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X chromosome in males (
Figure 10.1
). The resulting histone H4 acetylation leads to
hypertranscription (
Marin et al. 2000, Smith et al. 2000, Lucchesi 2009, Laverty et al.
2010
). By contrast, the SXL protein in females prevents removal of a female-specific
intron in the
msl-2
+
mRNA; without this MSL-2 protein the other MSL proteins fail
to assemble on the X chromosome and hyperactivation is prevented.
10.5.2 Somatic-Sex Determination
The relative number of X chromosomes and autosomes (A) in
D. melano-
gaster
was considered to be responsible for the primary step in sex deter-
mination immediately after fertilization. It was thought that cells with two
X chromosomes and two sets of autosomes (2X:2A, or a ratio of 1.0) develop
into females, while diploid cells (with 1X:2A, or a ratio of 0.5) develop into
males (
Figure 10.2
). Flies with equal numbers of X chromosomes and autosomes
(XX:AA, XXX:AAA, or X:A, or a ratio of 1.0) develop as females. Flies with an
intermediate X:A ratio (XX:AAA) develop as intersexual flies that appear to
be mosaics of discrete patches of male or female tissues. However,
Erickson
and Quintero (2007)
suggest that X chromosome dose, not the X:A ratio, sig-
nals which sex will develop in
Drosophila
. Distinct boundaries between cells
exist in
Drosophila
and sex determination takes place in individual cells (
cell-
autonomous sex determination
). Cell-autonomous determination of sex has
been assumed to be due to the lack of
sex hormones
in insects. However,
DeLoof and Huybrechts (1998)
note that hormones are involved in the develop-
ment of secondary sexual characters in some insects.
The Y chromosome does not determine sex in
D. melanogaster
, although it
is required for normal spermatogenesis and fertility. At least six genes on the
Y chromosome are important in fertility, each performing a single, unique function
(
Hennig 1993, Hochstenbach et al. 1994
). The fertility genes are
>
1000 kb each and
are highly susceptible to mutations (
Hackstein and Hochstenbach 1995
). The Y chro-
mosome is important in sex determination in some insects (
Marin and Baker 1998
).
Numerator genes
communicate the relative number or ratio of X chromo-
somes and autosomes in
D. melanogaster
. Numerator genes in
D. melanogas-
ter
include
sisterless-A
+
(
sis-A
),
sisterless-B
+
(
sis-B
),
sisterless C
+
(
SisC
), and
runt
+
(
Cline and Meyer 1996
). Once the X:A ratio is assessed, the activities of a rela-
tively small number of major regulatory genes are triggered that ultimately lead
to male or female differentiation in the soma (
Figure 10.2
). Somatic sexual dif-
ferentiation is regulated through a cascade of sex-specific events in which RNA
transcripts are differentially spliced in males and females (
Figure 10.2
,
Table
10.1
). Note that in
Figure 10.2
gene products (proteins) are capitalized (SXL),
whereas the gene (
Sxl
+
) is italicized.
