Another study has explored the putative role of vitello-genin in the regulatory control of the decline in immunity that characterizes honeybee foragers (Amdam et al., 2004c). The data from this work demonstrate that vitellogenin is the main zinc carrier in honeybee workers, and that lack of zinc induces pycnosis of honeybee hemocytes in vitro. The authors therefore suggested that depletion of vitellogenin, a protein that normally constitutes 30-50 % of the plasma protein pool in hive bees and diutinus workers but is down-regulated to undetectable levels in foragers (see Nutritional State), causes the plasma level of zinc to become so low that it triggers a loss of immunity. This phenomenon may further elucidate how natural selection works on circuits that control aging in social insect societies (reviewed by Omholt and Amdam, 2004): turning off vitellogenin synthesis in foragers can serve to economize the colony's protein household, as it prevents build-up of a storage protein that will be lost when the forager perishes in the field. Furthermore, as immune system maintenance is costly in solitary as well as social insects, the down-regulation of the honeybee forager immune system fits well into an explanatory framework where foragers constitute a disposable caste (O'Donnell and Jeanne, 1995) with an accelerated aging rate and level of physiological frailty (reviewed by Amdam, 2005).
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