Biology Reference
In-Depth Information
Bownes (1992)
used their model to compare the sex-determination system in
the house fly (
Figure 10.4
). According to the
Nothiger and Steinmann-Zwicky
(1985)
model, the male-determining factor (
M
) in
M. domestica
would corre-
spond to the repressor (R) of
Sxl
+
. The genes
tra
+
and
Ag
+
may be equivalent to
da
+
in
Drosophila
. The
F
gene of
M. domestica
could be equivalent to
Sxl
+
.
Sxl
+
is involved in dosage compensation in
Drosophila
, but dosage compensation is
not needed in species such as
M. domestica
with heterochromatic sex chromo-
somes (which usually contain few coding regions) or no sex chromosomes. As a
result, insects with heterochromatic sex chromosomes or no sex chromosomes
can survive mutations of
tra
+
,
Ag
+
, and
F
; such mutations can alter sex determi-
nation, but are not lethal to one sex.
Different genera of mosquitoes have several different sex-determination
systems, but these systems still may conform to the Nothiger and Steinmann-
Zwicky model (
Bownes 1992
).
Anopheles gambiae
and
Anopheles culicifacies
have XY males and XX females. Sex in
Aedes
is determined by a dominant male-
determining factor. Intersex flies with phenotypes similar to the
ix
,
dsx
, and
tra
mutants of
Drosophila
have been found in
Aedes aegypti
and
Culex pipiens
.
A single gene located on an autosome determines sex in
Culex
;
Culex
gynan-
dromorphs have been found, suggesting that sex determination is cell autono-
mous, as it is in
Drosophila
. The sex of some northern strains of
Aedes
depends
upon the temperature at which they are reared, with males transformed into
intersexes at higher temperatures. This suggests that an allele equivalent to
ix
+
is temperature sensitive in these populations. In
Cx. pipiens
, a sex-linked gene
cercus
(
c
) changes females into intersexes; these intersexes are sterile and fail
to take blood meals.
Bownes (1992)
speculated that it is possible that
cercus
+
is
similar to
tra
+
,
ix
+
, or
dsx
+
of
Drosophila
.
Sexual differentiation among different organisms (flies, nematodes, and
mammals) has superficially similar patterns of hierarchical control (for reviews,
see
Marin and Baker 1998, McAllister and McVean 2000
). Comparative genetic
analysis suggests that the functions of
tra
+
,
tra-2
+
, and
dsx
+
may be conserved
throughout higher eukaryotes (
Verhulst et al. 2010a
). Sex-determining mecha-
nisms are, however, variable and the function of
Sxl
+
is not conserved among
all arthropods.
Verhulst et al. (2010a)
evaluated what is known about insect sex
determination and conclude
“The primary signal that starts sex determination is processed by a cascade
of genes ending with the conserved switch doublesex that controls sexual
differentiation. Transformer is the doublesex splicing regulator and has
been found in all examined insects, indicating its ancestral function as a sex-
determining gene.”
