Biology Reference
In-Depth Information
3.2. Growth and nutrient utilization
Growth regulation at the cellular, tissue, and organismal level is critical for
proper size development in all multicellular organisms, and it is affected by
several environmental factors including nutrient availability (
Mirth &
Riddiford, 2007
). In
Drosophila
, the feeding larva grows an astounding
amount, increasing its size by
200-fold during the 3.5-day period
(
Church & Robertson, 1966
). Without this accumulation of body mass,
the fly may have reduced reproductive success as an adult or it may not even
be able to survive metamorphosis from the larva to adult.
For the adult fly to reach its proper size, the larva must pass three weight
checkpoints. The first checkpoint occurs near the second instar to third instar
molt and is called the threshold size for metamorphosis (
Zhou, Zhou,
Truman, & Riddiford, 2004
). This size assessment determines whether the
next molt will be a larval or metamorphic molt (
Nijhout, 1975
). The second
checkpoint is the minimal viable weight which is the minimum body mass
that is necessary to complete larval and pupal development in the absence
of nutrients (
Bakker, 1959
). The final checkpoint, critical weight, occurs dur-
ing the last larval stage (
Nijhout, 2003; Nijhout &Williams, 1974
). Reaching
critical weight ensures that the animal will pupate within a certain amount of
time regardless of nutrient availability (
Bakker, 1959; Mirth & Riddiford,
2007; Nijhout, 2003; Robertson, 1963
). Of these three size assessment check-
points, critical weight is the most studied and best understood in
Drosophila
.
Once larvae reach their critical weight, environmental factors have a
large impact on adult size. Larvae that starve before they achieve critical
weight will delay their development until the nutrient supply improves.
If nutrients are still abundant after larvae reach critical weight, they will con-
tinue to accumulate body mass (
Mirth & Riddiford, 2007; Tennessen &
Thummel, 2011
). On the other hand, if postcritical weight larvae starve,
they will stop growing in size. Since these starved larvae have reached their
critical weight, they will enter metamorphosis within a similar time frame as
fed larvae, but they will be smaller and will mature into smaller adults than
the fed animals. This suggests that the mechanisms that regulate develop-
ment and puparium formation must coordinate with nutrient utilization.
The endocrine cascade that follows critical weight achievement was
originally described in the tobacco hookworm,
Manduca sexta
(
Nijhout &
Williams, 1974; Truman &Riddiford, 1974
). Briefly, once larvae reach crit-
ical weight, juvenile hormone (JH) titers drop, causing a release of pro-
thoracicotropic hormone, which signals to the prothoracic gland (PG) to
produce ecdysone. However, this function of JH does not seem to be
