Biology Reference
In-Depth Information
Figure 10.2
The general features of somatic sex determination in
D. melanogaster
. The ratio of X
chromosomes to autosomes (A) determines whether
Sxl
+
is ON.
Sxl
+
produces a protein, SXL, that
acts as a splicing factor on the RNA produced by the
tra
+
gene, resulting in the production of active
TRA protein. TRA, together with the product of the
tra-2
+
gene, determine the female-specific splic-
ing of the
dsx
+
and
fruitless
+
RNAs, which results in a cascade of genes functioning to produce a
female. If
Sxl
+
is OFF, the individual becomes a male because male-specific products (DSX
M
and
FRU
M
) of the
dsx
+
and
fru
+
genes are produced.
Sex-lethal
+
is a key switch gene that, very early in development, affects both
somatic sexual differentiation and dosage compensation in
D. melanogaster
(
Figure 10.2
).
Sex lethal
+
codes for an RNA-splicing enzyme. Its action on the next
gene in the cascade,
transformer
+
, is restricted to females in its role in sexual dif-
ferentiation.
Sex-lethal
+
must be ON in females and OFF in males (
Figure 10.2
).
Once the X:A ratio is read and the
Sxl
+
gene is turned ON or OFF early in embryonic
development, the developmental path chosen is stable (
Cline and Meyer 1996
).
Sxl
+
is transcribed in
D. melanogaster
females in a complex manner. Two dif-
ferent promoters function in somatic cells; one, the establishment promoter,
Sxl
Pe
, acts very early and only for a brief period during nuclear cell cycle 12 to
