" specific genes. This is an incredibly useful genetic technique, and this paper marks the first time that it has been used in honeybees.
The researchers used their vitellogenin RNAi tool to silence the gene in a group of bees, which were then housed in observation hives with about 5,000 other adult bees of various ages and social statuses (the same was done with "normal" bees, injected with a green dye instead of the RNAi tool, as a control). (One of the cool things about honeybee research is it looks at natural behaviors in natural environments, as opposed to, say, testing rodent memory by forcing them to swim around in a giant pool of milk.)
The researchers found that bees in which vitellogenin had been silenced initiated foraging behavior significantly earlier than normal bees. This suggests that vitellogenin inhibits the onset of foraging, implying that it may be a molecular mediator for environmental cues that inhibit foraging behavior. The paper goes further to associate behavioral/social maturation with actual lifespan. Remember that the transition from nursing to foraging is a "maturation" process, and that the inhibition of foraging behavior is coupled with the perpetuation of nursing behavior. In a fascinating corroboration of this connection, the authors found that bees without vitellogenin had shorter lifespans than "normal" bees.
I tend to be excited about anything that draws attention to honeybee social behavior, but was particularly happy with this paper. Honeybees offer the unique opportunity to identify genes that influence social behavior and may be involved in social evolution, and the use of RNAi by Amdam is a tremendous step in furthering this fascinating field.