Axis Formation and Germline Determination 
in Drosophila 
Ruth Lehntann, Ph.D. — Assistant Investigator 
Dr. Lehmann is also Associate Member of the Whitehead Institute of Biomedical Research, Associate 
Professor of Biology at the Massachusetts Institute of Technology, and Assistant Molecular Biologist at 
Massachusetts General Hospital, Boston. She received her M. Sc. degree from the University of Freiburg, 
where she worked with Jose Campos-Ortega on early neurogenesis in Drosophila. She received her Ph.D. 
degree from the University of Tubingen, where she worked with Christiane Ntisslein-Volbard at the Max 
Planck Institute for Developmental Biology on the genetics of pattern formation in the Drosophila embryo. 
After postdoctoral training in Tiibtngen and at the MRC in Cambridge with Michael Wilcox, she joined 
the Whitehead Institute. 
HOW does a developing embryo know where 
to form a head and where to put a tail? In 
Drosophila, basic information about the "coordi- 
nates" of the embryo is supplied to the egg cell 
by the mother. Mutations in maternal genes have 
led to the identification of a small number of 
genes that are required for the establishment of 
anterior-posterior (head-to-tail) polarity. In these 
mutants the lack of a particular gene product in 
the mother is lethal to its progeny. 
Three signals are required for the establish- 
ment of pattern along the anterior-posterior axis. 
The anterior signal controls the development of 
head and thorax, the posterior signal determines 
the abdominal region, and the terminal signal is 
required at both ends of the embryo. The anterior 
and posterior signals are localized to the respec- 
tive poles (see figure) . 
We have concentrated on the dissection of the 
pathway leading to normal development of the 
posterior region. Nine maternal genes, called the 
posterior group, set the basic posterior pattern. 
These genes share the abdominal phenotype: ho- 
mozygous mutant females produce offspring that 
lack abdominal segmentation. One gene, nanos, 
encodes the localized signal required for the de- 
velopment of the abdomen. Another gene, pumi- 
lio, regulates the activity of NANOS protein, 
while the remaining seven genes — oskar, vasa, 
tudor, valois, staufen, cappuccino, and spire — 
are required for the localization of nanos RNA to 
the posterior pole. 
These seven genes are also required for germ 
cell formation. Embryos from mutant females 
lack the specialized posterior pole plasm that 
normally contains the polar granules. These em- 
bryos therefore fail to produce pole cells, the 
Drosophila germ cell precursors. Our molecular 
analysis of the posterior group is aimed at under- 
standing how nanos and other RNA species be- 
come localized to the posterior pole and how the 
pole plasm, composed of RNA and protein, is 
assembled. 
Synthesis and Function 
of the Abdominal Signal 
Through genetic experiments as well as cyto- 
plasmic transfers between wild-type and mutant 
embryos, we concluded that the specialized cyto- 
plasm at the posterior pole is the source of an 
abdominal signal, and that embryos derived from 
females with mutant posterior group genes lack 
this signal in the abdominal region. The signal is 
encoded by nanos. We have isolated and cloned 
this gene and have shown that nanos mRNA is 
transcribed during oogenesis and becomes local- 
ized to the posterior pole plasm of the mature 
oocyte. The role of nanos as a signal for posterior 
pattern formation is demonstrated by the finding 
that injection of synthetic nanos mRNA into the 
anterior pole of early embryos leads to suppres- 
sion of head formation and induction of a second 
abdomen in mirror image to the normal "poste- 
rior" abdomen. 
Seven of the posterior group genes affect the 
localization of nanos mRNA to the posterior 
pole. Since these seven mutants also lack pole 
plasm, we can conclude that their abdominal seg- 
mentation defect is a consequence of a lack of 
localized nanos mRNA. We have identified se- 
quences within the nanos mRNA required for its 
localization. In future experiments we will deter- 
mine which of the posterior group genes are di- 
rectly involved in tethering nanos mRNA to the 
posterior pole plasm. 
Regulation of Posterior Activity 
Several lines of evidence indicate that the pu- 
milio gene is required to regulate the activity of 
NANOS protein. After fertilization of the egg, 
NANOS is translated from the localized mRNA and 
emanates anteriorly toward the abdominal region 
to form a gradient of protein concentration. Cyto- 
plasmic transplantation experiments and analysis 
of the distribution of nanos RNA and NANOS pro- 
tein in pumilio mutant embryos show that pumi- 
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