Download Document

Chapter 16
The Genetic Basis of
Development
zygote  adult
4 and 6 April, 2005
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.
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.
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
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
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 NUMNUM 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)
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 femalespecific 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
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
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
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
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
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
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
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
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.
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
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
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)
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
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
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
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
Assignment: Concept map, Solved
Problems 1 and 2, All Basic and
Challenging Problems.