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PATTERN FORMATION AND GERM CELL DETERMINATION 
IN THE DROSOPHILA EMBRYO 
Ruth Lehmann, Ph.D., Assistant Investigator 
Essential cues for the establishment of the em- 
bryo's body pattern are provided to the egg cell dur- 
ing its maturation in the mother. Classical experi- 
ments that involved direct manipulation of early 
embryos indicated that some maternal information 
is localized within the egg cell. To understand how 
gene products can be spatially restricted within a 
single cell, Dr. Lehmann and her colleagues have 
focused on a specialized cytoplasmic region that 
forms during oogenesis at the posterior pole of the 
Drosophila oocyte. Nuclei that migrate into this cy- 
toplasmic region during early embryogenesis be- 
come determined to the germ cell fate. 
In addition to its role in germ cell determination, 
the posterior pole plasm also harbors a morphoge- 
netic signal required at a distance for the normal 
development of the embryonic abdomen. Removal 
of this signal either by experimental manipulation 
or by mutation results in the lack of abdominal seg- 
ments in the embryo. At least 10 maternal-effect 
genes (nanos, pumilio, oskar, staufen, cappu- 
ccino, spire, mago nashi, vasa, valois, and tudor) 
are involved either in the assembly of the pole plasm 
or in the synthesis, activity, and localization of the 
abdomen-specifying signal. All 10 maternal-effect 
genes share the abdominal phenotype, and all genes 
except nanos and pumilio affect germ cell 
formation. 
Abdomen Formation and Mirror Images 
The abdomen-specifying signal, encoded by the 
nanos gene, is localized in the posterior pole 
plasm. Charlotte Wang and Laura Dickinson showed 
that nanos RNA is synthesized during oogenesis and 
is localized to the posterior pole of the freshly laid 
egg. It is not localized to the posterior pole in those 
eight mutants that lack pole plasm. The abdominal 
segmentation defect of these mutants can thus be 
attributed to a lack of localized nanos activity. 
The Nanos protein is distributed in a posterior-to- 
anterior concentration gradient. This protein also 
inhibits the activity of another maternally derived 
protein, encoded by the hunchback gene. The 
Hunchback protein is a repressor molecule that 
blocks transcription of genes within the embryo. By 
establishing a concentration gradient of Hunchback 
protein complementary to that of Nanos, «a«05 indi- 
rectly promotes activation of the first tier of embry- 
onic genes required for abdomen formation. 
To test whether a localized source of the Nanos 
protein is sufficient to change polarity and pattern 
of the embryo. Dr. Elisabeth Gavis used a mix-and- 
match strategy to express this protein at a new posi- 
tion in the embryo. First, Dr. Gavis showed that the 
3'-untranslated region (3'-UTR) of nanos RNA con- 
tains an RNA localization signal and that this signal is 
sufficient to trigger localization of a reporter RNA to 
the posterior pole of the embryo. Then she removed 
the 3'-UTR of nanos and replaced it with the 3'-UTR 
from the bicoid gene, which is responsible for pat- 
tern formation in the head region of the Drosophila 
embryo. (Previously Dr. Paul Macdonald and Dr. 
Gary Struhl [HHMI, Columbia University] had shown 
that the 3'-UTR of bicoid RNA contains sequences 
necessary for localization of bicoid RNA to the 
anterior pole.) Finally, Dr. Gavis transferred the al- 
tered gene into flies and observed the effects on 
embryogenesis. 
All females carrying the altered gene produce 
headless embryos with two abdomens in mirror 
CELL BIOLOGY AND REGULATION 89 
