Biology Reference
In-Depth Information
feeding (
Mirth & Riddiford, 2007; Mirth & Shingleton, 2012
). Starvation
before minimal viable weight results in larval death without an attempt to
undergo metamorphosis. Minimal viable weight should not be mistaken
for critical weight, which is the size where starvation no longer delays meta-
morphosis. In
Drosophila
minimal viable weight and critical weight are
attained almost simultaneously, making the distinction difficult.
Several classical studies using lepidopteran insects suggest that attainment
of critical weight is followed by a drop in juvenile hormone (JH) levels,
which is permissive for prothoracicotropic hormone (PTTH) release at a
certain time defined by the photoperiod. According to this scheme, the
release of PTTH from the brain is the principal event committing the PG
to ramp up ecdysone production and release to trigger metamorphosis
(
Nijhout & Williams, 1974a,1974b
). Although the decline in circulating
JH levels following attainment of critical weight is believed to be the key
determinant for PTTH release in lepidopterans, a role for JH titer drop in
triggering PTTH release from the PG neurons in
Drosophila
is less clear since
ablation of the corpus allatum, the organ responsible for JH synthesis, does
not appear to alter critical weight or accelerate the timing of pupariation in
this species (
Riddiford, 2011; Riddiford, Truman, Mirth, & Shen, 2010
).
Instead, a body of recent evidence shows that, although PTTH release is
an important event, other factors converge on the PG to coordinate ecdy-
sone synthesis and release. For example, recent studies have demonstrated
the importance of insulin/TOR signaling in the PG for critical weight
assessment and final body size determination (
Caldwell et al., 2005;
Colombani et al., 2005; Gibbens et al., 2011; Layalle et al., 2008; Mirth
et al., 2005
), while other factors such as myosuppression appear to act
negatively to reduce PG activity (
Yamanaka et al., 2005; Yamanaka et al.,
2010; Yamanaka et al., 2006
; see
Marchal et al., 2010
for a review of factors
regulating PG synthesis of ecdysone). Together these studies place the PG in
a more central position compared to the classical scheme that focuses on the
PTTH-producing neurons.
Starvation does not change critical size, showing that it is genetically
determined and unaffected by environmental factors (
Beadle et al., 1938
).
For example, genetic reduction of insulin signaling before critical weight
slows larval growth rate and delays the attainment of critical weight and
pupariation (
Shingleton, Das, Vinicius, & Stern, 2005
). However, it does
not change the critical weight, which means that it does not alter final adult
body size. On the other end, reducing insulin signaling after attainment of
the critical weight reduces final adult size but does not delay pupariation.
