Development of the Drosophila Body Plan
Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J. (2001) Principles of Development. 2th ed. London: Oxford university press.
Gilbert SF. (2003) Development Biology. 7th ed. Sunderland: Sinaure Associates Inc.
Homeotic Gene Expression in Drosophila Overview of Drosophila development
Drosophila is the best understood of development system at gene level.
13600 gene (nematode is 19,000 gene) 1995 Nobel Prize (twice)
Genes that control development in Drosophila are very similar to those that control development in vertebrates.
Drosophilais the best understood developmental system with great impact Drosophilais the best understood developmental system with great impact
upon our knowledge of all development. (for example, Hox genes were first found in Drosophila.)
Drosophila larva is patterned along two distinct and largely independent axes: A/P and D/V. Fig.5.1
Bilateral symmetry is established by the A/P and D/V axes.
The larvae has an anterior acron, three thoracic and eight abdominal segments and a posterior telson. Each segment has its own unique character—external cuticular structure and internal organization.
E l tt i i th ti l bl t d d it b
Early patterning occurs in the syncytial blastoderm and it becomes multicellular at the beginning of segmentation.
At syncytial stage, the embryo contain multinucleus and many protein. The protein did not secreted from cells. It is transcription factors → enter nuclei
→ regulated gene expression.
The concentration gradients of protein → provide positional information for the nuclei → different develop and set up.
Fig. 5.1
Patterning of the Drosophila embryo
1. Body plan is patterned along A/P and D/V axes 2. A/P divided into 4 regions:
head throacic segments head, throacic segments, abdominal segments and tail
3. The future segments (14 parasegments, and 10 marked) regulated by specific gene activity. 14 parasegments converted into T1-3, and A1-8 only.
4. D/V divided into 4 regions:
mesoderm, ventral ectoderm, dorsal ectoderm and extra-embryonic membrane (aminoserosa)
syncytial blastoderm
Repeated rounds of nucleus division without any cytoplasmic division.
One cell contain many nucleus
Laser Confocal Micrographs of Stained Chromatin Showing Superficial Cleavage in a Drosophila Embryo (nucleus stain) Drosophila Embryo (nucleus stain) Number refer to the cell division cycle. In cycle 10 formed syncytial blastoderm; After cycle 13, the oocyte membranes ingress between the nuclei to form the cellular blastoderm
The formation of cellular blastoderm
(A) Development series showing progressive cellularization.
(B)Confocal fluoresecnce photomicrographs of nuclei dividing photomicrographs of nuclei dividing during the cellularization of the blastoderm; tubulin (mitotic spindle) is red, actin is green
(C)Cross section during cellularization. The actin expands
Early development is 2-dimensional : the superfical layer of the embryo that consists first of nuclei and later of cell. 3-dimensional
t l ti t h t f th f l i t th
:gastrulation stage → when parts of the surface layer move into the interior to form the gut, the mesodermal structures that will give rise to muscle, and the ectodermally derived nervous system.
Concentration gradients of proteins (transcription factors) can diffuse,
Drosophila development: maternal genes set up the body axes
Mother gene, must be expressed by the mother and not by the embryo.
Zogotic gene, contrast to mother gene. It expressed during embryo development.
M t l t bli h th b d t j (A/P D/V)
Maternal genes establish the body two major axes (A/P; D/V).
Maternal gene products (50 genes), mRNAs and proteins are expressed in the ovary and localized. Fig 5.2
About 50 maternal genes set up the A/P and D/V axes: the framework of positional information (spatial distributions of RNA and proteins).
Zygotic genes respond to maternal gene expression.
First broad regions are established, then smaller domains (with a unique g ( q set of zygotic gene activities) in a hierarchy of gene activity.
Mother gene → affect zygotic gene expression → step by step gene expression → body axes set-up……
Fig.5.2
The sequential expression of different sets of gene established the body plan along the A/P axis
Positional information After feritilzation, maternal
gene products laid down in
Regional difference
Parasegments foreshadow segmentation the egg (bicoid gene) →
bicoid mRNA translated → positional information → four classes of zygotic gene (Gap, pair rule, segment polarity and selector gene) expression → different gene determined different function
Segmentation
Segment identity determined different function
Selector gene also called Homeotic gene)
Morphogenetic determinants create
Bicoid protein
Gap protein → Hunchback protein (orange) and
( g )
Kruppel protein (green)
→ transcription factor (yellow)
Pair- rule gene Segme nt polarity gene
Drosophila development: the A/P axis Before fertilized, Maternal gene expression → egg along A/P.
Three classes of maternal genes set up the A/P axis. Fig 5.3
Anterior class: loss of head
d th ( ti
and thorax (sometimes replaced with posterior).
Posterior class: loss of abdominal segments.
Terminal class: missing acron and telson.
bicoid, hunchback, nanos and caudal are key to A/P
Maternally expressed genes distinguish the anterior from the posterior.
Maternal effect mutants result in females that can not produce Drosophila development: the A/P axis
normal progeny.
Three mutant classes are 1) Anterior class: bicoid 2) posterior class: nanos 3) terminal class: torso
Maternal genes(bicoid) provides an A/P morphogen gradient It group I morphogen
bicoid is formed in the oocyte during oogenesis, it is a maternal gene.
It localized at the anterior end. After fertilization → translated → positional information effect
bicoid sets up a A/P morphogenic gradient and controls the first steps in embryo development and, thus, is essential to the developing organism. Female flies lacking bicoid gene expression → proudced embryos that have distrupted anterior segments.
After fertilization, the mRNA is translated and a concentration gradient is formed along the A/P axis. (bicoid was the first evidence of a morphogen gradient )
morphogen gradient.)
Bicoid gradient distribution: un-fertilization, m-RNA attach cytoskeleton
→ after fertilization, translated to protein → gradient Injected bicoid → induced anterior segment.
Bicoid expression gradient → establishes a gradient in some substance (bicoid) → highest level are at the anterior end.
Fig. 5.4 The bicoid gene is necessary for the development of anterior structure
Bicoid gene/protein can induce a
t i t
anterior-segment
No head
1) bicoid (bcd) mutant females lay eggs that give rise to embryos missing the head and thorax (and have an anterior telson).
2) Embryos missing anterior cytoplasm resemble b
Maternal genes (bicoid) provides an A/P morphogen gradient Fig 5.5 The distribution of bicoid mRNA and protein above.
3) Bicod mutant embryos rescued by anterior cytoplasm injections.
4) Anterior cytoplasm can induce ectopic head &
thoracic segments by injection in the middle of a bicoid egg.
The posterior pattern is controlled by nanos & caudal protein gradients (group 2)
Maternal gene,nanos mRNA is localized to the posterior pole of the egg. It translated after fertilization.
nanos is NOT a morphogen (like bicoid) but acts to suppress translation of another maternal gene hunchback (hb)
another maternal gene, hunchback (hb).
hunchback is maternal (present at low levels in embryo) AND zygotic (the latter is activated by high bicoid levels).
nanos (and pumilio) bind hb mRNA to prevent translation.
caudal mRNA is distributed evenly (like hunchback distribution, before fertilization).
The P-A gradient of caudal protein is established by inhibition of caudal protein synthesis by bicoid. Anterior is high, and posterior is low.
Mutantation of caudal gene → abnormal development of abdominal segments.
bicoid and hunchback run in A to P gradients and caudal runs P to A.
Anterior and posterior extremes are specified by cell-surface receptor activation
Maternal distribution
↓
↓ Nano mRNA
↓
Inhibition of hunchback protein (hb)
Anterior bicoid
↓ Inhibited caudual protein synthesis
↓ G di t di t ib bti Transcription
Bind to hb mRNA
Morphogen
↓ A/P axis
Gradient distribubtion
Unfertilization egg After fertilization
LOW Two gradient-bicoid and
hunchback protein → run LOW
hunchback protein → run in an anterior to posterior direction.
Group 3 maternal genes specify the acron and telson regions
Specifies the end structure of A/P axis by specific maternal gene.
Terminal specification is due to cell-face receptor activation, at the two poles only.
The key gene is torso; torso mutants develop neither acron nor telson regions.
The key gene is torso; torso mutants develop neither acron nor telson regions.
torsoencodes a uniformly distributed receptor protein which is activated by ligand present only at the anterior and posterior parts of the vitelline membrane.
The ligand is released after fertilization.
torso (a receptor tyrosine kinase) signals to direct terminal zygotic gene expression.
Most anterior/posterior polarity is due to vitelline membrane proteins Most anterior/posterior polarity is due to vitelline membrane proteins.
At fertilization, a protein deposited on the ventral vitelline membrane initiates a series of reactions which, in part, activates (cuts) spätzle, the ligand for the uniformly distributed receptor Toll.
Fig 5.7 The torso receptor is involved in specifying the terminal regions of the embryo
Before fertilization, ligand in vitelline envelop → after fertilization → ligand move to perivitelline→ bind to torso receptor → function (set up acron or telson)
Different region, but the same Different region, but the same pathway;Two terminal regions, despite their topographical separation, are not specified independently but use the same pathway.
dorsal provides positional information along the D/V axis The D/V polarity of the egg is specified by localization of maternal
proteins in the vitelline envelope.
dorsal provides positional information along the D/V axis.
Different maternal gene from A/P axis. But use the same mechanism.
In the syncytial blastoderm, dorsal (a transcription factor) is activated and enters nearby nuclei, signal form ventral region. But positional information along the D/V axis is provide by dorsal. (ventral →signal
→Dorsal → structure) Fig 5.8
The key defines D/V polarity in the follicle cells is pipe, which transcribed into mRNA only in the follicle cells that surround the ventral region.
Dorsal is in highest concentration in ventral nuclei (little or none is present in the dorsal nuclei). When dorsal protein enter nucleus → no function, can not developed to dorsal region
Toll signals the degradation of the maternal protein cactus.
Without Toll signal, cactus binds dorsal to keep it in the cytoplasm.
Dorsal and cactus are homologues of vertebrate NF-B and I-B.
Fig 5.8
Maternal gene (pipe gene ) Only expressed in ventral
↓ Secreted enzyme
↓ Interaction
↓ Spatzle proteinp p
↓ Secreted form follicle To perivetelline space
↓
Spatzle fragment (ligand)
↓ Bind to Toll-receptor
↓
Still on ventra
Toll protein (receptor) activation → formed a gradient of dorsal protein alone D/V axis
↓ Dorsal protein enter
Nucleus
↓
Ventral-dorsal cytoplasma gradie less much
developed to dorsa
Fig. 5.9 The mechanism of localization of dorsal protein to the nucleus
Dorsal protein
No function Enter nucleus
↓ Ventral function
Polarization of the body axes during oogenesis How the maternal mRNA and proteins get into the egg during the
development in the ovary (oogenesis)? How are they localized in the correct places? After release from the ovary, it aleady has a well defined organization: bicoid mRNA is located at the anterior end and nanos and caudal mRNA at opposite end.pp
In the germarium, a stem cell gives rise to 16 cells by four mitotic divisions.
These become one oocyte and 15 nurse cells, all of which are connected by cytoplasmic bridges. Fig 5.10
A sheath of somatic follicle cells surround the nurse cells and oocyte to form the egg chamber which secrete the vitelline membrane and egg shell.
Follicle cell around the nurse cell and oocyte (more), and provide the patterning of egg’s axes.
Fig.5.11 Drosophila oocyte development
The first visible sign of A/P polarization during oogenesis is the movement of the oocyte toward one end of the egg chamber, where it comes into contact with the follicle cells.
Dorsal part
15 ll t
15 nurse cell oocyte
Follicle cell
The oocyte and follicle layer are define the future D/V axis of the egg and embryo. Follicle cells expressed gene → overlying the dorsal anterior region of the oocyte (blue staining).
Different follicle cell → expressed different gene/ secrete materials → vitelline envelope and eggshell
A/P and D/V axes of the oocyte are specified by interactions with follicle cells
The oocyte induces follicle cells to adopt posterior fate and the anterior follicle cells are not in contact with the oocyte. (follicle)
Posterior follicle cell and oocyte has high E-cadherin than other follicle and nurse cell
nurse cell.
The signal from the oocyte to the follicle cells is the gurken protein (secreted), a member of the TGF-alpha family.
gurken binds to torpedo (a receptor in follicle cell), a receptor tyrosine kinase similar to the EGF receptor. Torpedo high expressed in posterior of follicle cell.
After gurkin bind to torpedo receptor → Follicle cells signal back to reorganize the oocyte’s cytoskeleton which directs bicoid mRNA move reorganize the oocyte s cytoskeleton which directs bicoid mRNA move to the anterior and oskar mRNA (which specifies germ plasm) and nanosmRNA to the posterior. Fig 5.12
Later the D/V axis is set up by gurken (again) which signals to establish dorsal follicle cells.
Fig.5.12 Specification of A/P and D/V axes during oogenesis
Secreted
Back signal
Specifying the Anterior-Posterior Axis of the Drosophila Embryo During Oogenesis
Specifying the Anterior-Posterior Axis of the Drosophila Embryo During Oogenesis
From follicle cell back to egg
Three independent Genetic Pathways Interact to Form the Anterior-Posterior Axis of the Drosophila Embryo
Expression of the Gurken Message and Protein Between the Oocyte Nucleus and the Dorsal Anterior Cell Membrane
(A)The gurken mRNA is localized between the oocyte nucleus and the dorsal follicle cells of the ovary
(C)Cross section
Schematic Representation of Gastrulation in Drosophila
Zygotic geneexpression along D/V axis is controlled by dorsal protein After dorsal protein has entered the nuclei → gene expression → D/V
development. D/V axis formed is related with three layer development.
Form ventral to dorsal, are mesoderm, ventral ectoderm (prospective neurectoderm), dorsal ectoderm (prospective dorsal epidermis) and prospective amnioserosa (which is an extra-embryonic membrane on the dorsal side of the embryo).
Mesoderm → internal soft tissue (muscle, connective tissue) Neurectoderm → epidermis and nervous
Dorsal ectoderm → epidermis
dorsal drives gene expression to activate and inactivate a number of genes by binding to the regulatory regions of many genes it controls.
D/V axis, is regulated by dorsal protein. In ventral region, dorsal protein activated gene in ventral, but repress gene related with dorsal region.
Dorsal protein of the dorsal region is less than ventral region.
High levels of dorsal protein activates twist and snail (required for mesoderm induction and gastrulation).
Low levels of dorsal activate protein rhomboid (which is suppressed by snail) to give rise to the neuroectoderm.
decapentaplegic(dpp), tolloid and zerknult are suppressed by dorsal protein and are restricted to the most dorsal regions. In dorsal region, intra nucleus dorsal protein is less so inhibition X
intra-nucleus dorsal protein is less, so inhibition X
decapentaplegicIs specific to the development of dorsal region, and dorsal protein inhibition decrease, so the function can induced it.
zerknultspecifies the amnioserosa.
Intra-nucleus dorsal low
Dorsal morphogen Fig 5.13 Model for the
subdivision of the dorso-ventral axis into different regions by the gradient in nuclear dorsal protein
Intra nucleus dorsal high Intra-nucleus dorsal high
Developme nt of mesoderm and gastrulation
The most ventral region become mesoderm (muscle and connective tissue).
Ventral ectoderm becomes neurectoderm (some epidermis and all nervous tissue).
Dorsal ectoderm becomes dorsal epidermis and the amnioserosa (an extra- embryonic membrane).
The endoderm from the terminal regions, give rise to the midgut.
Fig.5.14 The nuclear gradient in dorsal protein is interpreted by the activation of
Dorsal is present at high concentra tion in all the nuclei
In ventral region, dorsal protein activated gene in ventral, but repress gene other genes
dpp (decapentaplegic) protein patterns the dorsal region (morphogen)
A/P and D/V axis is specified by different protein. High dorsal protein in ventral, low dorsal protein in dorsal region. However, the development of dorsal region, may not dorsal protein. It downstream of dorsal protein signal---dpp.
f G f f f
dpp is a member of the TGF-beta family of secreted growth factors.
It produces a gradient of activity by binding an inhibitory protein sog (short gastrulation). Middle region (neuroectoderm) →Sog →bind dpp → prevent dpp enter ectoderm → dpp gradient
Song degrade by tolloid ptoein → dpp high concentration and affect in dorsal region (positive feedback). Fig.5.15
A/P axis is divided into broad regions by gap genes The gap genes, the first genes expressed along
the A/P axis, are transcription factors.
The gap genes are initiated by bicoid gradient in the synctial blastoderm.
Bi id t i i ti t t i
Bicoid protein primary activates anterior expression of hunchback, it acts to help switch on the other gap genes (giant, Krüppel and knirps). Fig.5.16
Mutants of gap genes have large sections of the body pattern missing.
Gap gene proteins are short lived (half-life of minutes) and diffusible
minutes) and diffusible
Control of zygotic hunchback by biocid can set up A/P axis.
Different gap gene is controlled by different concentration of bicoid and hunchback
Fig.5.2
The sequential expression of different sets of gene established the body plan along the A/P axis
Positional informatio After feritilzation, maternal n
gene products laid down in
Regional difference
Parasegments foreshadow segmentation
the egg (bicoid gene) → bicoid mRNA translated → positional information → four classes of zygotic gene (Gap, pair rule, segment polarity and selector gene) expression → different gene determined different function
Segmentation
Segment identity
determined different function
Selector gene also called Homeotic gene)
bicoid protein signals anterior hunchback expression
Zygotic hunchback expression is in the anterior half of the embryo.
Suppression of hunchback (translation level) in the posterior half by nanos protein produces a gradient running A to P
a gradient running A to P.
Anterior expression is switched on by high levels of bicoid. Fig.5.17
Increased anterior bicoid expression will result in extending the hunchback gradient toward
bicoid
Fig 5.18 Zygotic hunchback expression is controlled by bicoid protein.
Hunchback gene Lac Z
promoter
Gene turn on and expressio
Gradient hunchback (hb) activates and represses other gap genes
Form Fig.5.16, found that bicoid combine hunchback can induced many type of gap genes
High hb
low
genes hb
Krüppelis activated by a combination of bicoid and low levels of hunchback but is repressed by high levels of hunchback. Fig. 5.19 TOP This locates Krüppel expression to the centre of
the embryo.
Other gap gene, by a mechanism involving thresholds for repression and activation for A/P thresholds for repression and activation for A/P axis.
In this way the initial gradients of morphogens can lead to the establishment of regions within the syncytial blastoderm which themselves lead to the beginning of segmentation.
Summary :early expression of zygotic genes
A/P
Bicoid protein turn on Gradient of intra-nucleus dorsal D/V
hunchback
↓
Bicoid and hunchback regulated gap genes
↓
Gap gene products and gap gene interaction to sharpen
expression
protein
↓
Ventral activation of twis, snial and decapentaplegic repressed
↓
Decapentaplegic expressed dorsally expression ↓
↓
Axis is divided into unique domains containing different combinations of transcription
factors
↓
Gradient of decapentaplegic activity patterns dorsal region
↓
D/V axis divide into mesoderm, neurectoderm, eidermis,
animoseroa
Technique: transgenic Drosophila
• P element transformation is accomplished by cloning a sequence of interest (genomic region, cDNA or control region/reporter gene fusion) and a marker gene (often the white gene) into a cloned transposable P element.
• The cloned DNA along with a source of transposase (helper plasmid) are injected into the pole plasm of an early embryo.
• If incorporated into the germline, progeny of the injected individual that express the marker gene can be selected.
• These will also carry the transgene.
• Transgenesis allows manipulation of developmental processes segmentation:
pair-rule gene activation
Parasegments (PS) are the basic module of fly development
Parasegments (early embryo) → segments (late embryo) Fig 5.20
Parasegments arise first and each segment is made from the posterior part of one PS and the anterior of the next.
Parasegments are delimited by periodic expression pair rule (PR) genes Parasegments are delimited by periodic expression pair-rule (PR) genes
(even-skipped).
Transient grooves on embryo surface (after gastrulation) define the 14 PS.
Each PS is independent developmental unit.
Each segment is made up of the posterior region of one parasegment (formed anterior part) and anterior region of the next (formed posterior part).
Pair-rule genes delimit the parasegments and are expressed in 7 transverse Pair rule genes delimit the parasegments and are expressed in 7 transverse
stripes (every 2nd parasegment). Fig 5.21
Pair-rule expression determined by gap gene activity (non-repeating pattern) to interpret a series of broad expression patterns to make a repeated series of stripes.
Fig 5.20
The relationship between parasegments and segments.
Pair rule gene: even-skipped Pair rule gene: even skipped Polarity selector gene: engrailed
Fig 5.21
The striped patterns of activity of pair-rule genes in the Drosophlia embryo just before cellularization
Parasegments segments Pair rule gene
pair rule gene even-skipped (blue) fushi tarazu (brown) 7
Gap gene activity positions stripes of pair-rule expression (gap gene regulated pair-rule expression)
Pair-rule genes (7) are expressed in alternate parasegments.
even-skippeddefines odd parasegments.
fushi-tarazudefine even parasegments.
Striped expression pattern of pair-rule genes begins just before cellularization (still a syncytium).
After cellularization, each pair-rule gene is restricted to a few cells in seven stripes. (Fig 5.21)
Each stripe is independently specified by different gap gene product regulated
The 2nd stripe of even-skipped (eve) requires bicoid & hunchback.
giant represses eve to form a sharp anterior border.
Krüppel represses eve to form a sharp posterior border.
Since each stripe is independently controlled by combinations of transcription factors (gap genes), each pair-rule gene has complex control regions with multiple binding sites for each of the different factors.
Some factors activate (activator) and other inactivate (repressor).
Some require the activity of the primary pair-rule genes (such as even and hairy).
Fi 5 23 Sit f ti f ti ti i t i ti f t i th
Fig 5.23 Sites of action of activating repressing transcription factors in the region of the even-skipped promoter involved in expression of the second even-skipped.
1070-1550 base pairs upstream of the transcription start site, directs formation of the second even-skipped stripe.
Repressor may act by preventing binding of activator
Specific Promoter Regions of the even-skipped (eve) Gene Control Specific Transcription Bands in the Embryo
Hypothesis for the formation of the Second Stripe of Transcription from the even-skipped Gene
Fig.5.2
The sequential expression of different sets of gene established the body plan along the A/P axis
Positional information After feritilzation, maternal
gene products laid down in
Regional difference
Parasegments foreshadow segmentation
the egg (bicoid gene) → bicoid mRNA translated → positional information → four classes of zygotic gene (Gap, pair rule, segment polarity and selector gene) expression → different gene determined different function
Segmentation
Segment identity
determined different function
Selector gene also called Homeotic gene)
Maternal mRNAs a) Nurse cells
deposit mRNAs into oocyte b) Bicoid mRNA
localized at anterior, nanos at posterior
Translation a) Hunchback mRNA translation suppressed in posterior by nanos b) Bicoid acts as transcription factor to activate hunchback translation
Gap-genes a) Gap genes are activated/represse d by maternal effect genes b) Gap genes and their products interact to sharpen expression
Pair-rule genes a) Protein products of gap genes interact with their neighboring gap gene’s proteins to activate transcription of pair-rule genes
Segment-polarity genes a) Segment polarity genes act to define an anterior and posterior part of each parasegment posterior
c) Bicoid mRNA translated and forms protein gradient from anterior to posterior d) Nanos mRNA translated and forms
translation c) Bicoid represses caudal translation d) Hunchback and caudaul form opposing gradients, with that of hunchback being strengthened by bicoid
expression boarders c) Embryo is diveded into unique domains
pair rule genes b) Pair-rule genes expressed in seven stripes c) Defines 14 parasegments, altenating between pair-rule expression and none and forms
posterior to anterior protein gradient e) Hunchback and
caudal expressed uniformly
by bicoid
Segment polarity (SP) genes and compartments
Segment polarity genes are a diverse group of genes (not just transcription factors). They:
1) are expressed in 14 stripes, 2) t ft ll l i ti d 2) act after cellularization and, 3) are activated by the pair-rule genes.
engrailed (a transcription factor) is expressed in the anterior of each parasegment to define a boundary of cell lineage restriction.
engrailedis a selector gene which confers identity by a duration of expression.
Pair-rules gene →activate segment polarity gene (engrailed gene)
→ transcription factor →→→ final segmentation
Expression of engrailed delimits a cell lineage boundary and defines a compartment
engrailedis expressed throughout the life of the fly (not transient like gap and other pair rule genes). The activity first appear at the time of cellularization as a series of 14 transverse stripes. Fig.5.24
Engrailedis only expression of a single line of cells at the anterior margin of each parasegment. engrailed defines anterior margin of parasegment and thus the posterior portion of the segment. Fig 5.25
A parasegment is a compartment that cells do not move between (cell lineage restriction).
Anterior part of a parasegment becomes the posterior part of a segment.
So engrailed only expressed in posterior part of the segmentg y p p p g
Engrailed is expressed at the anterior border of each parasegment
Compartments can be detected by marking
Segment vs. Cell lineage restriction
Fig 5.26 The boundary between anterior and posterior compartments in the wing can be demonstrated by marked cell clone
cells and following the fates of the clones (cell‘s descendants ). Gentic control which prevents them mixing with their neighbors and controls their later development.
Compartment boundaries can be studied in the adult wing which is normally divided
i t t i d t i t t
into anterior and posterior compartments.
Engrailed gene is required for the maintenance of the character of the posterior compartment, and for the formation of the boundary.
Segment polarity genes pattern the segments and stabilize parasegment and segment boundaries
Each larval segment has an A/P pattern: the anterior part has denticles while the posterior part has naked cuticle. Fig 5.27
Denticles Naked
Larval had been segment.
Mutations in segment polarity genes often alter the denticle pattern.
In wingless & hedgehog mutants, the naked cuticle is converted to a mirror image duplication of the anterior part to give the
“lawn of denticles" phenotype.
Segment polarity genes are expressed in a restricted subset of the cells of each parasegment.
Parasegment boundary depends on the intercellular signaling between cells on either side of the compartment boundary involving segment polarity genes. g g p y g Wingless gene encodes a secreted protein,
it regulated the formation of vetebrates.
Hedgehog signaling pathway: Fig 5.28 winglessand hedgehog encode highly conserved proteins and are part of a number of signaling systems.
Segment polarity genes are expressed in restricted domain within the parasegment
Parasegment boundary depends on an intercellular signaling circuit being set up between adjacent cell. Fig 5.29
The domain of expression of the segment polarity gene
Cellular blastoderm stage
Anterior expression posterior expression
After gastrulation, patched is expressed
p p
in all of the cells that express neither engrailed nor
hedgehogThe larval denticle patterns depend on the correct establishment and maintenance of parasegment boundaries
Segment polarity gene are expressed in restricted domains within the
Engrailed (transcription factor ) at anterior region → expressed and secreted segment polarity gene (protein) hedgehog (hh) → maintain or activate wingless (in adjacent cell across compartment) → feed back signal → engrailed and hedgehog
Wh i l t t it l d t
When wingless mutant, it leads to loss of the compartment boundary and segment
Fig.5.31
Gradients and asymmetry signal, results in different compartment.
Model for the Transcription of the Segment Polarity Genes engrailed and wingless (wg)
segment polarity gene
Model for the Transcription of the Segment Polarity Genes engrailed and wingless (wg)
Model for the Transcription of the Segment Polarity Genes engrailed and wingless (wg)
Figure 9.26(4) Model for the Transcription of the Segment Polarity Genes
engrailed (en) and wingless (wg)
Fig 5.32 Gradients could specify polarity in the segments of Oncopeltus. Compartment boundaries are involved in patterning and polarizing segments
Gradient –possible of a morphogen-running from the anterior boundary to the posterior boundary of each segment
Segmentation: selector and homeotic genes
Each segment has an unique identity. Fig 5.37
Homeotic selector genes (master gene) specify each segment to control other genes and maintain segment identity.
Homeotic selector genes into two complexes (Bithorax and Homeotic selector genes into two complexes (Bithorax and
Antennapedia: the HOM genes), together are homologous to the HOX gene complexes of vertebrates.
First identified by homeotic genes, mutations in which cause homeosis, the transformation of one structure into another structure (not partly transformation). Fig5.38
Antenna to leg (Antennapedia) or haltere to wing (Bithorax) haltere to wing (Bithorax)
Fig 5.37
The antennapedia and bithorax hometic selector gene complex
Fig 5.38
Homeotic transformation of the wing and haltere by mutations in the bithorax Bithoraz mutant: haltere →partly wing Antenniapedia mutant: antenna → legs
Homeotic genes of the bithorax complex (BX-C) are responsible for the posterior segments
Bithorax complex (BX-C) consists of three homeobox genes (Ubx, ultrabitoraz; abd-A, adbominal-Aand Abd-B, Adbominal-B).
Ubxis expressed in PS 5 and posterior.
abd-Ais expressed in PS 7 and posterior. p p
Abd-Bis expressed in PS 10 and posterior (and suppresses Ubx).
Expression is controlled by gap and pair-rule genes.
Larvae missing the complete bithorax complex develop PS 5-13 as PS4, thus BX-C diversifies PS5-13 and PS4 is the default state modified by the BX-C proteins.
BX-C genes impose a new identity to the segments (selector genes).
Experiment: Replace BX-C components in embryos missing the complex.
BX-C absence: PS1-4, plus ten more PS4 like segments.
Ubx only (missing abd-A and Abd-B):
PS1-6 followed by seven more PS6- like segments. g
Ubx plus abd-A (no Abd-B): PS1-9 plus 4 PS9 segments.
Parasegments must be acting in a combinatorial manner.
While gap and pair-rule genes control the original pattern of HOM gene expression, the polycomb and expression, the polycomb and trithorax gene groups maintain the correct expression of these genes after first four hours.
polycomb group maintain transcriptional repression of homeotic genes.
trithorax group maintain expression of homeotic genes.
Drosophiladevelopment (1)
• Early syncitial development – zygotic nucleus divides 9
times with no cell division
• some nuclei migrate to
• some nuclei migrate to posterior pole to give rise to germ line
– 4 more mitotic divisions without cell division
Drosophiladevelopment (2) At 10 hours, 14 segments
– 3 head – 3 thoracic – 8 abdominal
At 12 hours organogenesis begins At 12 hours, organogenesis begins At 15 hours, exoskeleton begins to form At 24 hours, larva hatches
Drosophiladevelopment (3)
Developmental fate determined through transcription-factor interactions
A-P cardinal genes = gap genes
– Kruppel and knirps (mutants have gap in normal segmentation)g )
– promoters have differential sensitivity to BCD and/or HB-M – establishes different developmental fields along embryo,
roughly defining segments
Bifurcation of development: targets of gap gene encoded transcription factors
– one branch to establish correct number of segments – one branch to assign proper identity to each segment
Drosophiladevelopment (4)
Segment number
– gap gene products activate pair-rule genes
• several different pair-rule genes
• expression produces repeating pattern of seven stripes,expression produces repeating pattern of seven stripes, each offset
– pair-rule products act combinatorially to regulate transcription of segment-polarity genes
• expressed in offset pattern of 14 stripes Segment identity
– gap gene products target cluster of homeotic gene complexes
complexes
• encode homeodomain transcription factors
• mutations alter developmental fate of segment – e.g., Bithorax (posterior thorax and abdomen) and
Antennapedia(head and anterior thorax)
Pattern formation
Transcriptional response to gradients (asymmetrical distribution) of transcription factors
Memory of cell fate
i t ll l d i t ll l iti f db k l – intracellular and intercellular positive-feedback loops – e.g., homeodomain protein binds to enhancer elements of its
own gene, ensuring continued transcription Cell-cell interactions
– inductive interaction commits groups of cells to same developmental fate
– lateral inhibition results in neighboring cells assuming g g g secondary fate
Oncopeltus Different mechanisms used by other insects for the body plan (exceptional case)
Long germ band development : as the blastoderm corresponds to the whole of the future embryo. All segments develop at once (Drosophila).
Short germ band development: (Tribolium, the flour beetle ): the anterior segments are formed in the blastoderm and the more posterior segments are added by growth of the posterior (after completion of the blastoderm stage and gastrulation). Fig 5.34
The mature germ bands appear to be similar (phylotypic stage, common to insects).
Although different growth processes are involved the same genes (i e Although different growth processes are involved, the same genes (i.e.
Krüppel, wingless and engrailed) have conserved functions. Fig 5.35
Tribolium, the flour beetle
Fig 5.34 H-head Th-thorax Ab- abdomen
Fig 5.35
Gap and pair –rule gene expression in long- germ and short-germ insect at the time of germ band formation
Not long-germ, not short- germ model: leaf-hopper Euscelis
Intermediate between short and lone germ modelg Only A/P gradient of morphogen in egg, can directly modulated the development of A/P axis
Summary : segment polarity gene expression defines segment compartment Pair-rule gene expression
↓
Segment polarity and selector gene (engrailed) activated in anterior of each
parasegment,
↓↓
defining anterior compartment of parasegment and posterior compartment of segment
↓
Engrailed-expressing cell also express segment polarity gene (hedghog)
↓
Cells on the other side of compartment boundary express segment polarity gene
( i l ) (wingless)
↓
Engrailed expression maintained and compartment boundary stabilized by wingless
and hedghog protein
↓
Compartment boundary provides signaling center form which segment is patterned
Antennapedia complex controls specification of anterior regions
• Antennapedia complex (Antp-C) consists of 5 homeobox genes.
• Antp-C control expression anterior parasegments in a manner similar to BX-C in the posterior segments (described above).
• deformed mutants affect PS0 and 1.
• Sex combs reduced mutants affect reduced PS2 and 3.
• Antennapedia mutants affect PS4 and 5.
• As with HOX genes in mammals, HOM gene expression order corresponds to the order of genes on the chromosome. p g
• The order of HOM gene expression corresponds to the order of genes along the chromosome
• HOM gene expression in visceral mesoderm control the strucutre of the adjacent gut
