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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
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
Drosophila syncitial stage embryo
Fig. 18-7
Chapter 18: Genetic basis of development
*
Fig. 18-8
Genes controlling early development
were discovered in
Drosophia mutant screens
(Nϋsslein-Volhard, Wieschaus, Lewis)
p. 584
A-P axis differentiation by gradients of two proteins
Fig. 18-9
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
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
• 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)
bcd mRNA is localized to the anterior pole
by sequences within its 3’ UTR
Fig. 18-11
The gradient of BCD protein determines
A-P axis cell fates (which cells form cephalic furrow)
Fig. 18-13
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)
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
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
DL nuclear localization is controlled by
a signal transduction cascade
Fig. 18-17
Known types of positional information in embryos
Fig. 18-19
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
Delayed cellularization of the Drosophila embryo
compartmentalizes factors and their gradients
Fig. 18-20
Compartmentalized factors direct
zone-specific development → segments
Fig. 18-21
Loss-of-function mutations of those factors
create segment-specific changes
Fig. 18-22
Gap gene expression determines zonal identity
Pair-rule gene expression drive segmentation
Fig. 18-23
Gap gene expression determines zonal identity
Pair-rule gene expression drive segmentation
ftz and eve expression patterns
Fig. 18-23
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
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
BX-C mutations
can transform the
identities of
posterior
segments
wild-type
bithorax
mutant
(T3
Fig. 18-24
T2)
Embryonic development is driven by a
hierachical cascade of transcription factors
and signalling systems
Fig. 18-26
Hox gene clusters are highly similar
to Drosophila HOM-C gene clusters
…..but, Hox clusters are repeated
Fig. 18-30
Hox gene clusters are highly similar
to Drosophila HOM-C gene clusters
…..but, Hox clusters are repeated
Fig. 18-30
Hox and HOM-C genes are expressed
in similar patterns during development
Fig. 18-30
Testing the role(s) of Hox genes
Hox C8 knockout mice
Homeotic transformation of vertebra L1
Fig. 18-32
Animals exhibit other skeletal defects
Sex determination in mammals vs. flies
Somatic sex differentiation
H. sapiens
Drosophila
XX
female
female
XY
male
male
Sex determination in mammals vs. flies
Somatic sex differentiation
H. sapiens
Drosophila
XX
female
female
XY
male
male
XO
female
male
XXY
male
female
Determined by Y
determined by # of Xs
(Turner)
(Klinefelter)
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)
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
Sex determination in humans directed by
intra- and extra-cellular gene interactions
Fig. 18-33
How are cell fates “sealed” in development?
Models for cellular “memory”
(feedback loops)
Fig. 18-27
Recommended problems in Chapter 18: 11, 15, 21, 24, 32