Nutrient Signaling and Developmental Timing of Maturation - Current Topics in Developmental Biology - page 41

and in Drosophila the clock neurons, producing the pigment dispersing factor
(PDF), impinge on the PTTH-producing neurons ( McBrayer et al., 2007 ).
The photoperiodic mechanism gating PTTH release therefore imposes an
additional checkpoint control on the release of ecdysone.
Here, we review the system responsible for developmental timing of
maturation based on research on Drosophila with focus on the role of con-
served molecules related to insulin in connecting onset of maturation with
the consumption of calories and growth.
2. INTERPLAY BETWEEN NUTRIENTS, SYSTEMIC
GROWTH, AND DEVELOPMENTAL TRANSITIONS
2.1. Switching to a nutrient-independent developmental
timing program
Ecdysone is produced and released from the PG in response to cues that
serve as nutritional and developmental checkpoints. This means that the
neuroendocrine systemmonitors these cues to ensure that the larva has accu-
mulated sufficient mass and completed enough growth before the release of
ecdysone that triggers metamorphosis ( Mirth & Riddiford, 2007; Rewitz
et al., 2013; Yamanaka et al., 2013 ). In early L3, Drosophila larvae reach crit-
ical weight, a nutritional checkpoint that mediates a switch in the develop-
mental response to starvation ( Fig. 2.1 A). Critical weight is defined as the
time when the larva is committing itself to undergo metamorphosis even
in the absence of any further nutritional uptake. If starved before critical
weight, larvae extend the growth period by delaying pupariation, whereas
starvation after the critical weight point will result in cessation of growth but
without affecting the timing of pupariation ( Mirth & Riddiford, 2007;
Rewitz et al., 2013 ). Much of the larval growth takes place during the
while nutrient poor environments reduce the growth rate. Critical weight in the begin-
ning of L3 marks a nutritional checkpoint that switches the larva to a nutrient-
independent schedule for the release of ecdysone and initiation of metamorphosis. This
fixes the period between the critical weight and pupariation, called the terminal growth
period (TGP). Critical weight is related to size, and not time, which means that the larva
adjusts the duration of the growth depending on nutrient conditions pre-critical weight.
Conversely, post-critical weight, the TGP is fixed and cannot be adjusted according to
nutrition supply. The amount of growth during this period therefore determines the
final body size of the adult. (B) Opposing effects of high protein and high sugar concen-
trations in the diet on development timing.