gap proteins and activated by a general transcription 
factor. 
The pair-rule gene products regulate overall seg- 
mental pattern by controlling the initial expression 
of the segment polarity loci, a large group of genes 
that are usually expressed in one band of cells 
within each segment. James Skeath has shown that 
other genes involved in the formation of structures 
within segments are also regulated by pair-rule 
genes. The proneural genes achaete and scute, for 
example, are expressed in several anteroposterior 
rows of ectodermal cell clusters from which individ- 
ual neuroblasts will segregate. The registration of 
these rows is achieved by combinations of pair-rule 
gene products directly regulating proneural gene ac- 
tivity. Thus global (segmentation) and fine-scale 
(intrasegmental) pattern formation are integrated 
by a common set of regulatory genes. 
To dissect the molecular regulatory mechanisms 
underlying the global control of achaete and scute 
gene expression, Jim Skeath, Dr. Grace Panganiban, 
and Jane Selegue have isolated certain cis-acting re- 
gions that regulate both genes simultaneously. This 
common intergenic region (> 1 5 kb in size) is neces- 
sary for spatial regulation by the pair-rule genes. 
Different sets of pair-rule gene products establish at 
least two different anteroposterior domains of 
achaete and scute expression. Along the dorsoven- 
tral axis, other regulatory proteins restrict proneural 
gene activity to the ventral neurogenic region. Thus 
the initial two-dimensional pattern of proneural cell 
clusters expressing achaete/scute is carved out by 
the combined action of two sets of axis-patterning 
genes acting through a common regulatory region. 
A Genetic Hierarchy Guides Wing Formation 
One of the least well understood aspects of animal 
development is the formation of the various adult 
limbs and appendages. In Drosophila the adult ce- 
phalic and thoracic structures develop from imag- 
inal discs, small groups of cells set aside during 
embryogenesis that proliferate during larval devel- 
opment and undergo morphogenesis during the late 
larval and pupal stages. Recently, Dr. Jim Williams 
showed that the vestigial gene plays a key role in 
wing and haltere formation. This nuclear protein is 
first expressed very early during the development of 
the wing and haltere imaginal discs and is expressed 
at high levels in the late third instar imaginal cells 
that will give rise to wing and haltere structures, but 
not expressed in those disc cells that give rise to 
structural components of the thorax. In the absence 
of vestigial, the fly completely lacks wings and hal- 
teres. These results suggest that the vestigial gene 
product acts to distinguish cells that will become 
pan of the flight appendages from those that will be 
part of the thorax proper. 
To identify other genes involved in wing forma- 
tion that may regulate the remarkable pattern of yex- 
f/gz'fl/ expression, Dr. Williams and Dr. Stephen Pad- 
dock have analyzed the developmental roles of and 
interactions between vestigial and three other 
genes that are required for wing formation. The dy- 
namic patterns of wingless, apterous, and vestigial 
expression reflect the progressive division of the 
growing imaginal disc into several distinct subre- 
gions that will give rise to different parts of the wing 
and notum. The wingless gene — the earliest acting 
member of the genetic hierarchy guiding wing de- 
velopment — is required to set up the distinct pat- 
terns of apterous and vestigial expression and that 
of the gene scalloped, which appears to act in con- 
cert with vestigial to promote the differentiation of 
wing tissue. 
Evolution of Developmental Mechanisms 
It is not known whether the developmental mech- 
anisms found in Drosophila are conserved through- 
out different taxa. Julie Gates has shown that within 
the order Diptera (two-winged insects), the above- 
mentioned hairy, achaete/scute, and vestigial pro- 
teins appear to exhibit very similar, if not identical, 
patterns of expression throughout the development 
of nearly a dozen species that are up to 80 million 
years diverged from D. melanogaster. To compare 
segmentation, neurogenesis, and wing formation in 
much more divergent insects. Dr. Lisa Nagy, Jim 
Skeath, and Dr. Williams have isolated segmenta- 
tion, proneural, and "pro-wing" genes from a num- 
ber of species representing key insect orders. The 
expression of these genes in different species may 
reveal fundamental diff'erences in how insects are 
organized. For example, there is some doubt as to 
whether pair-rule genes, so critical for the Drosoph- 
ila mode of development (and highly conserved 
within these flies), function as segmentation genes 
in other insects. The availability of molecular 
probes for key regulatory genes may shed light on 
the evolutionary history of insects and the genetic 
basis of morphological diversification. 
Advances in Imaging Technology 
Research in many cell and developmental biology 
laboratories is being aided by more powerful tech- 
niques for imaging large and complex biological 
specimens. Dr. Paddock has refined and developed 
new techniques for visualizing gene expression by 
immunofluorescence labeling and laser scanning 
confocal microscopy. The relative spatial and tem- 
poral patterns of up to three different proteins can 
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