The Questions of Developmental Biology

(Sxl) gene,* while the autosomes produce transcription
factors that compete for these activators. When the X-toautosome
ratio is 1 (i.e., when there are two X
chromosomes per diploid cell), the Sxl gene is active,
and the embryo develops into a female fly. When the
ratio is 0.5 (i.e., when the fly is XY with only one X
chromosome per diploid cell), the Sxl gene is not active,
and the embryo develops into a male (Figure 5.30).
But what is the Sxl protein doing to determine
sex? It is acting as a differential splicing factor on the
nRNA of the transformer (tra) gene. Throughout the
larval period, the tra gene actively synthesizes a
transcript that is processed into either a general mRNA
(found in both females and males) or a female-specific
mRNA. The female-specific message is made when Sxl
binds to the nRNA and inhibits spliceosome formation
on the general 3´ splice site of the first intron
(Sosnowski et al. 1989; Valcárcel et al. 1993).
Instead, the spliceosome forms on another, less efficient
3´ site and allows splicing to occur there. As a result, the female form of the transformer mRNA
lacks a portion of RNA that is found in the general form. And that is a crucial difference, for this
portion contains a translational stop codon (UGA) that causes the message to make a small,
nonfunctional, protein. Therefore, the general transcript has no bearing on sex determination
(Belote et al. 1989). However, in the female-specific message, the UGA codon is spliced out
during mRNA formation and does not interfere with the translation of the message. In other
words, the female transformer transcript is the only functional transcript of this gene.
The Transformer protein is, itself, an alternative splicing factor that regulates the splicing
of the nuclear transcript of the doublesex (dsx) gene. This gene is needed for the production of
either sexual phenotype, and mutations of dsx can reverse the expected sexual phenotype, causing
XX embryos to become males or XY embryos to become females. During pupation, the
doublesex gene makes a transcript that can be processed in two alternative ways. It can generate a
female-specific mRNA or a male-specific mRNA (Nagoshi et al. 1988). In females and males, the
first three exons of the doublesex mRNA are the same (Tian and Maniatis 1992, 1993). But if the
Transformer protein is present, it converts a weak 3´ splicing site into a strong site (a more
efficient binder of U2AF), and exon 4 is retained, resulting in female-specific doublesex mRNA.
If Transformer protein is not present, U2AF will not bind to this 3´ site, and exon 4 will not be
included, resulting in the male-specific doublesex message. The Doublesex proteins made by the
male and female mRNAs are both transcription factors, and they recognize the same sequence of
DNA. However, while the female Doublesex protein activates female-specific enhancers (such as
those on the genes encoding yolk proteins), the male Doublesex protein inhibits transcription
from those same enhancers (Coschigano and Wensink 1993; Jursnich and Burtis 1993).
Conversely, the female Doublesex protein can inhibit transcription from genes that are otherwise
activated by the male Doublesex protein. Research into Drosophila sex determination shows that
differential RNA processing plays enormously important roles throughout development.
*The name of this gene, Sex-lethal, comes from the deadly decoupling of the dosage compensation mechanism that
arises when this gene is mutated. When this happens, the fly will become male, even if it is XX. However, its two X
chromosomes will be instructed to transcribe their genes at the higher (male) rate. This will create regulatory defects
that will kill the embryo (Cline 1986).