Biology Reference
In-Depth Information
Before the cellularized-blastoderm stage, the dividing nuclei are equivalent
and totipotent, but after the cellularized blastoderm stage, specific body seg-
ments have been determined. The cellularized blastoderm stage is a key tran-
sition point in embryogenesis in
D. melanogaster
because this is when the
products of maternal genes become less important. It is thought that only a few
zygotic genes are active before cellularization. After the cellularized blastoderm
stage, the genes in the zygote begin to dominate in directing the development
of the embryo (
Walser and Lipshitz 2011
). After additional development, the
insect embryo gives rise to a segmented larva with three major tagmata: the
head, thorax, and abdomen.
4.13.3 Postembryonic Development
Drosophila melanogaster
is a holometabolous insect with sequential life stages:
egg
→
larval stage 1
→
molt
→
larval stage 2
→
molt
→
larval stage 3
→
molt
→
pupa
→
molt
→
adult. The larva hatches (
=
ecloses) from the egg, grows, and molts after
each larval stadium. After the third larval stadium, the insect molts, pupates,
and undergoes metamorphosis to the adult form. During metamorphosis, most
of the larval tissues are digested.
Adult structures develop from cells in structures called
imaginal discs
and
abdominal histoblast nests that will give rise to the abdominal epithelium. The
cells that give rise to the 19 imaginal discs became segregated from surrounding
cells during the first half of embryogenesis. By the time the larva hatches, the
imaginal discs and histoblast cells are visibly distinct from the surrounding lar-
val cells because they have smaller nuclei and an undifferentiated appearance.
The labial, clypeolabral, antennal
+
eye, thoracic, three leg, wing, and haltere
(
=
wings on metathorax of other insects) imaginal discs are paired. In addition,
there is a single fused genital disc. Imaginal cells are diploid and able to divide.
By contrast, most of the 6000 somatic cells of
D. melanogaster
grow in size, but
they do not undergo cell division. The chromosomes of the larval cells continue
to undergo replication and become polytene.
At the end of the third larval stage, the larva transforms into a pupa. During
the pupal stage, the imaginal discs, each consisting of about 40 cells, develop
into adult structures such as legs, wings, eyes, ovaries or testes, and antennae
(
Larsen-Rapport 1986
). Because the imaginal discs were determined during
embryonic development, the basic body plan of the adult fly was laid down
before the larva eclosed from the egg. The wings, halteres, and legs of the
adult, with as many as 50,000 cells each, are formed from the imaginal discs.
Not only is segmentation in
Drosophila
larvae based on cues obtained from the
