General Research Interests
My research interests are focused on the genetic and molecular analysis of chromosome and chromatin structure in Drosophila melanogaster.
Through examining and understanding mutations associated with gene
position effects, my work will help elucidate the role that chromosome
and chromatin structure plays in regulating gene expression. I am
currently concentrating on the cubitus interruptus (ci)
locus on Drosophila chromosome 4 and its unusual position effects. I am
also involved in the large scale physical mapping of chromosome 4.
Mapping chromosome 4 of Drosophila melanogaster
The model organism Drosophila melanogaster has two sex
chromosomes and 3 autosomes.The smallest chromosome is chromosome 4, ~5
Mbp in length (Locke and McDermid, 1993), which appears as a "dot"
chromosome in metaphase spreads. Chromosome 4 has two major regions. The
centromeric domain is a-heterochromatic and consists primarily of about
~3-4 Mbp of short, satellite repeats. This region forms part of the
highly condensed chromocenter seen in polytene chromosome spreads. The
remaining ~1.2 Mbp constitutes cytogenetic regions 101E to 102F on
salivary gland chromosomes (the banded region). Of the ~80 genes
expected on chromosome 4, only 15-20 of the genes have been mapped and
so far all to this domain.
The fourth is an atypical Drosophila chromosome in several respects,
all of which relate to the long-standing presumption that this
chromosome is, in some ways, heterochromatic in nature. First, the
banded region of chromosome 4 often shows a diffuse and poorly defined
appearance similar to regions of b-heterochromatin. Second, a similarity
to b-heterochromatin is shown in that the chromosomal protein HP1,
thought to be an important constituent of heterochromatin, binds to
several sites along the chromosome. Third, repetitive DNA sequences
normally confined to b -heterochromatin are distributed throughout the
banded region of 4. Fourth, further similarities to b -heterochromatin
are indicated by the behavior of P element transgenes inserted into this
region since they frequently show variegated expression of a white+
marker gene, a characteristic of insertion sites near heterochromatic
boundaries.Fifth, chromosome 4 lack crossing over during meiosis.
The loci on chromosome 4 have been shunned in the Drosophila
literature. Most genetic screens have been designed to recover mutants
on only the X, 2, or 3 and ignored chromosome 4 loci. This evasive
action was probably due to its small size and the last feature above,
lack of crossing over. Without crossing over was perceived as hard, if
not impossible, to characterize chromosome 4 genes. Nevertheless,
Hochman in 1976 identified ~ loci and roughly mapped them using the few
deletions available at the time.
Chromosome 4, which constitutes cytogenetic regions 101E to 102F on
salivary gland chromosomes, is the region we have decided to map.
Beginning with many entry points and many chromosomal walks we have
recovered overlapping cosmid and BAC clones that together constitute a
contig that spans the length of the "banded region" defined by
cytogenetic bands 101E to 102F. Chromosome walking has been hindered by
the abundance of moderately repeated sequences dispersed along the
chromosome. The clones have been correlated to the cytogenetic map by in
situ hybridization to polytene chromosomes. The locations of previously
cloned genes have also been mapped on the contig. A minimal tiling set
of clones from the contig will provide templates for sequencing this
chromosome as part of the Drosophila Genome Project.