Biology Reference
In-Depth Information
This ensures a certain amount of disc growth before the larvae commits to a
fixed schedule for metamorphosis. Since imaginal discs continue to grow even
when nutrients are restricted, this checkpoint is, at least partly, nutrient inde-
pendent and different from the nutritional checkpoint associated with the
attainment of critical weight. This view is supported by a recent study showing
that disc damage can delay the ecdysone-mediated development program
until the mid-L3 (
Hackney, Zolali-Meybodi, &Cherbas, 2012
), after the crit-
ical weight switch in the early L3 where limiting nutrition has no influence on
timing. Altogether, these studies show that at least two different growth-
related checkpoints control the developmental transition to adulthood, and
that both are based on crosstalk between tissues and the ecdysone system
via secreted insulin-like molecules (
Fig. 2.4
): (I) a nutritional checkpoint dur-
ing early L3 that depends on systemic nutrient-dependent insulin signaling
regulated by dietary factors and primarily amino acids via the fat body; (II)
a mid-L3 checkpoint that allows developmental progression depending on
growth status of adult precursor tissues. Interestingly, imaginal discs grow
to a final size independently of the developmental stage of their host
(
Hackney et al., 2012
). This suggests that their target size is controlled by a
Figure 2.4 Control of developmental progression to adulthood by checkpoints. Devel-
opmental progression is controlled by a surveillance systemmonitoring the completion
of events associated with the accumulation of nutrients/mass and discs growth.
A circadian checkpoint is superimposed on this system, to gate the release of PTTH
according to the photoperiod. The time-line indicates the third larval instar (L3). The
predicted translation of passing these checkpoints into ecdysone pulses that coordi-
nates and control developmental progression is shown. DILPs, Drosophila insulin-like
peptides; DILP8, Drosophila insulin-like peptide 8; PDF, pigment dispersing factor.
