Biology Reference
In-Depth Information
Figure 10.5
Is there a general model for sex determination in insects? This model assumes that acti-
vators are produced by the mother that activate an
Sxl
+
-like gene in the zygote. Other activators may
be produced by the zygote, and the combination results in a functional
Sxl
+
-like product in females.
Male zygotes produce a repressor of the
Sxl
+
-like gene and no functional product is produced in
males. Next, a
dsx
+
-like gene is turned on in both females and males but different products are pro-
duced in the two sexes. The different
dsx
+
gene products turn on a subsequent series of genes that
result in the differentiation of either males or females. Current data suggest the lower level of the
cascade is more conserved. (Redrawn from
Bownes 1992
.)
Recent studies indicate that this model is partly correct, especially at the lower
levels of the hierarchy.
Sxl
+
is apparently not as conserved as genes at the lower
level of the hierarchy:
tra
+
,
tra-2
+
,
dsx
+
, and
intersex
+
are more conserved among
the insects studied so far (
Siegal and Baker 2005
).
Wilkins (1995)
proposed the
hypothesis that sex-determination pathways have evolved from the bottom up:
the later a gene acts in the pathway, the more likely it will be conserved. Thus,
a new genetic function inserted into the top of the pathway would be initiated
by natural selection to maintain an optimal sex ratio. If the sex ratio deviates too
much from a 1:1 ratio, there will be selective pressure to shift the ratio toward the
less-frequent sex and this new gene at the top of the pathway would then affect
all downstream genes. The
doublesex
+
gene is functionally conserved in Diptera
and Lepidoptera, and
tra
+
appears to be at the top of the sex-determination hier-
archy in the Mediterranean fruit fly and the honey bee (
Siegal and Baker 2005
).
