Genetic Control of Pattern Formation and Germline Determination 
in Drosophila 
and the trans-acting factors required for nanos 
RNA localization. 
We are just beginning to understand the pro- 
cess through which the nanos protein affects the 
development of abdominal structures. The gene 
appears to inhibit the activity of another mater- 
nally derived protein, encoded by the hunch- 
back gene. Maternally derived hunchback pro- 
tein seems to act as a repressor molecule; it 
blocks the transcription of genes within the em- 
bryo. The nanos gene acts negatively on the 
mRNA stability and/or translation of the mater- 
nally provided product of hunchback, such that 
the maternal hunchback transcript and protein 
product are absent from the posterior of the em- 
bryo. Thus, by interfering with hunchback activ- 
ity, nanos indirectly promotes activation of the 
first tier of embryonic genes required for abdo- 
men formation. 
Distribution of the Posterior Signal 
The pumilio gene is required for the distribu- 
tion of the abdominal signal, encoded by the 
na«05 gene, during early embryogenesis. It there- 
fore provides an excellent opportunity to study 
the redistribution of localized cytoplasmic deter- 
minants. Initial cytoplasmic transplantation ex- 
periments indicated that in the absence of the 
maternally provided pumilio gene product, the 
abdominal signal nanos is properly localized to 
the posterior pole but does not reach its target, 
the prospective abdominal region. In situ hybrid- 
ization studies have demonstrated that nanos 
mRNA is localized to the posterior pole plasm in 
wild-type embryos and that its distribution is un- 
affected by a pumilio mutation. This suggests that 
the pumilio gene affects the distribution of the 
nanos protein. 
We have cloned the pumilio gene and have 
shown that it encodes several transcripts that are 
present throughout development. In early em- 
bryos, pumilio transcripts are localized to the 
posterior pole. This observation is consistent 
with the possible interaction between pumilio 
and nanos gene products. The localization of the 
transcript is disrupted by the same set of mutants 
that disrupt nanos localization, and it is possible 
that the same mechanism of RNA localization acts 
on both RNAs. 
Assembly of the Pole Plasm 
The remaining seven genes in the posterior 
group are associated with pole plasm defects. We 
are interested in their role in pattern formation 
and thus in nanos RNA localization. We are also 
very excited about the potential for learning 
more about the origin of the germline. 
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 the zone actually con- 
tains specialized cytoplasm packed with mito- 
chondria and numerous donut-shaped organelles 
that do not occur anywhere else in the embryo. 
These organelles, called polar granules, must be 
present for the formation of pole cells, the pre- 
cursors of the germ cells. 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 pole cells has provided a new ave- 
nue for research. 
Our hypothesis is that the seven remaining pos- 
terior group genes do not have a direct role in 
transmission of the abdominal signal. Instead, 
they provide the structural framework that local- 
izes nanos mRNA in the pole plasm. This frame- 
work may be the polar granule or components 
thereof. 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 are optimistic that new information about 
the assembly and composition of the pole plasm 
and polar granules will lead to greater under- 
standing of their various functions in the early 
embryo. Structures similar to the polar granules 
in Drosophila have been observed in association 
with germ plasm in many different invertebrate 
and vertebrate animals. Thus the Drosophila 
model could reveal basic concepts underlying 
the establishment of germline tissues in all 
species. 
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