Biology Reference
In-Depth Information
Table 10.1: Some Genes Involved in Somatic Sex Determination and Dosage Compensation in
D. melanogaster.
Gene
Function
Maternal genes
daughterless
+
Necessary for numerator genes to act appropriately;
da
+
and
her
+
activate
Sxl
+
in
hermaphrodite
+
female embryos.
emc
+
and
gro
+
negatively regulate
Sxl
+
in female embryos.
extramachrochaetae
+
groucho
+
Numerator genes
sisterless-A
+
Communicate X-chromosome dose in dosage compensation. X-linked genes
sisterless-B
+
involved in activating
Sxl
+
in females; they “count” the number of
sisterless-C
+
X chromosomes and turn on
Sex-lethal
+
.
runt
+
Zygotic genes
Sex-lethal
+
Major control gene; produces a full-length protein in females; no protein produced
in males.
transformer
+
Active, with
tra-2
+
; in regulating
dsx
+
in females.
transformer-2
+
Active in females to induce female-specific
dsx
+
expression and repress male-
speciic
dsx
+
expression. Needed for spermatogenesis.
doublesex
+
Active in males to repress female differentiation; in females
dsx
+
represses male
differentiation; loss of function mutants result in intersexes in both males and
females; a pivotal terminal-differentiation switch.
intersex
+
Active in females with
dsx
+
product to repress male differentiation, not needed
in males.
fruitless
+
Gene product is necessary in nervous system of males to elicit normal mating
behavior and development of male muscle (muscle of Lawrence).
male-specific lethal
+
All four genes regulate X chromosome transcription in males;
msl-2
+
has no
msl-1
+
function in females. Absence of the MSL-2 protein in females prevents formation
msl-2
+
of the compensasome.
msl-3
+
(Derived from
Bownes 1992, Cline and Meyer 1996, Li and Baker 1998, Marin and Baker 1998
.)
involves three major components: dosage compensation, somatic-cell differentia-
tion, and germ-line differentiation. More is known about dosage compensation
and somatic-cell differentiation than about germ-line differentiation.
10.5.1 Dosage Compensation of X Chromosomes
A basic aspect of sex determination in insects with an XY sex-determining system
is dosage compensation of the X chromosomes because Y-chromosomes typically
have few genes and consist primarily of heterochromatin (
Baker et al. 1994
). The
mechanism of dosage compensation varies in arthropods.
Dosage compensation
equalizes the amount of gene products produced by individuals with an XX/XY
