(050509 revised, 050509 created)
Laboratory of Molecular Entomology and Baculovirology, The
Institute of Physical and Chemical Research (RIKEN) 2-1Hirosawa, Wako, Saitama 351-0198, Japan.
The sex determination pathway is different between
Drosophila melanogaster and Bombyx mori in the initial
signal. Here we show evidence that the sex determination
pathway in B. mori is similar to that of D. melanogaster
at the level of the terminal regulator, doublesex (dsx),
which is essential for the proper differentiation of the
sexually dimorphic somatic features of D. melanogaster. In
B. mori, a homolog of dsx (Bmdsx) is expressed in various
tissues, and its primary transcript is alternatively
spliced in males and females to yield sex-specific mRNAs
that encode male-specific (BmDSXM) and female-specific
(BmDSXF) polypeptides. In the studies reported here,
transgenic silkworms carrying a construct with a Bmdsx
male cDNA placed under the control of either an hsp70
promoter or a Bombyx actin3 promoter were generated by
piggyBac-mediated germline transformation. Ectopic
expression of the male cDNA in females resulted in
abnormal differentiation of certain female-specific
genital organs and caused partial male differentiation in
female genitalia. Transgenic analysis also revealed that
the expression of BmDSXM in females caused repression of
the female-specifically expressed gene, the vitellogenin
gene, and also resulted in activation of the
pheromone-binding protein gene that is dominantly
expressed in males. These results provide evidence that
the role of BmDSXM includes the activation of some aspects
of male differentiation as well as the repression of
female differentiation. Taken together with our previous
data on the function of BmDSXF, we can conclude that Bmdsx
is a double-switch gene at the final step in the
sex-determination cascade of B. mori.