1. The Genetic Basis of Development
Chapter 16
The Genetic Basis of Development
zygote  adult
4 and 6 April, 2005

3. Overview
Instructions in the genome establish the developmental fate of cells in multicellular organisms.
Developmental pathways consist of sequences of various regulatory steps.
The zygote is totipotent, giving rise to all body cells.
Gradients of maternally-derived regulatory proteins establish polarity of the body axis and control transcriptional activation of zygotic genes.
Transcriptional regulation and cell signaling mediate development in animals and plants.
The same set of genes appears to regulate early development in all animals.

4. Development In multicellular organisms, life begins as a single cell.
With few exceptions, somatic cells contain the same genetic information as the zygote.
In development, cells commit to specific fates and differentially express subsets of genes.
Cells identify and respond to their position in developmental fields.
Daughter cells may differ with respect to regulatory instructions and developmental fate.

5. Building the embryo Developmental decisions
made at specific times during development
many are binary, e.g., male or female, germ line or somatic.
most are irreversible
many involve groups of cells rather than single cells
In animals decisions are made to
establish anterior-posterior and dorsal-ventral axes
subdivide anterior-posterior axis into segments
subdivide dorsal-ventral axis into germ layers
produce various tissues and organs
Most decisions involve changes in transcription

6. Sex determination
XX-XY chromosomal systems for sex determination have evolved many times
Different molecular pathways for sex determination in different groups of animals
Drosophila
each cell lineage makes sexual decision
ratio of X chromosomes to autosomes determines sex
cascade of differential mRNA splicing
Mammals
TDF gene on Y chromosome determines maleness
endocrine hormonal system

9. Sxl toggle
Ratio of NUM bHLH proteins to DEM bHLH proteins measures X:A ratio by competing for dimer formation
DNA binding domain of NUM proteins recognizes Sxl early promoter
twice as much NUM protein in females with two X chromosomes as males with one X results in more NUM-NUM homodimers
Sufficient NUM-NUM homodimers activate Sxl early promoter resulting in SXL protein that alternatively splices larger Sxl transcript from late promoter
sets up autoregulatory loop in flies with X:A ratio of 1.0
in flies with X:A ratio of 0.5, insufficient NUM-NUM homodimers results in no SXL protein and late transcript is normally processed (yields nonfunctional protein)

11. Sxl downstream target
SXL protein activates downstream shunt that leads to female development
SXL protein binds to primary transcript of tra (transformer) resulting in spliced transcript that produces TRA protein
TRA protein in turn is RNA-binding protein that produces female-specific splicing of dsx (doublesex) transcript
DSX-F transcription factor represses male-specific gene expression resulting in female development
In absence of SXL, there is no functional TRA protein, and dsx is spliced to produce DSX-M transcription factor which represses female-specific genes, leading to male development

13. Sex determination in mammals
Presence of Y chromosome determines maleness
SRY gene in humans encodes transcription factor (testis-determining factor)
expression of SRY in developing gonad causes it to develop into testis
testis secretes testosterone resulting in male development
In XX individuals, absence of SRY protein and subsequent absence of testosterone results in default female shunt pathway

15. Role of cytoskeleton in development
Consists of highly organized rods and fibers
microfilaments (actin)
intermediate filaments
microtubules
Such structures are polar, with distinct “+” and “–” ends
Serve as highway system for intracellular transport
Asymmetry of cytoskeletal elements plays fundamental roles
location of mitotic cleavage plane
control of cell shape
directed transport of molecules

16. Origin of germ line
In animals, germ line is set aside from soma in early development
only germ cells can undergo meiosis
somatic cells form body of organism
Asymmetric distribution of cytoplasmic particles (e.g., P granules of Caenorhabditis elegans) by cytoskeleton
cells receiving particles develop into germ line
particles anchored to actin in some organisms, to microtubules in others

18. Drosophila anterior-posterior axis
Determined by gradients of BCD (product of bicoid) and HB-M (product of hunchback)
mRNA maternally deposited in egg
BCD mRNA tethered to “–” ends of microtubules via 3’ UTR
HB-M protein gradient depends on NOS protein
nos mRNA tethered to “+” end of microtubule via 3’ UTR
NOS protein gradient blocks translation of hb-m mRNA, resulting in HB-M gradient
Resulting opposite gradients of BCD and Nos determine axis

22. Drosophila dorsal-ventral axis
Determined by gradient of transcription factor DL (encoded by dorsal)
gradient established by interaction of spz and Toll gene products deposited in oogenesis and released during embryogenesis
SPZ-TOLL complex triggers signal transduction pathway in cells that phosphorylates inactive DL
Phosphorylated DL migrates to nucleus, activating genes for ventral fates

25. Positional information
Localization of mRNAs within cell establishes positional information in cases where developmental fields begin as a single cell
Formation of concentration gradients of extracellular diffusible molecules establishes positional information in multicellular developmental fields
works by signal transduction
diffusible molecules are known as morphogens

27. Complex pattern: Drosophila
Successive interpretation of established, changing, and new gradients
Largely due to changes in transcription
Genes targeted by gradients of maternal A-P and D-V transcription factors are cardinal genes
respond to these factors at enhancers and silencers
similar genes in other animals

29. Drosophila development (1)
Early syncitial development
zygotic nucleus divides 9 times with no cell division
some nuclei migrate to posterior pole to give rise to germ line
4 more mitotic divisions without cell division
Nuclei migrate to surface of egg cytoplasm
membrane forms around them (cellularization)
begin responding to positional information in A-P and D-V transcription factor gradients.

31. Drosophila development (2)
At 10 hours, 14 segments
3 head
3 thoracic
8 abdominal
At 12 hours, organogenesis begins
At 15 hours, exoskeleton begins to form
At 24 hours, larva hatches

33. Drosophila development (3)
Developmental fate determined through transcription-factor interactions
A-P cardinal genes = gap genes
Kruppel and knirps (mutants have gap in normal segmentation)
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

34. Drosophila development (4)
Segment number
gap gene products activate pair-rule genes
several different pair-rule genes
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
encode homeodomain transcription factors
mutations alter developmental fate of segment
e.g., Bithorax (posterior thorax and abdomen) and Antennapedia (head and anterior thorax)

40. Pattern formation
Transcriptional response to gradients (asymmetrical distribution) of transcription factors
Memory of cell fate
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 secondary fate

43. Generalizations
Asymmetry of maternal gene products establishes positional information used for early development
Successive rounds of expression of genes encoding transcription factors establish axes and body part identity
Positive feedback loops maintain differentiated state
Components of one developmental pathway are also found in many others
Differences in types and concentrations of transcription factors result in different outputs

44. Developmental parallels
Early animal development follows fundamentally similar pattern
Remarkable similarity among homeotic genes
one HOM-C cluster in insects
four HOX clusters in mammals
paralogous to insect cluster
expressed in segmental fashion in early development
Knockout and genome studies suggest animal development uses same regulatory pathways

47. Development in plants
Plants have different organ systems than animals and plant cells can not migrate
Plants do not separate soma from germ-line until flower development
Plants too have hormones and signaling pathways
Arabidopsis thaliana is model system
Transcriptional regulators control fate of four whorls (layers) giving rise to flower
process similar action of homeotic genes in animal development

48. Assignment: Concept map, Solved Problems 1 and 2, All Basic and Challenging Problems.