Biology Reference
In-Depth Information
rich environments. Insulin-like peptides also affect the decision between
delaying or continuing development to the adult stage in both
Caenorhabditis
elegans
(
C. elegans
) and mammals (
Kjaer, Hagen, Sando, & Eshoj, 1992;
Liu & LeRoith, 1999; Tennessen & Thummel, 2011
).
Drosophila
has three larval instars where it rapidly accumulatesmass and nutri-
ents. At the end of the third instar (L3), when the larva has accumulated enough
mass and stored sufficient nutrients to survive the non-feeding metamorphic
development to the adult stage, it exits the food and initiates a characteristicwan-
dering behavior before it pupariates. This marks the onset of metamorphosis, a
process that transforms the juvenile larva into a sexually mature adult by the
destruction of obsolete larval tissues and the morphogenesis and differentiation
of adult precursor tissues (
Baehrecke, 2000; Siaussat, Porcheron, & Debernard,
2009; Thummel, 2001
). Pulses of ecdysone that causes the larva to shed its old
cuticle and form a new and bigger one that allows continued growth trigger
developmental transitions during the larval stages. In
Drosophila
,asingleecdy-
sone pulse triggers the transition through the first two larval stages, while
pupariation (onset ofmaturation) is initiatedinresponsetothree low-level pulses
followed by a high-level pulse during L3 (
Rewitz et al., 2013; Rewitz,
Yamanaka, & O'Connor, 2010; Yamanaka, Rewitz, & O'Connor, 2013;
Warren et al., 2006
).
Ecdysone is produced and released from the prothoracic gland (PG), the
major endocrine tissues of insects, in response to a brain-derived neuropep-
tide called prothoracicotropic hormone (PTTH;
McBrayer et al., 2007;
Smith & Rybczynski, 2012
). PTTH activates its receptor Torso, a receptor
tyrosine kinase that signals through the mitogen-activated protein kinase
(MAPK) pathway to stimulate the production and release of ecdysone from
the PG (
Rewitz, Yamanaka, Gilbert, & O'Connor, 2009
). Stimulation of
the PG by PTTH activates the ecdysone biosynthetic pathway that converts
dietary cholesterol into ecdysone. The biosynthesis of ecdysone is mediated
by several cytochrome P450 enzymes, a rieske-domain protein and a short-
chain dehydrogenase/reductase (
Niwa et al., 2010; Petryk et al., 2003;
Rewitz, Rybczynski, Warren, & Gilbert, 2006a, 2006b; Warren et al.,
2002, 2004; Yoshiyama, Namiki, Mita, Kataoka, & Niwa, 2006;
Yoshiyama-Yanagawa et al., 2011
). Lack of either PTTH or Torso attenu-
ates ecdysone synthesis, which delays development by prolonging primarily
the L3 stage. This extends the duration of the growth period and increases
adult body size. Therefore, PTTH triggers the onset of maturation once
the animal has completed enough growth during larval development. More-
over, the release of PTTH is regulated by the photoperiod (
Nijhout, 1981
)
