Sex Determination in Insects - Insect Molecular Genetics - page 420

(producing the male gene product, DSX M ), it represses female differentiation.
When doublesex + is active in females (producing DSX F ), and the intersex + gene
product is present, male development is suppressed. If dsx + is inactivated, both
male and female genes are active within a cell, which results in an intersexual
phenotype at the cellular level.
The determination of sex during embryogenesis in D. melanogaster is trans-
mitted through a hierarchy of regulatory genes to the terminal differentiation
genes, whose products are responsible for the sexually dimorphic traits of the
adult fly ( Bownes 1992 ). The different activities of the regulatory genes in males
and females are largely due to sex-specific differences in RNA splicing that lead
to the production of functionally different transcripts in the two sexes ( Baker
1989 ). The individual genes in this regulatory hierarchy are not only themselves
controlled at the level of RNA splicing but, in turn, specify the splicing pattern
of the transcripts of genes down stream in the hierarchy, producing a cascade of
RNA splicing reactions. Thus, RNA processing, involving alternative splicing, is a
crucial regulatory mechanism in this developmental pathway.
In addition to the sex-determination genes, there are genes whose products
are responsible for the structure and function of sexually dimorphic somatic tis-
sues ( Kopp et al. 2000 ). The sex-determination regulatory pathway regulates
many structural genes, such as the yolk polypeptide genes, which are expressed
in the fat body in a female-specific manner.
10.5.3 Germ-Line Determination
Sex determination in the development of germ-line tissues in D. melanogaster is
different from that in the soma ( Pauli and Mahowald 1990, Janzer and Steinmann-
Zwicky 2001, Vincent et al. 2001 ). Pole cells in the embryo are segregated into the
posterior pole of the insect embryo before cellular blastoderm, and they include
the progenitors (stem cells) of the germ cells ( Xie and Spradling 2000 ).
Components of the germ plasm ( = pole plasm) are synthesized in the mother
during oogenesis by a cluster of 15 nurse cells, which are connected to the
oocyte at its anterior by cytoplasmic bridges. Pole plasm components are trans-
ported into the oocyte and translocated to the posterior pole of the egg.
Maternally active genes important in the production of pole cells include cap-
puccino + , spire + , staufen + , oskar + , vasa + , valois + , mago nashi + , and tudor +
( Ephrussi and Lehmann 1992 ). These genes also are important in the formation
of normal abdomens in Drosophila .
During embryogenesis, prospective male- and female-germ cells are indistin-
guishable, but differentiation begins during the larval stage, when male gonads