DEVELOPMENTAL CONTROL OF CHROMOSOME FUNCTION 
Allan Spradling, Ph.D., Investigator 
The genetic information for development is 
stored on chromosomes. Each generation this in- 
formation must be accurately copied, segregated to 
daughter cells, and recombined within the germ 
cells. Dr. Spradling's laboratory is interested in the 
molecular mechanisms involved in these chromo- 
somal functions and how they relate to the actual 
use of developmental information through regu- 
lated gene expression. 
L A Model Chromosome. 
Dr. Spradling's group has selected as a model the 
smallest known Drosophila chromosome. Dpi 187. 
This minichromosome was produced (in the 1950s) 
by deleting 97% of the internal sequences from a 
normal X chromosome. Although just 10 genes 
from the X chromosome tip are appended to the 
basal 10% of the centromeric heterochromatin in 
Dp 1187, it replicates and is transmitted with high 
fidelity. Pulsed-field gel electrophoresis revealed 
that Dpi 187 contains only —1.4 million base pairs 
(bp) of DNA, far less than any other known chro- 
mosome in a multicellular animal. The distal 300- 
400 kb encodes the known genes on the 
minichromosome, while the remaining 1 million bp 
represents heterochromatin, the little-understood 
chromosome domain associated in most higher or- 
ganisms with centromeres. 
A restriction map was constructed using pulsed- 
field gel electrophoresis of most of the gene-con- 
taining portion of the minichromosome and —100 
kb of the adjacent heterochromatin. This map pro- 
vided sufficient information to analyze a long-stand- 
ing problem in chromosome replication. Many Dro- 
sophila cells, such as the larval salivary gland, 
undergo polytenization, a specialized type of chro- 
mosome replication that sometimes produces giant 
polytene chromosomes. During polytenization the 
heterochromatic chromosome regions replicate 
much less than the remainder of the chromosome. 
As expected. Dp 1187 heterochromatin was found 
to replicate less than the distal euchromatin in the 
salivary gland. Replication of the euchromatin de- 
pended on its distance from the junction with het- 
erochromatin; the copy number of sequences in- 
creased in a gradient over at least a 100 kb region. 
Surprisingly, however, the replication of Dpi 187 
also varied from cell to cell within the salivary 
gland, and probably in many other adult tissues. 
These studies suggested that replication of hetero- 
chromatin may not always be as tightly controlled 
as the coding part of chromosomes. 
These observations also provided insights into 
the phenomenon of position-effect variegation. 
Genes that are relocated next to a heterochromatic 
region as the result of a chromosome rearrange- 
ment frequently are expressed only in some of the 
cells within a tissue. The expressing cells are scat- 
tered, apparently at random, throughout the af- 
fected tissues. Genetic evidence shows that this is a 
long-range effect that spreads from the heterochro- 
matic breakpoint into the adjoining euchromatin. 
The construction of Dp 1187 created such a euchro- 
matin-heterochromatin junction. Several genes on 
Dpll87, including yellow, show typical position-ef- 
fect variegation in flies with the replication defects 
that were measured. Addition of a Y chromosome 
suppressed variegated yellow expression. The add- 
ed Y chromosome also greatly increased the repli- 
cation of Dp 1187 euchromatin (including the yel- 
low locus) in the salivary gland. Variable replication 
of Dpi 187 therefore appears to be the underlying 
cause of the variegated expression of genes located 
near its euchromatin-heterochromatin junction. 
n. Oogenesis. 
Dr. Spradling's group has initiated genetic stud- 
ies of several aspects of Drosophila oogenesis. The 
technique of single-? element insertional mutagen- 
esis was used to generate and screen 5,000 lines for 
female sterile mutations. The expression patterns 
of an enhancer-sensitive lacZ-fusion gene residing 
on a mobilized P element were also studied within 
ovaries from an additional 9,000 lines. These stud- 
ies have allowed Dr. Spradling's group to identify 
candidate genes involved in several processes in oo- 
genesis, including stem cell maintenance, oocyte 
determination, follicle cell specialization, transmis- 
sion of material between nurse cells and oocyte, 
and follicle cell-germ cell communication. A signifi- 
cant current task is to analyze the molecular pro- 
cesses giving rise to the common mutant pheno- 
types so that mutations involved in specific 
processes can be identified. 
Studies of the bag-of-marbles (bgm) gene re- 
vealed a possible connection between sex determi- 
nation and the ability of a germ cell to become an 
oocyte. Mutants at bgm are female sterile and lack 
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