Axis Formation and Germline Determination in Drosophila 
Ho does not interfere with nanos RNA localiza- 
tion or NANOS protein distribution, synthesis, 
and stability. We conclude that pumilio is re- 
quired for optimal NANOS activity in regions of 
the embryo where NANOS protein concentrations 
are low. 
We have cloned the pumilio gene and have 
shown that pumilio mRNA is localized to the pos- 
terior pole in early embryos. This observation is 
consistent with a possible interaction between 
the pumilio and nanos gene products. Localiza- 
tion of pumilio mRNA, like that of nanos mRNA, 
is disrupted in mutants that lack the specialized 
posterior pole plasm (see below) . It is thus possi- 
ble that the same mechanism of RNA localization 
acts on both mRNAs. 
Assembly of the Pole Plasm 
The remaining seven genes in the posterior 
group are associated with pole plasm defects. We 
are studying these genes from two perspectives. 
One is their role in pattern formation and body 
segmentation (see above) ; the other, their role in 
pole plasm assembly and germ cell formation 
(see below) . 
During the first hour of embryogenesis, the 
pole plasm appears as a distinct clear zone at the 
posterior end of the newly fertilized egg. Closer 
examination reveals that this zone contains spe- 
cialized cytoplasm packed with mitochondria 
and numerous donut-shaped organelles, called 
polar granules, that do not occur anywhere else 
in the embryo. The zone of specialized cytoplasm 
coincides with the site of germ cell formation, 
and polar granules may therefore contain factors 
that control germ cell fate. Polar granules were 
first recognized in Drosophila embryos 30 years 
ago, but until recently little was known about 
their biochemical structure or function. Now the 
link between defects in abdomen formation and 
the absence of germ cells has provided a new ave- 
nue for research. 
We have begun extensive studies of the pole 
plasm in mutant and normal embryos, using 
probes designed to recognize mRNA and proteins 
derived from the posterior group genes. We sus- 
pect that oskar, one of the genes required for 
abdominal and germline development in the Dro- 
sophila embryo, is an essential component of the 
pole plasm. We have isolated this gene. In situ 
hybridization to whole-mount embryos reveals 
that oskar mRNA distribution is restricted to the 
posterior pole of early embryos. Genetic studies 
suggest that pole plasm formation occurs in a se- 
ries of steps, with each step dependent on the 
previous one, and that oskar is required early in 
the assembly pathway. Future experiments will 
address questions about the role of oskar mRNA 
and protein for pole plasm formation and for lo- 
calization of the abdominal signal. 
We are optimistic that new information about 
the assembly and composition of the pole plasm 
and polar granules will lead to better understand- 
ing of their functions in the early embryo. Their 
primary role may be to protect or sequester infor- 
mation required by the future germ cells. In addi- 
tion, however, they could provide a convenient 
anchor for substances like the nanos mRNA that 
must be retained in the posterior region. Struc- 
tures similar to the polar granules in Drosophila 
have been observed in association with germ 
plasm in many different invertebrate and verte- 
brate animals. Thus the Drosophila model could 
reveal basic concepts underlying the establish- 
ment of germline tissues in all species. 
The National Institutes of Health provided fi- 
nancial support for two projects that are summa- 
rized here. These projects focus on the molecular 
and functional characterization of the nanos and 
oskar genes. 
Localization of positional signals in the Dro- 
sophila embryo. In situ hybridization experi- 
ment, using biotinylated cDNA probes, local- 
izes RNA of the anterior signal bicoid and of the 
posterior signal nanos respectively to the ante- 
rior and posterior poles of an early ( stage 2 ) 
embryo. Hybridization to nanos at the posterior 
also demarcates the position of the germ plasm. 
Research by Laura Dickinson. Photograph by 
Ruth Lehmann. 
254 
