Biology Reference
In-Depth Information
divisions and relatively insensitive to nutrient uptake controls disk growth.
In contrast, growth of endoreplicative larval tissues is tightly coupled with
nutrient availability and relies on the increase in cell size rather than number.
Because larvae pupariate even in the absence of disks, Dilp8 is a permissive
signal, rather than a positive “trigger” of pupariation, which provides a
checkpoint to ensure that the adult precursor tissues are ready for metamor-
phosis. Conversely, nutrient sensitive growth influenced by the fat body
likely provides positive signals to the endocrine system.
Based on recent insights discussed here, we propose that developmental
timing is regulated by a series of checkpoints that verify a certain amount of
nutrient stores and disk growth before a circadian input allows developmen-
tal progression (
Fig. 1.5
). The first checkpoint, which probably corresponds
to critical weight, is the key decision whether to enter or delay pupariation if
nutrient becomes limited. Another checkpoint verifies that the imaginal
disks are ready, and finally the photoperiod is monitored to impose a circa-
dian gate on the developmental transition. Although the mechanisms
responsible for generating each peak of ecdysone are poorly understood,
CHECKPOINTS
Mass
(nutrients)
Disc
growth
Photo
gate
All
PTTH
Dilp8
?
PDF
?
Insulin
PTTH
PTTH
PTTH
0
8
20
28
44
THIRD LARVAL INSTAR
PREPUPA/PUPA
(hours after L3 ecdysis)
Figure 1.5 Checkpoint controls of developmental timing of metamorphosis. The figure
shows a proposed model for checkpoint control of ecdysone release and timing of
pupariation. Accumulation of nutrients corresponding to the critical weight presumably
results in the insulin dependent production of the critical weight ecdysone peak 8 h
after L3 ecdysis. Maturation and patterning of the imaginal disks and the photoperiod
are also verified by the endocrine system before development can proceed. These
checkpoints may potentially translate into to the other low-level ecdysone peaks 20
and 28 h after L3 ecdysis. When all checkpoints are cleared, developmental transition
to the metamorphic stage can take place.
