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New behavior may use old genes

October 26, 2006

TEMPE, Ariz. - Though you may not be able to teach an old dog new tricks, ASU researchers have found that evolution may have taught old genes new tricks in the development of social behavior in honeybees.

The genetic basis of social behavior is being deciphered through the efforts of ASU researchers and their work with the honeybee, Apis mellifera. The recently completed honeybee genome sequence is aiding that effort by providing DNA sequences of genes that may be involved in social behavior, according to ASU assistant professor Gro Amdam.

Amdam and ASU professor Robert Page, founding director of ASU's School of Life Sciences, were part of a consortium of researchers that sequenced and analyzed the honeybee genome. The results are reported in this week's issue of Nature (Oct. 26), in the article, "Insights into social insects from the genome of the honeybee Apis mellifera."

Amdam explained that the honeybee is a particularly well-suited organism for studying the genetic basis and evolution of social behavior. "As first determined by Page, the genetic recombination rate in Apis mellifera is the highest of all animals, and 10 times greater that that of flies or humans," Amdam said.

The scientists have used the high recombination rate, a natural process involving the shuffling of DNA, to their advantage in mapping behavior precisely to genome regions. Essentially, the more frequently genes are "shuffled around" in each generation of honeybee, the smaller the identified region and the better chance the researchers have of identifying specific genes related to behavior.

"An early expectation in the sequencing project was that we would find many new genes responsible for social behavior in the honeybee genome," Amdam said. "However, we didn't find much diversification of such social genes and, in fact, the number of honeybee genes overall was smaller than in the genome of the fly, which has a solitary lifestyle."

The consortium authors explain that the relatively small number of genes may be a reflection of the highly specialized and self-managed life of the honeybee. However, the lack of social genes makes sense in the context of research that Amdam and Page have done in parallel with the sequencing project.

For example, Page showed that "social behavior could be mapped to the genome," Amdam said. He accomplished this by first breeding two different strains of bees that exhibit starkly different foraging behavior. Then, by a series of crosses involving pollen-favoring strains, nectar-favoring strains and their hybrids, behavior differences in the resulting worker offspring were linked to genetic variation, according to Amdam.

Initially, the worker bees' foraging behavior was mapped to general regions of the genome called linkage groups. Now that the entire Apis mellifera genome has been sequenced, the linkage groups have been identified with specific regions of chromosomes. The ASU researchers next identified genes encoding proteins in regulatory pathways based on their genome location as potentially responsible for differences in the social foraging behavior of the honeybee.

These pathways support the ongoing work of Amdam and Page on the evolutionary basis for social behavior, and also are consistent with the lack of new social genes in the honeybee genome. So, instead of finding novel genes devoted to a specific behavior, Amdam and colleagues have found that ancestral genes from solitary predecessors are likely to be connected to social behavior.

The researchers have found that the important conserved pathway of insulin signaling may be involved in deciding whether a bee collects pollen or nectar, Amdam said. According to the genome sequence, these genes - which the researchers have found involve regulation of the female reproductive cycle and lifespan in insects - are much more common in the genomic regions of foraging behavior than expected by chance alone.

Foraging behavior is just one aspect of the complex social network developed in honeybee colonies, but the ASU scientists have found that preferential foraging of pollen rather than nectar is associated with a complex system of physiology, behavior and longevity that affect bees throughout their life, Amdam said. Genetic variation that allows for the specialization of worker bees into pollen foraging or nectar foraging roles appears to be important for the social structure of the honeybee society and for the general understanding of social evolution and aging.

Amdam plans to further refine the genetic analysis of social behavior by using a new research technique she pioneered for use in adult honeybees. This technique, called RNA interference, involves silencing targeted genes in the honeybee genome and observing the affect on social behavior.

Through the use of RNA interference, Amdam and colleagues expect to be able to target the genes they have mapped and, by silencing them, precisely determine what aspects of social behavior they affect.

Arizona State University