Biology Reference
In-Depth Information
patients often exhibit clinodactyly, the bending of the small finger towards
the other fingers, which represents the survival of more primitive fetal struc-
tures to adulthood. Down syndrome also causes delays in liver and bone tis-
sue development and is associated with a range of malformations affecting
facial features; heart formation and brain development, all of which are con-
sistent with the idea that heterochronic aberrations are the underlying theme
of these symptoms (
Wilson, 1988
).
In this review we will focus on developmental timing in insects, and in
particular, we will explore the relationship between endocrine control and
circadian rhythms. Heterochronic phenotypes have been described for some
mutations in insect genes, but they are not linked to a group of genes such as
the “lineage” genes in
C. elegans
. Instead, the timing of developmental tran-
sitions in insects is associated with hormonal and circadian control. Studying
developmental timing in insect model organisms provides certain advantages
over other model systems. In particular, the presence of clearly delineated
developmental transitions, in combination with well-described stage-
specific events throughout the insect's life cycle, provides a unique platform
for isolating and characterizing mutations that affect developmental timing.
While
C. elegans
development is also characterized by molts, they lack the
well-characterized endocrine and circadian systems found in insects. Here,
we will review a string of recent advances from disparate research fields to
highlight how endocrine control and circadian rhythms affect developmen-
tal timing in
Drosophila
and other model insects.
3. DEVELOPMENTAL TIMING IN INSECTS IS UNDER
HORMONAL CONTROL
In insects, three classes of hormones act as key determinants of devel-
opmental timing: (1) Ecdysteroids, exemplified by the insect molting hor-
mone ecdysone, which are closely related steroid hormones that promote
developmental transitions such as the molts. (2) The prothoracicotropic hor-
mones (PTTH), which are neurosecretory peptide hormones that stimulate
the synthesis of ecdysteroids. (3) The juvenile hormones (JHs), which when
present at high levels ensure that molts result in another immature stage, such
as larvae or nymphs. For simplicity, we will refer to these hormones gener-
ically as ecdysone, PTTH, and JH.
JH, as the name suggests, keeps the developing insect in a juvenile state
by repressing metamorphosis. A decline of JH titers during the last instar
allows a subsequent pulse of ecdysone to trigger metamorphosis, defining
