Sci. Aging Knowl. Environ., 25 January 2006
The Way of the Honeybee
Bees turn reproductive protein into antioxidant
When it comes to aging, bees have flown away from other invertebrates, according to a new study. The work reveals that an egg protein that cuts survival in worms shields honeybees from destructive oxidants. The insects' social habits might have allowed them to divert the life-shortening protein to a beneficial function.
In worms and fruit flies, longevity and fecundity often don't go together. Turning up the insulin/insulin-like growth factor 1 pathway boosts reproduction, and quashing the pathway extends life (see, for example "Paying the Price"). Honeybees invert this survival-fertility relationship. Sterile workers last a few months at the most, whereas queens, which can pump out more than 1500 eggs a day, persist for several years. One link between aging and reproduction is the protein vitellogenin, which shepherds other molecules into the egg yolk, among other functions. Cutting the activity of vitellogenin genes, which fall in the insulin/insulin-like growth factor pathway, adds time for worms. By contrast, vitellogenin appears to prolong survival in bees (see Omholt and Amdam Perspective ). Queens brim with the protein. Furthermore, young workers that remain in the hive to tend the larvae harbor much more vitellogenin than do older workers that leave the colony to collect honey, and these venturesome insects quickly fall apart. The protein's structure suggests an explanation for how it promotes longevity. Vitellogenin latches onto zinc, as do many life-extending enzymes that combat the harmful byproducts of metabolism called reactive oxygen species (ROS). So evolutionary biologist Gro Amdam of Arizona State University, Tempe, and colleagues tested whether the yolk protein protects bees from the destructive molecules.
The researchers started with colony-bound and forager workers, which naturally manufacture different amounts of the protein, and gauged their response to paraquat, a pesticide that spurs ROS production. The smaller an insect's vitellogenin quantities, the faster it died. The team then used tiny RNA molecules to trim vitellogenin output in nonforaging workers. Again, bees that carried less vitellogenin died sooner after a blast of the pesticide. Furthermore, three proteins showed more oxidative damage in the vitellogenin-depleted bugs. The finding suggests that vitellogenin protects worker bees by squelching ROS. Previous studies suggested that vitellogenin in other species has the potential to become an antioxidant, says Amdam, but the indication that it extends survival is new. The researchers hypothesize that bees' queen-centered social system allowed them to adapt a death-promoting protein into a life-sustaining one, Amdam says. Unless the queen survives and keeps squeezing out eggs, the hive perishes. The switch might have helped bees, which originated in the tropics, to spread to temperate climates, because the protein enables some workers to survive the winter and restart the hive in the spring, she says. One difference that might have propelled vitellogenin's personality change, Amdam notes, is that bees scrupulously control the protein's quantities, whereas worms don't.
The paper is exciting because it shows that a protein that evolved to stock the yolk with nutrients can take on an entirely different function--antioxidant protection--says evolutionary geneticist Olav Rueppell of the University of North Carolina, Greensboro. Even if vitellogenin has antioxidant properties in the other species, the overall effect is to shorten life. The authors gauged the effects of natural variation in vitellogenin and manipulated its quantities, which makes the results doubly convincing, says neurogeneticist Gene Robinson of the University of Illinois, Urbana-Champaign. However, geneticist Jay Evans of the U.S. Department of Agriculture in Beltsville, Maryland, cautions that the decreased vitellogenin might harm the creatures in other ways. Because vitellogenin is a worker's main source of protein, cutting its quantities "might pull the plug on other physiological processes that affect longevity." Further work on aging in additional species might determine whether any others break away from the swarm.
January 25, 2006
Science of Aging Knowledge Environment. ISSN 1539-6150