Biology Reference
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by doctoral candidates. In their system, however, the external reviewer
is called an opponent and plays a special role in the process. I agreed
and soon at erward received Amdam's dissertation. My role was to
read it and develop questions to ask her in a public defense. h e central
part of her dissertation examined how juvenile hormone and vitello-
genin coregulate each other and as a consequence regulate the onset of
foraging behavior in bees. She and her doctoral advisor, Stig Omholt,
had worked out an elegant model for how it can work and supplied
some critical data in support of it. h ey called their model the double-
repressor hypothesis.
7.2 h
e Double-Repressor Model
h e double-repressor model proposes a new role for vitellogenin (Vg)
in the life history of honey bees and a new interaction between vitello-
genin and juvenile hormone (JH). In most insects, juvenile hormone
signals the production of vitellogenin by globular structures in the he-
molymph that are attached to the wall of the abdomen collectively called
the fat body, and prepares the ovaries to incorporate Vg (Figure 7.1). (In-
sects do not have a closed, vascular system for moving blood around.
h ey have an open system where the organ tissues l oat in the blood,
called hemolymph.) h e fat body serves the role of the liver and biosyn-
thetic factory for insects. Vitellogenin is released from the fat body into
the hemolymph, binds to the ovaries, is incorporated, and is used to
make eggs. Amdam suggested that instead of just initiating the produc-
tion of Vg, JH also suppresses it. But Vg also suppresses JH. At er the
initial surge in blood titer of JH, Vg production begins and suppresses
JH. As Vg is fed to larvae, Vg titers decrease, and Vg loses its suppres-
sion of JH. As JH titer increases, it drives down Vg even faster, resulting
in a sharp switch where Vg goes low and JH goes high (Figure 7.2).
Workers then initiate foraging in response to the changes. Amdam and
her colleagues were able to demonstrate that when Vg titers were sup-
pressed by interference with production of Vg, JH titers increased, and
bees initiated foraging earlier in life.
Sometimes clarity really does come in the shower. At er reading the
dissertation of Gro Amdam, I felt that the double-repressor model must
