Developmental Genetics of Drosophila
In Principle:
All cells contain the same DNA genome
Genome: the complete set of genes in a cell - constant
Differences among cells reflect differences in genome expression
timing, amount, & location of RNA transcription & protein translation
Proteome: the complete set of proteins in a particular cell,
in a particular tissue, or at a given developmental stage
The proteome is therefore highly variable
Developmental cascades of genes that activate other genes (MGA2 16 Chapter Overview)
Basic patterns are the result of relatively few genes
Variable patterns arise from simple 'switches'
Overview of insect development:
insects are tripartite (MGA2 16-26a)
Early development forms monolayer blastoderm (MGA2 16-14, 22)
Subsequent "fate" of cells predictable from position in fate map
3 Head + 3 Thorax + 9 Abdomen segments (MGA2 16-23)
anterior & posterior compartments in each
Cuticular structures develop from imaginal disks
12 disks are associated with segments
e.g., wings from T2, halteres from T3
Genetics of early
embryogenesis
The genetic approach:
(C. Nüsslein-Volhard and E. Wieschaus: Nobel
Prize in
1995)
Infer
nature of wild-type by
observation of mutant phenotype
Induce recessive alleles
with embryonic-lethal
effects:
Mm x Mm
1 / 4 mm
& fail to
develop (lethal) or
Mm [f] x Mm [p]
all embryos are 'M' : maternal effect (MGA2 Foundations 3)
e.g.:
at bicoid locus:
bcd / bcd has no head
or thorax
structures (MGA2 16 Foundations 4)
gene
affects anterior / posterior (A / P) axis formation
increasing
dose of bcd+ increases "cephalization" (MGA2
Foundations 4)
displaces "head" to more
posterior position
Conclusion:
Drosophila development is determined by a limited number of gene
sets
Activated
as ordered
cascades
(MGA2 16-28)
Maternal-Effect genes (Table 16.3)
Gene
products (mRNA & protein) are present in egg
(maternal)
cytoplasm
anterior /
posterior
(A / P) gradients form in egg
(mRNAs
are anchored to microtubules in cytoskeleton) (MGA2 16-21a)
bicoid locus affects formation of anterior
head & thoracic structures
bcd+ mRNA localized at anterior pole of
egg (MGA2 16-16 a,b)
BCD protein forms A / P
gradient
nanos locus affects formation of posterior
abdominal segments
nos+ mRNA expressed at posterior pole
of egg (MGA2 16 c,d)
NOS protein inhibits
translation of hunchback (hb-m)
locus
expression
of HB-M protein in zygote is
enhanced by bicoid,
inhibited by nanos
=>
Gradient of HB-M concentration
exists in zygote
Cells
"read" positional
information as to
their place on
A / P axis (MGA21 16-21b)
cells can be "fooled" to misread the
information (MGA2 16-17)
Zygotic genes (Overview of
phenotypes)
(MGA2 16-24)
Transcription of biparental genes is induced by maternal-effect genes ("French
Flag" model)
Gap genes
determination of tri-partite body
pattern:
roughly, head,
thorax,
& abdomen (MGA2 16-25a)
(MGA2 16 Foundations 4)
Pair-Rule genes
determination of segments
segment patterns are repetitive
(iterative) (MGA2 16-25b)
Segment patterns are determined by multiple,
independent
"rules"
Segment Polarity genes
determination of anterior & posterior compartments within
segments
Homeotic selector genes
Determine phenotypic expression of segment
Control differentiation of imaginal
discs
Transdetermination
Homeotic mutants
alter fate of imaginal
discs (E. B. Lewis 1945:
Nobel
Prize 1995)
Bithorax
transforms halteres (T3) 
wings
(T2)
Note that flies are "Diptera", but bithorax has four
wings
(MGA2 16-26c)
alternative expression of Ubx
gene
complex
antennapedia
(nasobemia)
transforms antennae legs
alternative expression of Antp
gene
complex
All text material © 2008 by Steven M. Carr