Biology Reference
In-Depth Information
Garelli, Gontijo, Miguela, Caparros, &Dominguez, 2012; Mirth, Truman, &
Riddiford, 2005; Ou, Magico, & King-Jones, 2011; Walkiewicz & Stern,
2009
). At the cellular level, two things determine body dimensions: the num-
ber of cells and their size. It is clear that the great size diversity among animals is
largely determined by differences in cell number rather than cell volume.
However, insects utilize both strategies to grow as many larval tissues sustain
their rapid growth and metabolic activity by endoreplication cycles, a process
of chromosomal replications without cell division (
Edgar & Orr-Weaver,
2001; Lee, Davidson, & Duronio, 2009; Smith & Orr-Weaver, 1991
).
Increasing the DNA content allows cells to dramatically increase in volume
without dividing and is responsible for most of the size increase during larval
growth. During metamorphosis, the breakdown of obsolete endoreplicated
larval tissues provides nutrients for growth and differentiation of mitotic neu-
roblasts and imaginal disk tissues that form the adult body.
Unlike the well-characterized genetic control of embryonic develop-
ment, we are only now beginning to understand the developmental timing
system that ensures unidirectional progression of developmental transitions.
Recent genetic studies in the fruit fly
Drosophila
have provided new insight
into how the developmental timing system evaluates larval growth and en-
ergy stores and integrates external cues such as photoperiod to time the onset
of metamorphosis (
Caldwell et al., 2005; Colombani et al., 2005, 2012;
Garelli et al., 2012; Layalle, Arquier, & Leopold, 2008; McBrayer et al.,
2007; Mirth et al., 2005
). All of these signals are eventually communicated
to the endocrine system responsible for producing the pulses of the steroid
hormone 20-hydroxyecdysone (for simplicity, hereafter referred to as
ecdysone) that triggers molting and metamorphosis (
Gilbert, Rybczynski,
& Warren, 2002
). In
Drosophila
, a single pulse of ecdysone triggers each
of the first two larval molts (
Warren et al., 2006
). During the terminal third
instar stage, three low-level pulses followed by a high-level peak of ecdysone
initiate the physiological and behavioral changes necessary to transform a
food seeking larva into a nonfeeding immobile pupa. Although the roles
of the initial low-level ecdysone peaks are not completely understood,
the final major high-level peak triggers pupariation and the onset of
metamorphosis.
Ecdysone is produced in the prothoracic gland (PG) by a series of
reactions mediated by a Rieske oxygenase and enzymes encoded by the Hal-
loween family of genes that include several cytochrome P450s and one dehy-
drogenase/reductase (
Niwa et al., 2010; Ono et al., 2006; Petryk et al., 2003;
Warren et al., 2002, 2004; Yoshiyama, Namiki,Mita, Kataoka, &Niwa, 2006;
