Morphogen Gradients and the Control of Body Pattern in Drosophila 
early embryo. The second source is maternal hb 
mRNA synthesized during oogenesis and depos- 
ited uniformly throughout the egg. These tran- 
scripts are translated only after fertilization, and 
their translation is repressed in more-posterior 
portions of the egg, owing to nos activity emanat- 
ing from the posterior pole. 
We recently found that the ability of nos to 
repress hb translation depends on the presence of 
two copies of a short cis-acting regulatory se- 
quence in the 3' noncoding portion of the hb 
mRNA. As in the case of the DNA sites mediating 
the binding and regulation of hb by bed protein, 
the number and quality of the target sites in hb 
mRNA can determine the pattern of expression of 
hb protein. 
However, in contrast to bed, which has many 
other signaling roles, nos appears to have no role 
other than to block posterior expression of hb 
protein. Indeed, we have been able to show that 
nos activity is completely dispensable if the 
translation of maternally derived hb mRNAs is 
blocked by other means (such as mutation) . Thus 
the control of posterior body pattern may depend 
solely on the repression of maternal hb tran- 
scripts by nos. 
Posterior Body Pattern Is Controlled 
by hb Protein Acting 
as a Classical Gradient Morphogen 
The bed and nos determinants have opposite 
effects on hb protein expression. The bed gene 
triggers the synthesis of hb transcripts, and hence 
protein, anteriorly; nos blocks the translation of 
both maternal and zygotic hb transcripts posteri- 
orly. Together the actions of bed and nos gener- 
ate a graded pattern of hb protein in which the 
concentration declines from uniformly high lev- 
els in the anterior half of the body to undetect- 
able levels in the posterior half. 
As described above, our analysis of the interac- 
tion between nos and hb indicates that nos itself 
can be rendered dispensable, provided that ma- 
ternal hb transcripts can be inactivated by other 
means. In these unusual embryos, hb protein is 
still expressed differentially along the anteropos- 
terior axis, owing to zygotic activation of the hb 
gene by bed. This finding, taken together with the 
critical role nos normally plays in repressing the 
translation of hb mRNAs, suggests that the distri- 
bution of hb protein may be the critical determi- 
nant of posterior body pattern. 
We tested this possibility by generating em- 
bryos in which the differential expression of hb 
protein along the anteroposterior axis has been 
systematically altered while all other known sig- 
naling systems are eliminated or held constant. By 
examining the expression of the subordinate reg- 
ulatory genes Kriippel, knirps, and giant, each of 
which is normally responsible for controlling a 
particular subdomain of thoracic and abdominal 
pattern, we have been able to show that the gra- 
dient of hb protein provides a series of distinct 
concentration thresholds that govern where these 
genes are expressed. 
Thus hb appears to control posterior body pat- 
tern by acting as a classical gradient morphogen. 
In this regard, it functions as the posterior coun- 
terpart to the anterior morphogen bed. 
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