Finding helps explain how honeybees developed their distinctive social behavior
By Graciela Flores | March 6, 2007
A gene involved in egg production also helps honeybees exhibit some crucial social behaviors that distinguish them from solitary insects, researchers report in PLoS Biology this week.
The gene vitellogenin, which is involved in egg production in all egg laying animals, coordinates three core aspects of bees' social life: "It paces the onset of foraging behavior, it primes bees for specialized foraging tasks, and it influences longevity, three very important life-history characteristics for honeybees," senior author Gro Amdam of Arizona State University told The Scientist.
The results confirm predictions drawn from the sequence of the honey bee genome, which was published last October. The sequence suggested that new, distinguishing social behaviors likely developed from old genes and mechanisms.
"This is a very exciting paper," Thomas Flatt of Brown University, who did not participate in the study, told The Scientist. "It's the first mechanistic study to look at the genetic and hormonal regulation of complex social behaviors by a single gene using honeybees as a model organism."
In a previous paper, in which Amdam and her colleagues devised and validated vitellogenin silencing through RNAi in honeybees (Apis mellifera), knockdowns expectedly showed reduced hemolymph levels of vitellogenin. However, they also showed high levels of juvenile hormone. "This combination is typical of bees in the foraging stage, in contrast to bees in the earlier nursing stage, in which the insects have high levels of vitellogenin and low levels of juvenile hormone," Amdam explained. "Therefore, we expected that in field experiments, the knockdown bees would become foragers earlier in life than controls, that the shift from nest tasks to foraging would occur earlier," Amdam added.
Because selectively bred bee strains specialized in nectar collection have lower vitellogenin hemolymph levels than those bred to specialize in pollen collection, the authors also predicted that vitellogenin knockdowns would more likely become nectar collectors. They predicted, as well, that knockdowns would live shorter than controls. "It's known that honeybee vitellogenin affects longevity because it can scavenge free radicals, acting as an antioxidant shield, and thus can extend life," Amdam explained.
To test these hypotheses, the authors injected vitellogenin double stranded RNA (dsRNA) in a subgroup of bees and compared the bees' behavior and lifespan to two other groups that had received either dsRNA derived from a different gene (green fluorescent protein) or no injections (reference group).
The results confirmed all of the researchers' hypotheses. The vitellogenin knockdown bees started foraging at a younger age than the controls, showed a preference for nectar, and died earlier than the controls. "We tested our three hypothesis in one go and all of them turned out to be correct," Amdam said. "We found this single gene that has multiple coordinating effects in social organization."
"It seems that vitellogenin, which is typically used as a yolk precursor protein in reproduction by insects, including the non-social Drosophila, has been co-opted in social honeybees for the regulation of social behaviors," Flatt said. "It makes sense to assume that evolution has used a previously existing mechanism for a new function, which is very economical," Flatt added.
The paper also reports an important technical breakthrough, marking the first time that RNA interference (RNAi) is used to manipulate the behavior of bees out in the field, as opposed to in the laboratory, Gene Robinson of the University of Illinois at Urbana-Champaign, who was not involved in the study, told The Scientist. "This technique promises to be extremely helpful in identifying the no-doubt many other genes involved in regulating division of labor," Robinson predicted.
Links within this article
CM Nelson et al., "The gene vitellogenin has multiple coordinating effects on social organization," PLoS Biol 5(3), (2007)
Gro V. Amdam
M. E. Watanabe, "Honeybee sequencing: one honey of an idea," The Scientist, June 24, 2002.
M. L. Phillips, "Honeybee genome sequenced," The Scientist, October 25, 2006.
G. V. Amdam et al., "Disruption of vitellogenin gene function in adult honeybees by intra-abdominal injection of double-stranded RNA," BMC Biotechnol. 3:1, 2003.
J. Perkel, "Shhh: Silencing genes with RNA interference," The Scientist, April 7, 2003.
Gene E. Robinson