Genetic Control of Pattern Formation 
in Drosophila 
Sean B. Carroll, Ph.D. — Assistant Investigator 
Dr. Carroll is also Associate Professor of Molecular Biology, Genetics, and Medical Genetics at the 
University of Wisconsin-Madison. He obtained his B.A. degree in biology from Washington University in 
St. Louis and his Ph.D. degree in immunology from Tufts University School of Medicine in Boston. He 
received postdoctoral training in developmental genetics working with Matthew Scott at the University 
of Colorado. In addition to his central work on pattern formation in Drosophila, Dr. Carroll has 
also conducted basic research on new types of snake antivenoms, which are now under evaluation 
as potential pharmaceuticals. His honors include the NSF Presidential Young Investigator Award. 
INSECTS are the dominant group of animals on 
earth today. Nearly 1 million species have been 
classified among an estimated 20 million species 
extant. The innumerable sizes, shapes, and colors 
of insects have evolved from a basic segmented 
body plan consisting of three broad divisions — 
head, thorax, and abdomen — with three pairs of 
ambulatory legs and, in some orders, wings. 
One insect, the fruit fly Drosophila melano- 
gaster, has emerged as a key model for investiga- 
tions into the genetics and molecular biology of 
animal development. An enormous amount of in- 
formation has been gained about the organization 
of the Drosophila egg, the dynamic regulatory 
mechanisms guiding embryonic development, 
and the cellular and molecular processes in- 
volved in tissue differentiation and organogene- 
sis. By addressing the intricacies of Drosophila 
development at the genetic, cellular, and molecu- 
lar levels, one of the central puzzles of biology is 
being deciphered: How do complex animals 
form from a simple egg? 
In addition, the stage is being set for compara- 
tive studies that will integrate the growing knowl- 
edge of Drosophila development with contempo- 
rary views of animal evolution. Efforts in our 
laboratory are aimed at both a detailed under- 
standing of Drosophila embryology as a model 
for animal development and comparative studies 
of other insects (e.g., beetles and butterflies) as 
models for evolution. 
Pattern Formation in Drosophila Embryos 
The genetic control of pattern formation can be 
broken down conceptually into at least three 
phases. The first consists of a molecular prepat- 
tern, revealed as chemical changes that take place 
in different regions of the animal and foreshadow 
the cellular events to follow. For example, cer- 
tain key regulatory proteins in Drosophila come 
to be expressed in stripes of cells encircling the 
embryo — stripes representing future segmental 
divisions. The second phase involves the specifi- 
cation of groups of precursor cells that populate 
the different tissues of the animal. For example. 
genes such as those of the achaete- scute complex 
(AS-C) are activated in clusters of cells that give 
rise to the central and peripheral nervous sys- 
tems. Finally, in the third phase, these precursor 
cells divide and differentiate, giving rise to the 
full complement of specialized cells that make 
up different tissues and organs and express dis- 
tinct structural genes to carry out their individual 
tasks. 
From molecular prepattern to the formation of 
stem cells to the differentiation of their progeny, 
there is a flow of genetic information. The pre- 
pattern specifies the spatial domains of genes that 
are activated in stem cells, and these genes in turn 
regulate cell type-specific gene expression. Our 
laboratory is interested in the genetic basis of this 
information flow and focuses on three aspects of 
pattern formation. 
First, we are studying the molecular regulation 
and function of the pair-rule genes, the first genes 
expressed in a segmentally repeating prepattern. 
Second, we are studying how the early prepattern 
genes regulate the expression of the stem cell- 
specific proneural genes of the AS-C. Third, we 
are trying to identify some of the molecular pre- 
patterns that govern pattern formation in imagi- 
nal discs — the distinct pouches of cells that are 
set aside during embryogenesis, proliferate exten- 
sively during larval development, and give rise to 
adult structures such as the wing, leg, and eye. 
In addition, we have recently initiated studies 
of other insects (guided by our knowledge of Dro- 
sophila genes) to compare the molecular aspects 
of segmentation in species that exhibit different 
styles of oogenesis and early development and to 
explore the genetic basis of wing development 
and evolution among the flying insects. 
The Pair-Rule Segmentation Genes 
During early Drosophila development, small 
batteries of genes are expressed in rapid succes- 
sion to establish the segmental prepattern. The 
first genes expressed in a periodic pattern are the 
pair-rule genes, deployed in one stripe every two 
segments. The striped patterns of certain pair- 
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