Biology Reference
In-Depth Information
nucleus itself, which is active briefly approximately halfway through development.
The total amount of mRNA in the oocyte is equal to
≈
10% of the single-copy
DNA of the
Drosophila
genome and corresponds to
≈
8000 distinct protein-coding
sequences. Most of the mRNA codes for proteins that required early in embryo-
genesis, including tubulins and histones. Products from a few maternal genes con-
tinue to affect development in
D. melanogaster
during the larval stage.
4.13.2 Embryogenesis in
D. melanogaster
Fertilization occurs when the mature oocytes are released into the oviducts.
A single sperm enters the egg cytoplasm through a special channel in the
anterior region of the oocyte called the micropyle. Fertilization initiates the
completion of meiosis I and II. Two polar body nuclei and the female pronucleus
are produced. After the haploid male and female pronuclei unite (
syngamy
),
early embryogenesis takes place so rapidly there is no time for cell growth
(
Figure 4.5
). Initial mitoses are atypical because the first nine divisions result in a
syncytium
containing
×
512 nuclei that lack cellular membranes.
After seven nuclear divisions, and when there are 128 nuclei in the central
region of the egg, most of the nuclei and their surrounding cytoplasm migrate
outward as they continue to divide. A few nuclei are left behind which divide
once to become yolk nuclei that do not become incorporated in the embryo
(
Figure 4.5
). After nine divisions, most of the nuclei have migrated to the egg
surface. At this time, the soma and germ-line nuclei segregate when
≈
15 nuclei
move to the posterior region of the egg, bud off, and eventually become the
nuclei in the pole cells. These nuclei divide about twice more and become
pole
cells
that will give rise to the germ-line tissues of the fly. Meanwhile, the other
nuclei migrate to the surface of the egg and divide four times more in syn-
chrony to produce a syncytial blastoderm.
Finally, the membrane covering the egg invaginates to enclose each nucleus
in a separate membrane, to form a cellularized
blastoderm
(
Figure 4.5
). The
blastoderm is the layer of cells in an insect embryo that completely surrounds
an internal yolk mass. The cellular blastoderm develops from a syncytial blasto-
derm by partitioning the cleavage nuclei with membranes derived from infold-
ing of the oolemma (egg membrane). During the cellular blastoderm stage,
D. melanogaster
exhibits the
long-germ band
type of development, in which the
pattern of segmentation is established by the end of blastoderm. Some other
insects exhibit a
short-germ band
type of development, in which all or most of
the metameric pattern is completed by the sequential addition of segments dur-
ing elongation of the caudal region of the embryo.
