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

Fig. 18-7

Drosophila syncitial stage embryo

Fig. 18-8

Chapter 18: Genetic basis of development

*

p. 584

Genes controlling early developmentwere discovered in

Drosophia mutant screens

(Nϋsslein-Volhard, Wieschaus, Lewis)

Fig. 18-9

A-P axis differentiation by gradients of two proteins

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

Fig. 18-10

bcd & nos mRNAs are tightly localized- BCD and NOS proteins form concentration gradients

bcd mutation → double-posterior embryonos mutation → double-anterior embryo

• 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)

Fig. 18-11

bcd mRNA is localized to the anterior poleby sequences within its 3’ UTR

Fig. 18-13

The gradient of BCD protein determinesA-P axis cell fates (which cells form cephalic furrow)

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)

Fig. 18-15

D-V polarity is determined by distributionof the DL protein (transcription factor)

DL quantity is similar in all cells

Nuclear localization differs in D-V axis

Nuclear DL activates “ventralizing” genes

DL nuclear localization is controlled bya signal transduction cascade

Fig. 18-17

Loss-of-function mutations thatproduce “dorsalized” embryos(nuclear DL nowhere):

•spz•toll•dorsal

Loss-of-function mutations thatproduce “ventralized” embryos(nuclear DL everywhere):

•cact

DL nuclear localization is controlled bya signal transduction cascade

Fig. 18-17

Fig. 18-19

Known types of positional information in embryos

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

Fig. 18-20

Delayed cellularization of the Drosophila embryocompartmentalizes factors and their gradients

Fig. 18-21

Compartmentalized factors directzone-specific development → segments

Fig. 18-22

Loss-of-function mutations of those factorscreate segment-specific changes

Fig. 18-23

Gap gene expression determines zonal identityPair-rule gene expression drive segmentation

Fig. 18-23

Gap gene expression determines zonal identityPair-rule gene expression drive segmentation

ftz and eve expression patterns

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

Fig. 18-24

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

BX-C mutations can transform theidentities of posterior segments

wild-type

bithorax mutant

(T3 T2)

Fig. 18-24

Fig. 18-26

Embryonic development is driven by ahierachical cascade of transcription factors

and signalling systems

Fig. 18-30

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

Hox and HOM-C genes are expressed in similar patterns during development

Fig. 18-30

Fig. 18-32

Testing the role(s) of Hox genesHox C8 knockout mice

Homeotic transformation of vertebra L1 Animals exhibit other skeletal defects

Sex determination in mammals vs. flies

Somatic sex differentiationH. sapiens Drosophila

XX female female

XY male male

Sex determination in mammals vs. flies

Somatic sex differentiationH. sapiens Drosophila

XX female female

XY male male

XO female male (Turner)

XXY male female(Klinefelter)

Determined by Y determined by # of Xs

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

Fig. 18-33

Sex determination in humans directed byintra- and extra-cellular gene interactions

How are cell fates “sealed” in development?

Fig. 18-27

Models for cellular “memory”(feedback loops)

Recommended problems in Chapter 18: 11, 15, 21, 24, 32