REGULATION OF POSITIONAL INFORMATION IN DROSOPHILA 
Tulle Hazelrigg, Ph.D., Assistant Investigator 
The production of different cell types at particu- 
lar places in the body depends on generalized sig- 
nals, usually termed positional information. In 
Drosophila, positional information residing in the 
egg helps determine the initial distribution of cell 
types along both the anterior-posterior and dorsal- 
ventral axes. Later in development positional infor- 
mation is used to generate the normal patterns of 
gene expression within individual tissues. The long- 
term goal of research in Dr. Hazelrigg's laboratory 
is to understand how such positional information is 
established and how it instructs cells to assume 
their particular identities during development. The 
use of positional information is being studied in the 
early embryo and the developing eye. 
L Maternal Genes That Regulate Formation of Ante- 
rior-Posterior Polarity in the Developing Embryo. 
Over the past few years, work in a number of lab- 
oratories has identified a set of maternal and zy- 
gotic genes that act in a hierarchical fashion to es- 
tablish the normal anterior-posterior segmentation 
pattern of the embryo. The general model that has 
emerged is that maternal gene products, deposited 
in the developing oocyte, control the spatial pat- 
tern of expression of a set of zygotically acting 
genes (the gap genes) so that large, contiguous do- 
mains are established in the early embryo. Succes- 
sively smaller developmental fields are established 
as the gap genes effect the expression of the pair- 
rule genes, which in turn may regulate the expres- 
sion of the segment polarity genes. In concert with 
the action of the homeotic genes, this cascade of 
gene expression is necessary to establish anterior- 
posterior polarity, segmentation, and the assump- 
tion of segmental identities. 
The key maternal component that initiates this 
cascade of anterior-posterior development is the 
product of the bicoid (bed) gene. In newly depos- 
ited eggs, maternally expressed bed RNA is present 
as a tight cap at the anterior end of the egg. During 
early cleavage divisions a steep gradient of bed pro- 
tein develops, with highest concentrations in the 
anterior-most 30% of the egg. The bed protein gra- 
dient appears to be formed as a consequence of the 
translation of the localized bed RNA, followed by 
diffusion. Thus the question of how developmental 
polarity is established becomes one of how the bed 
RNA is localized within the egg. This question is 
part of the larger one of how developmental deter- 
minants are established within cells. 
The maternal-effect gene exuperantia (exu) ap- 
pears to be required for the proper positioning of 
the bed RNA. In the absence of maternally encoded 
exu gene product, embryos die with loss of specific 
head structures. Gastrulation events, normally seen 
only at the posterior end of the embryo, occur also 
at the anterior end and are correlated with the for- 
mation of posterior structures at the anterior end. 
The bed RNA is evenly distributed and not localized 
in eggs produced by mothers deficient for the exu 
gene. These observations demonstrate that the lo- 
calization of the bed RNA is a key event in deter- 
mining the anterior-posterior polarity of the em- 
bryo and that the exu gene product is essential for 
localization. 
In Dr. Hazelrigg's laboratory, the goal of work on 
the exu gene is to understand how the product of 
exu affects the anterior localization of the bed RNA. 
Work has progressed on two fronts, genetic and 
molecular. X-ray mutagenesis screens for new exu 
alleles have yielded three mutations, two of which 
were small chromosomal deficiencies that localized 
exu to a region containing three polytene chromo- 
some bands. Previous experiments had yielded a 
marked P-element insertion in one of these bands. 
Retrieval of the DNA flanking this P element pro- 
vided the entry point for a chromosomal walk of 
120 kb, which covered the region known to con- 
tain exu by the genetic analysis. 
The location of the exu gene in this 120 kb walk 
has been determined by Southern blot analysis of 
DNA from mutant flies and from analysis of tran- 
scripts produced from this region. Particularly infor- 
mative has been a hybrid-dysgenesis-induced muta- 
tion of the exu gene that contains a deletion of 
~700 bp. Analysis of transcripts in this region in 
adults reveals a male-specific 2.6 kb mRNA and a fe- 
male-specific 1.8 kb mRNA. Three exu mutants, in- 
cluding the one with the 700 bp deletion, are miss- 
ing these transcripts. 
Several exu cDNAs have been isolated from an 
ovary cDNA library and are being sequenced to de- 
termine the nature of the exu products. The one fe- 
male cDNA that has been most extensively studied 
also hybridizes to the male transcript on Northern 
blots. Thus the differences in the sex-specific 
mRNAs appear to be due to alternative splicing of 
the exu transcript. cDNAs for the male-specific 
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