Biology Reference
In-Depth Information
(2n). Oocyte formation in
D. melanogaster
is complex, involving both somatic
and germ-line cells. The ovaries contain oocytes that are formed from the “pole
cells,” but the cells that surround each egg chamber and make up the walls of
the egg chambers are derived from mesoderm (somatic tissues). The pro-oocyte
arises in a set of cell divisions within the ovary from an oogonial stem cell. Each
oogonial stem cell divides to give a daughter stem cell and a cystoblast cell.
The cystoblast cell gives rise to a set of 16 sister cells after four mitotic divisions,
which provides a cyst. One of these 16 cells becomes the pro-oocyte, and even-
tually the oocyte, whereas its 15 sister cells become nurse cells whose function
is to synthesize materials to supply the growing oocyte. The 16-cell cyst, sur-
rounded by a layer of somatic cells is termed the egg chamber. The final stages
of egg chamber development involve covering the cyst with a monolayer of pre-
follicle cells; these cells are somatic in origin. These 80 somatic cells divide an
additional four times to give 1200 follicle cells that cover each cyst.
Initially
Drosophila
oocytes and nurse cells are roughly the same size, but
they increase in volume by
≈
40-fold when vitellogenin begins to accumulate
approximately halfway through development of the oocyte. Some vitellogenin
is derived from the follicle cells, but most is produced in the fat body and trans-
ported to the ovary (
Raikhel and Dhadialla 1992
). The later stages of oocyte
development involve very rapid growth, with the oocyte increasing in volume
1500-fold. While the oocyte is increasing in size, the nurse cells are decreasing
because their contents are being deposited in the oocyte. Nurse cells, derived
from the germ line, are polyploid, containing 512 and 1024 times the hap-
loid DNA content. These polyploid nurse cells synthesize proteins, ribosomes,
and mRNAs. These products, and mitochondria, are transferred to the oocyte
by intercellular channels. Thus, the oocyte contains products produced by the
mother, which means that initial development in the oocyte is highly dependent
upon the genome of the mother (
=
maternal effects
). Finally, the vitelline mem-
brane and the chorion are secreted around the oocyte by follicle cells and the
oocyte enters metaphase of meiosis I. The oocyte remains arrested at metaphase
of meiosis I until after fertilization.
The oocyte increases in total volume during its development by
×
90,000-fold.
Oogenesis is a complex developmental pathway that is estimated to require the
function of 70-80% of all genes in the
Drosophila
genome, although the major-
ity is expressed during other stages of development as well. Only
≈
75 genes are
expressed exclusively during oogenesis (
Perrimon et al. 1986
). The egg of
D. mela-
nogaster
is rich in stored RNA, including rRNA and mRNA. The bulk of the mater-
nally produced, stored mRNA is derived from transcription of nurse-cell nuclei
during egg-chamber growth, but some mRNA may be derived from the oocyte
