mRNA are being sought to confirm this hypothesis. 
Recently a number of genes in the sex-determina- 
tion pathway in Drosophila have been implicated 
in the process of alternate splicing. The exu mRNAs 
may represent the first example of genes down- 
stream of the sex-determining genes that are a tar- 
get for alternative splicing. 
The different sex-specific transcripts from exu are 
particularly interesting, given the finding that exu 
mutations affect spermatogenesis as well as oogen- 
esis. Six of seven exu mutations cause male sterility. 
Abnormalities are seen in meiosis I of spermato- 
cytes, and subsequent stages of sperm differentia- 
tion are disrupted. A detailed analysis of the precise 
defect in spermatogenesis is in progress. 
The sequences of the cDNAs should help differ- 
entiate among a number of models of exu function. 
It is possible that different products are generated 
from the different mRNAs. Alternatively, the mRNAs 
may encode the same product in the two sexes. 
Even if the same product is made, it is not known 
whether it serves a similar function in the two 
sexes. One possibility is that the exu product local- 
izes components within both spermatocytes and 
oocytes. 
II. Genomic Position Effects That Alter the Expres- 
sion of Adjacent Genes. 
Expression of the Drosophila white gene is re- 
quired for deposition of the red and brown pig- 
ments that produce the wild-type eye color of the 
adult. Usually the white gene is expressed in all re- 
gions of the eye, but in one case in which a P ele- 
ment carrying the white gene was inserted into the 
genome by transformation, the white gene shows a 
striking pattern of expression. In animals with this 
D-V insertion, white expression is limited to the 
ventral half of the eye. The D-V white gene contains 
all of the normal cis-acting regulatory sequences, 
since it is expressed throughout the eye when 
transposed to a new genomic location. Thus the 
D-V white expression appears to be repressed by 
flanking DNA in the dorsal half of the eye. 
About 40 kb of DNA flanking the D-V element 
was cloned. Molecular analysis of five of six x-ray- 
induced revertant strains (where white is expressed 
throughout the eye) revealed rearrangements in ei- 
ther the 3' or 5' DNA flanking the D-V insertion. 
These data suggest that the repression exerted by 
the flanking DNA is not due to a single, simple en- 
hancer sequence. 
Under certain conditions the repressive action of 
the flanking DNA can extend into the ventral region 
of the eye. This extension occurs with a particular 
x-ray-induced derivative, D-V^^^, in which the eye 
has only a small quadrant of pigmented tissue. The 
D-V^*^ mutation has recently been shown to delete 
~3.5 kb of DNA overlapping the insertion site at 
the 5' end of the D-V insertion. The same effect is 
seen when an allele of the zeste locus, z^, is present 
in D-V flies. The product oiz^ can bind to the white 
promoter and has been hypothesized to act by 
looping DNA so as to bring together distant reg- 
ulatory sites. Since the D-V^'*^ mutation removes 
DNA, the more severe phenotype may result from a 
similar juxtaposition of distant elements. This hy- 
pothesis predicts that there should be zeste-bind- 
ing sites in the DNA flanking D-V and that a tran- 
script from the region in wild-type flies should also 
be expressed differentially in the eye. Current ef- 
forts are directed toward confirming these predic- 
tions. 
Dr. Hazelrigg is also Assistant Professor of Biology 
at the University of Utah. 
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