1. 1 * egg: generate the system * larva: eat and grow
Developmental biology:
Drosophila segmentation and repeated units
* egg: generate the system
* larva: eat and grow
* pupa: structures in
larvae grow out to form
adult fly: metamorphosis
(Drosophila is a
holometabolous insect)
February 06 1

2. Segmentation pattern
Obvious segmentation begins to develop by germ band extension stage.
The embryonic segmentation pattern has direct analogs to the final segments of the adult.
Segmentation pattern can be thought of as classical segments or midsegment-to-midsegment intervals called parasegments.
Some early embryonic segments become incorporated into the complex structures of the head and mouth.

3. * Drosophila embryo: larva
Developmental biology:
Drosophila segmentation and repeated units
* Drosophila embryo: larva
February 06 3

5. The Bcd gradient
mRNA (in situ)
Protein (Ab staining)

6. bicoid protein gradient
gradient is interpreted at least at four different levels (thresholds).

7. 4 Embryo stage: developmental patterning:
Developmental biology:
Drosophila segmentation and repeated units
Embryo stage: developmental patterning:
most obvious characteristics in resulting larvae: segments
Segments are partially coated
with hairs: denticles
Fate map
How do you get from “empty bag”
to organized (segmented) larva?
What is required? What gene
Activities? Where and When?
February 06 4

8. 5 Loss of gene activity will tell us what it is required for:
Developmental biology:
Drosophila segmentation and repeated units
Loss of gene activity will tell us what it is required for:
Mutagenize flies and screen for segmental phenotypes
(Nüsslein-Volhard and Wieschaus):
Idea is to find all possible genes that when ko’d could lead to
a developmental anomaly…
Approach: randomly mutagenize whole genome;
Isolate single gene mutants;
Study their phenotypes (in embryo)
individual mutations created:
150 very clear segmentation phenotypes….
February 06 5

9. 6 Segmental phenotypes (Nüsslein-Volhard and Wieschaus): gap mutants
Developmental biology:
Drosophila segmentation and repeated units
Segmental phenotypes
(Nüsslein-Volhard and Wieschaus):
* found 150 mutants that disrupt
patterning in embryo on basis
of cuticle defects.
* amongst these, some have
remarkable phenotypes:
segments disappear in blocks
gap mutants
February 06 6

10. 6 Developmental biology: Drosophila segmentation and repeated units
February 06 6

11. 7 In gap mutants blocks of mutant phenotype caused consecutive
Developmental biology:
Drosophila segmentation and repeated units
In gap mutants
blocks of
consecutive
segments are
deleted.
mutant phenotype caused
by loss of gene activity
Clone gap genes:
determine expression
domain
gap gene B
Area of gene expression corresponds to affected body area in mutant
February 06 7

12. * From gradients to large blocks of expression
Developmental biology:
Drosophila segmentation and repeated units
* From gradients to large blocks of expression
February 06 8

13. 9 gap genes expressed in broad regions.
Developmental biology:
Drosophila segmentation and repeated units
gap genes expressed in broad regions.
targets?
all encode transcription factors:
Answer comes from group of mutants found in
same mutagenesis screen:
February 06 9

14. 10 Developmental biology: Drosophila segmentation and repeated units
Knirps protein
Fushi tarazu pair rule protein
Only three blocks
of gap gene expression:
how to get seven
stripes?
knirps action
induces expression
of one ftz stripe
February 06 10

15. 11 And we have functional test: in knirps mutant, stripe
Developmental biology:
Drosophila segmentation and repeated units
And we have functional test:
in knirps mutant, stripe
should disappear….
in fact kruppel mutant
February 06 11

16. 12 Gap transcription factor proteins, each induce one stripe
Developmental biology:
Drosophila segmentation and repeated units
Gap transcription factor
proteins, each induce one
stripe
Combined action of two
gap proteins (or gap +
head/tail proteins)
define stripe each again.
Hunchback protein
Kruppel protein 3 To 7
Stripes!
February 06 12

17. Enhancer trapping in Drosophila P element recognition sites
Use transposon P element
Carries reporter gene e.g.
b-galactosidase
Hops into genome
When lands near enhancer, activates gene expression
Expression similar to that of neighboring gene
P element recognition sites
TATA
b-galactosidase
P element vector
enhancer
gene Y
enhancer
b-galactosidase
gene Y

18. Regulation of eve stripes 3 and 7
eve 3 to 7
eve 3 and 7
hb-Z
kni
February 06

19. The even-skipped (eve) stripes: a model to study how transcriptional regulation establishes positional information
Regulatory elements are fused to a lacZ reporter gene and their expression is followed in transgenic flies. Eve has a long promoter element, which can be divided into regions that drive expression of various lacZ-transgenes (and indeed expression of eve) in different stripes (stripe-elements).
February 06

20. A great model for in vivo study of Transcription Factors and Their Binding Sites: The eve Stripe 2 Module
Determine binding sites by footprinting
Note that these transcription factors are the products of patterning genes higher in the regulatory hierarchy to eve.
Determine importance of enhancer elements by mutagenesis.

21. 13 In fly early embryo: transcription factor proteins
Developmental biology:
Drosophila segmentation and repeated units
Transcription factor proteins
are not transported out of cell:
influence transcription in
own nucleus
In fly early embryo:
transcription factor proteins
can influence next door nuclei
because there are no cell
membranes to stop the proteins
from diffusing.
Trancript is made in nucleus:
translated into cytoplasm.
In fact, the diffusion is important,
creates a gradient of activity:
fine patterning.
February 06 13

22. Regulatory
Protein Gradients That
Control
the Eve
Stripe 2
Module
The sharp Hunchback (green) and Kruppel (red) expression boundary.

23. Interaction Between Gene Products Sharpens
Expression Boundaries
early
late
Sharpening eve-ftz stripes.
Autoactivation and mutual inhibition of the eve and ftz transcription factors
Ftz
Eve

24. 14 Transcription regulation of typical pair rule gene: coding region
Developmental biology:
Drosophila segmentation and repeated units
Transcription regulation of typical pair rule gene:
high affinity
coding region
of pair rule gene
low affinity
February 06 14

25. pair rule genes = pair rule mutants! 16 * in mutant embryo,
Developmental biology:
Drosophila segmentation and repeated units
pair rule genes =
pair rule mutants!
* in mutant embryo,
every other segment is
deleted.
* pair rule gene expressed
in seven stripes. In mutant:
expression domains are
deleted.
* pair rule proteins are
transcription factors (again)
February 06 16

26. Targets of pair rule transcription factor proteins?
Developmental biology:
Drosophila segmentation and repeated units
Targets of pair rule transcription factor proteins?
wingless
expression
wildtype
paired
mutant
Pair rule protein
segment polarity gene
February 06 17

27. 18 pair rule stripes How: segment polarity stripes
Developmental biology:
Drosophila segmentation and repeated units
pair rule stripes
How:
segment polarity stripes
Each stripe of pair rule protein defines a stripe of segment polarity
gene expression.
Each segment polarity gene is thus induced by two different sets of
pair rule proteins (7 + 7 makes 14).
Fine tuning of stripe limits (anterior) by overlap of pair rule proteins
14 stripes of segment polarity gene expression: 14 segments!
February 06 18

28. 19 So far, transcription factors were able to define areas surrounding
Developmental biology:
Drosophila segmentation and repeated units
So far, transcription factors were able to define areas surrounding
the nuclei were gene is expressed: no cell membranes.
Expression domains of genes
Expression domains of proteins
February 06 19

29. 20 Developmental biology: Drosophila segmentation and repeated units
February 06 20

30. 21 Summary of early zygotic development in Drosophila embryos:
Developmental biology:
Drosophila segmentation and repeated units
Summary of early zygotic development in Drosophila embryos:
Bicoid protein
hunchback RNA
Simple axis derived maternal system, divides egg into discrete units
(mutants display loss of exactly the units).
Transcription factor gradients create sets of “seven” identical units.
February 06 21

31. 22 The action of the pair rule genes generates segmental stripes of
Developmental biology:
Drosophila segmentation and repeated units
The action of the pair rule genes generates segmental stripes of
expression of each segment polarity gene.
In fact, pair rule protein action defines segments!
Up until and including the pair rule proteins: all act within non-cellular
environment, all are transcription factors.
Segment polarity proteins act in cellular environment (not all transcription
factors, become expressed at time of cellularization).
February 06 22

32. 23 Further patterning within segments: next lecture
Developmental biology:
Drosophila segmentation and repeated units
Further patterning within segments:
next lecture
February 06 23

33. 1 time * transcription factors expressed in large blocks
Developmental biology:
segment polarity genes and cell signalling b i c o d g a p e n s
time
* transcription factors expressed in large blocks
* induce expression of downstream transcription factors in smaller blocks
* generates segmental expression (14) of segment polarity genes.
February 06 1

34. Maternal Coordinate Genes Maternal Coordinate Genes
A Gene’s Sphere of Influence Extends Only To Where Its Product is Expressed
Maternal Coordinate Genes
bicoid (bcd)
caudal (cad)
hunchback (hb)
nanos (nos)
Maternal Coordinate Genes
bicoid (bcd)
caudal (cad)
hunchback (hb)
nanos (nos)
hunchback (hb)
Krüppel (Kr)
knirps (kni)
giant (gt)
tailless (tll)
hunchback (hb)
Krüppel (Kr)
knirps (kni)
giant (gt)
tailless (tll)
Gap Genes
Gap Genes
Pair-Rule Genes
even-skipped (eve)
odd-skipped (odd)
hairy (h)
runt (run)
fushi-tarazu (ftz)
paired (prd)
Pair-Rule Genes
even-skipped (eve)
odd-skipped (odd)
hairy (h)
runt (run)
fushi-tarazu (ftz)
paired (prd)
Segment Polarity Genes En
Segment Polarity Genes
engrailed (en)
wingless (wg)
engrailed (en)
wingless (wg)

35. Summary Initial asymmetry for AP axis set up during oogenesis
Pattern organised by maternal proteins soon after fertilization
Localised maternal proteins activate or repress zygotic genes
Transcriptional and post-transcriptional regulation
Gradients of transcription factors are refined by mutual activation and repression and cell-cell signalling.