Major questions in developmental biology

1
Major questions in developmental biology
Single genome Diverse cell
types Totipotent zygote Fate
refinement Diverse cell fates Cell
commitments are largely driven by cell positions
within a developmental field
2

• Major cellular developmental decisions
• Establish basic body plan coordinates
• (anterior-posterior, dorsal-ventral)
• Subdivision of anterior-posterior axis
• (segmentation into metameres, specification
• of fates for each segment)
• Subdivision of dorsal-ventral axis
• (differentiation of primary germ layers
• endoderm, mesoderm, ectoderm)
• Organ/tissue differentiation

3
Drosophila syncitial stage embryo
Fig. 18-7
4
Chapter 18 Genetic basis of development

Fig. 18-8
5
Genes controlling early development were
discovered in Drosophia mutant
screens (N?sslein-Volhard, Wieschaus, Lewis)
p. 584
6
A-P axis differentiation by gradients of two
proteins
Fig. 18-9
7

• Major morphogens directing
• A/P axis formation in Drosophila
• BCD (bcd gene) directs anterior development
• transcription factor mRNA is localized
  mutations are
• tail duplications (bicaudal embryos)
• HB-M (maternal hb gene) differentiates axial
• development transcription factor mRNA
  unlocalized
• NOS (nos gene) directs posterior development
  
• translation repressor mRNA is localized
  mutations are
• head duplications
• All three are present in gradients in embryos

8
bcd nos mRNAs are tightly localized - BCD and
NOS proteins form concentration gradients
bcd mutation ? double-posterior embryo nos
mutation ? double-anterior embryo
Fig. 18-10
9

• BCD gradient results from diffusion of localized
  RNA
• (NOS gradient is similar)
• HB-M gradient results from translational
  repression
• by NOS protein
• Net effect cells along the A-P axis of the
  embryo
• have distinctive combinations of
  concentrations of
• BCD and HB-M transcription factors
• (Experimental perturbations of the gradients
• demonstrate their roles in determining the A-P
  axis)

10
bcd mRNA is localized to the anterior pole by
sequences within its 3 UTR
Fig. 18-11
11
The gradient of BCD protein determines A-P axis
cell fates (which cells form cephalic furrow)
Fig. 18-13
12

• D-V axis is specified by cell-cell signalling
• system in Drosophila
• DL protein (dl gene) transcription factor
  uniform
• distribution but localization gradient
  highest nuclear
• localization in ventral areas
• SPZ protein (spz gene) extracellular ligand
  for TOLL
• receptor secreted assymetrically by follicle
  cells during
• embryogenesis gradient most concentrated in
  ventral area
• TOLL protein (Tl gene) transmembrane receptor
  
• activates signal cascade resulting in
  phosphorylation
• of CACT protein uniform distribution
• CACT protein (cact gene) cytosolic protein
  uniform
• distribution unphosphorylated form binds DL
  phosphorylated
• form releases DL (permitting DL nuclear
  localization)

13
D-V polarity is determined by distribution of the
DL protein (transcription factor)
DL quantity is similar in all cells Nuclear
localization differs in D-V axis Nuclear DL
activates ventralizing genes
Fig. 18-15
14
DL nuclear localization is controlled by a signal
transduction cascade

• Loss-of-function mutations that
• produce dorsalized embryos
• (nuclear DL nowhere)
• spz
• toll
• dorsal
• Loss-of-function mutations that
• produce ventralized embryos
• (nuclear DL everywhere)
• cact

Fig. 18-17
15
DL nuclear localization is controlled by a signal
transduction cascade
Fig. 18-17
16
Known types of positional information in embryos
Fig. 18-19
17
A-P and D-V axes are defined by morphogens (BCD,
HB-M, DL) encoded by maternal-acting
genes These transcription factors differentially
activate a set of zygotic-acting genes the
cardinal genes A-P axis cardinal genes are
called gap genes (specify general body
regions) Gap genes encode transcription factors
and activate the set of pair rule genes
(cardinal genes specifying alternating
segments creating segments) Pair rule genes
encode transcription factors and activate
the set of segment polarity genes (cardinal genes
that distinguish anterior/posterior
compartments of each segment) Segment polarity
genes differentially activate the segment
identity genes
18
(No Transcript)
19
Delayed cellularization of the Drosophila
embryo compartmentalizes factors and their
gradients
Fig. 18-20
20
Compartmentalized factors direct zone-specific
development ? segments
Fig. 18-21
21
Loss-of-function mutations of those
factors create segment-specific changes
Fig. 18-22
22
Gap gene expression determines zonal
identity Pair-rule gene expression drive
segmentation
Fig. 18-23
23
Gap gene expression determines zonal
identity Pair-rule gene expression drive
segmentation
ftz and eve expression patterns
Fig. 18-23
24
A-P and D-V axes are defined by morphogens (BCD,
HB-M, DL) encoded by maternal-acting
genes These transcription factors differentially
activate a set of zygotic-acting genes the
cardinal genes A-P axis cardinal genes are
called gap genes (specify general body
regions) Gap genes encode transcription factors
and activate the set of pair rule genes
(cardinal genes specifying alternating
segments creating segments) Pair rule genes
encode transcription factors and activate
the set of segment polarity genes (cardinal genes
that distinguish anterior/posterior
compartments of each segment) Segment polarity
genes differentially activate the segment
identity genes
25
Segment identity genes are mostly found in
the homeotic gene complexes ANT-C
(Antennapedia complex) genes for anterior
segment identity BX-C (Bithorax
complex) genes for posterior segment
identity
Fig. 18-24
26
BX-C mutations can transform the identities of
posterior segments
wild-type
bithorax mutant (T3 T2)
Fig. 18-24
27
Embryonic development is driven by a hierachical
cascade of transcription factors and signalling
systems
Fig. 18-26
28
Hox gene clusters are highly similar to
Drosophila HOM-C gene clusters ..bu
t, Hox clusters are repeated
Fig. 18-30
29
Hox gene clusters are highly similar to
Drosophila HOM-C gene clusters ..bu
t, Hox clusters are repeated
Fig. 18-30
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Hox and HOM-C genes are expressed in similar
patterns during development
Fig. 18-30
31
Testing the role(s) of Hox genes Hox C8 knockout
mice
Homeotic transformation of vertebra L1
Animals exhibit other skeletal defects
Fig. 18-32
32
Sex determination in mammals vs.
flies Somatic sex differentiation H.
sapiens Drosophila XX female female XY male
male
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Sex determination in mammals vs.
flies Somatic sex differentiation H.
sapiens Drosophila XX female female XY male
male XO female male (Turner) XXY male
female (Klinefelter) Determined by
Y determined by of Xs
34

• Sex determination in mammals
• General biological context
• Hormonally mediated (androgens)
• Individual cells do not determine their own sex
• (no mosaicism)
• Early gonad indifference (to about two months
• gestation)

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• Sex differentiation controlled by
• Y-linked transcription factor gene
• Y-linked gene (SRY in humans) directs
  testosterone
• production in Leydig cells of indifferent gonad
• (loss-of-function SRY- develops female)
• Testosterone activates steroid receptors (e.g.,
  Tfm receptor)
• that lead to male differentiation of
  target organs/tissues
• Failure to activate receptors leads to female
  
• differentiation (default pathway)
• Translocation of Sry (mouse) to other
  chromosomes transfers
• sex determination cue to those chromosomes
• Binary switch is presence/absence of
  functional SRY gene
• copy in Leydig cells of the indifferent gonad

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Sex determination in humans directed by intra-
and extra-cellular gene interactions
Fig. 18-33
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How are cell fates sealed in development?
Models for cellular memory (feedback loops)
Fig. 18-27
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Recommended problems in Chapter 18 11, 15, 21,
24, 32