Biology Reference
In-Depth Information
The genes that control
Drosophila
embryonic development can be divided
into three classes: 1)
maternal-effect genes
that specify egg polarity and the
spatial coordinates of the egg and future embryo; 2)
segmentation genes
,
including the gap, pair-rule, and segment-polarity classes of genes that deter-
mine the number and polarity of the body segments; and 3)
homeotic genes
that determine the identification and sequence of the segments. Although most
genes with a homeo domain are in the homeotic class, a few are found among
the segmentation genes (
Figure 4.6
).
4.14.1 Maternal-Effect Genes
Maternal-effect genes are transcribed in the mother, and their messenger
RNAs influence the development of oocytes and embryos. Maternal-effect
genes were discovered by determining whether the mother carries a mutant
gene that results in an inability to rescue her embryo when the father con-
tributes a wild-type gene at the time the egg is fertilized. Maternal-effect
genes are important because the basic organization of the oocyte (front-
back, top-bottom) has been accomplished even before the egg has been fer-
tilized. At least 38 maternal-effect genes have been studied;
≈
20 affect the
anterior-posterior pattern and
≈
18 affect the dorsal-ventral polarity of the
embryo (
Figure 4.6
).
The 20 genes influencing differentiation into proper anterior-posterior polar-
ity can be subdivided into a group affecting the anterior half of the embryo, a
group affecting the posterior half, and a group affecting both the anterior and
posterior ends. Of 18 genes affecting dorso-ventral polarity, several are required
for the polarity of both the eggshell and embryo, and several are required for
the polarity of the embryo only (
Wilkins 1993
).
Determining the dorsal-ventral and anterior-posterior polarity in the embryo
is a highly significant step. The
Toll
+
gene is a dorsalizing gene, and mutations
produce embryos lacking both ventral and lateral structures. The
Toll
+
gene
product appears be a
morphogen
(molecules whose local concentration directly
determine
the local pattern of differentiation), because when the wild-type
gene product is injected into mutant embryos it can make the affected region
become the dorsal region of the fly.
A major determinant of anterior-posterior polarity is the product of the
bicoid
+
gene. It is transcribed in the nurse cells of the ovary, and the mRNA
passes into the oocyte where it becomes localized in the anterior of the egg,
apparently aided by components of the cytoskeleton that are encoded by prod-
ucts of the genes
swallow
+
and
exuperentia
+
.
