Developmental Biology

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Developmental Biology

• AP Bio
• 184
• 216
• 472 (part), 3

Wild-type mouse embryo 9.5 days post coitum
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Our focus

• Timing and Coordination
• Gene Expression
• Interactions, Cell Signaling

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Insights into development have been obtained
froms studying

• Slime Molds
• Nematode worm C. elegans
• Fruit Flies
• Zebrafish
• Frog Embryos
• Chick Embryos
• Mice

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Fig. 18-14
(a) Fertilized eggs of a frog
(b) Newly hatched tadpole
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Zebrafish are often used for embryological
research.
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Their embryos are transparent.
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A program of differential gene expression leads
to the different cell types in a multicellular
organism

• During embryonic development, a fertilized egg
  gives rise to many different cell types
• Cell types are organized successively into
  tissues, organs, organ systems, and the whole
  organism
• Gene expression orchestrates the developmental
  programs of animals

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A Genetic Program for Embryonic Development

• The transformation from zygote to adult results
  from cell division, cell differentiation, and
  morphogenesis

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• Cell differentiation is the process by which
  cells become specialized in structure and
  function
• The physical processes that give an organism its
  shape constitute morphogenesis
• Differential gene expression results from genes
  being regulated differently in each cell type
• Materials in the egg can set up gene regulation
  that is carried out as cells divide

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Cytoplasmic Determinants and Inductive Signals

• An eggs cytoplasm contains RNA, proteins, and
  other substances that are distributed unevenly in
  the unfertilized egg
• Cytoplasmic determinants are maternal substances
  in the egg that influence early development
• As the zygote divides by mitosis, cells contain
  different cytoplasmic determinants, which lead to
  different gene expression

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Fig. 18-15a
Unfertilized egg cell
Sperm
Nucleus
Fertilization
Two different cytoplasmic determinants
Zygote
Mitotic cell division
Two-celled embryo
(a) Cytoplasmic determinants in the egg
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• The other important source of developmental
  information is the environment around the cell,
  especially signals from nearby embryonic cells
• In the process called induction, signal molecules
  from embryonic cells cause transcriptional
  changes in nearby target cells
• Thus, interactions between cells induce
  differentiation of specialized cell types

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• Work with the nematode C. elegans has shown that
  induction requires the transcriptional regulation
  of genes in a particular sequence.

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Fig. 18-15b
NUCLEUS
Early embryo (32 cells)
Signal transduction pathway
Signal receptor
Signal molecule (inducer)
(b) Induction by nearby cells
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Sequential Regulation of Gene Expression During
Cellular Differentiation

• Determination commits a cell to its final fate
  it is the progressive restriction of
  developmental potential as the embryo develops
• Determination precedes differentiation
• Cell differentiation is marked by the expression
  of tissue-specific proteins

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For example

• Myoblasts produce muscle-specific proteins and
  form skeletal muscle cells
• MyoD is one of several master regulatory genes
  that produce proteins that commit the cell to
  becoming skeletal muscle
• The MyoD protein is a transcription factor that
  binds to enhancers of various target genes

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Fig. 18-16-1
Nucleus
Master regulatory gene myoD
Other muscle-specific genes
DNA
Embryonic precursor cell
OFF
OFF
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Fig. 18-16-2
Nucleus
Master regulatory gene myoD
Other muscle-specific genes
DNA
Embryonic precursor cell
OFF
OFF
OFF
mRNA
MyoD protein (transcription factor)
Myoblast (determined)
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Fig. 18-16-3
Nucleus
Master regulatory gene myoD
Other muscle-specific genes
DNA
Embryonic precursor cell
OFF
OFF
OFF
mRNA
MyoD protein (transcription factor)
Myoblast (determined)
mRNA
mRNA
mRNA
mRNA
Myosin, other muscle proteins, and cell
cycle blocking proteins
MyoD
Another transcription factor
Part of a muscle fiber (fully differentiated cell)
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Why doesnt myoD change any type of embryonic
cell?

• Probably a combination of regulatory genes are
  necessary for differentiation is required.

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Pattern Formation Setting Up the Body Plan

• Pattern formation is the development of a spatial
  organization of tissues and organs
• In animals, pattern formation begins with the
  establishment of the major axes
• Positional information, the molecular cues
  (cytoplasmic determinants and inductive signals)
  control pattern formation, and tell a cell its
  location relative to the body axes and to
  neighboring cells

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• Pattern formation has been extensively studied in
  the fruit fly Drosophila melanogaster
• Combining anatomical, genetic, and biochemical
  approaches, researchers have discovered
  developmental principles common to many other
  species, including humans

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The Life Cycle of Drosophila

• In Drosophila, cytoplasmic determinants in the
  unfertilized egg determine the axes before
  fertilization
• After fertilization, the embryo develops into a
  segmented larva with three larval stages

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Fig. 18-17a
Thorax
Head
Abdomen
0.5 mm
Dorsal
Right
BODY AXES
Posterior
Anterior
Left
Ventral
(a) Adult
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Fig. 18-17b
Follicle cell
Egg cell developing within ovarian follicle
1
Nucleus
Egg cell
Nurse cell
Egg shell
Unfertilized egg
2
Depleted nurse cells
Fertilization Laying of egg
Fertilized egg
3
Embryonic development
Segmented embryo
4
0.1 mm
Body segments
Hatching
Larval stage
5
(b) Development from egg to larva
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Genetic Analysis of Early Development Scientific
Inquiry

• Edward B. Lewis, Christiane Nüsslein-Volhard, and
  Eric Wieschaus won a Nobel 1995 Prize for
  decoding pattern formation in Drosophila
• Homeotic genes control pattern formation in the
  late embryo, larva, and adult.

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A mutation in regulatory genes, called homeotic
genes, caused this.
Fig. 18-18
Eye
Leg
Antenna
Wild type
Mutant
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Homeotic Genes

• One example are the Hox and ParaHox genes which
  are important for segmentation, another example
  is the MADS-box-containing genes in the ABC model
  of flower development.
• Chap 216

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The Homeobox

• Homeotic genes contain a 180 nucleotide sequence
  called a homeobox found in regulatory genes .
• This homeobox has been found in inverts and verts
  as well as plants.

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• The homeobox DNA sequence evolved very early in
  the history of life and has been conserved
  virtually unchanged for millions of years.
• Differences arise due to different gene
  expressions.

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ABC model of flower development
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Molecular basis of differentiation

• The A, B, and C genes are transcription factors. 
  Different transcription factors are needed
  together to turn on a developmental gene
  program--such as A and B needed to initiate the
  program for petals. What turns on the different
  transcription factors in different cells?
• Induction and inhibition by one cell signaling to
  a neighboring cell.

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Fate Mapping

• Fate maps are general territorial diagrams of
  embryonic development
• Classic studies using frogs indicated that cell
  lineage in germ layers is traceable to blastula
  cells
• Chap 47 (3)

http//education-portal.com/academy/lesson/how-fat
e-mapping-is-used-to-track-cell-development.html
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Fig. 47-21
Epidermis
Centralnervoussystem
Epidermis
64-cell embryos
Notochord
Blastomeresinjected with dye
Mesoderm
Endoderm
Neural tube stage(transverse section)
Larvae
Blastula
(b) Cell lineage analysis in a tunicate
(a) Fate map of a frog embryo
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Fig. 47-21a
Epidermis
Centralnervoussystem
Epidermis
Notochord
Mesoderm
Endoderm
Neural tube stage(transverse section)
Blastula
(a) Fate map of a frog embryo
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Axis Establishment

• Maternal effect genes encode for cytoplasmic
  determinants that initially establish the axes of
  the body of Drosophila
• These maternal effect genes are also called
  egg-polarity genes because they control
  orientation of the egg and consequently the fly

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Bicoid A Morphogen Determining Head Structures

• One maternal effect gene, the bicoid gene,
  affects the front half of the body
• An embryo whose mother has a mutant bicoid gene
  lacks the front half of its body and has
  duplicate posterior structures at both ends

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Fig. 18-19a
EXPERIMENT
Tail
Head
A8
T1
T2
A7
T3
A6
A1
A5
A2
A3
A4
Wild-type larva
Tail
Tail
A8
A8
A7
A7
A6
Mutant larva (bicoid)
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Fig. 18-19b
RESULTS
Fertilization, translation of bicoid mRNA
Anterior end
100 µm
Bicoid protein in early embryo
Bicoid mRNA in mature unfertilized egg
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Fig. 18-19c
CONCLUSION
Nurse cells
Egg
bicoid mRNA
Developing egg
Bicoid mRNA in mature unfertilized egg
Bicoid protein in early embryo
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Bicoid mRNA, Bicoid Protein (red)
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• This phenotype suggests that the product of the
  mothers bicoid gene is concentrated at the
  future anterior end
• This hypothesis is an example of the gradient
  hypothesis, in which gradients (amounts) of
  substances called morphogens establish an
  embryos axes and other features

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• The bicoid research is important for three
  reasons
• It identified a specific protein required for
  some early steps in pattern formation
• It increased understanding of the mothers role
  in embryo development
• It demonstrated a key developmental principle
  that a gradient of molecules can determine
  polarity and position in the embryo

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Cancer results from genetic changes that affect
cell cycle control

• The gene regulation systems that go wrong during
  cancer are the very same systems involved in
  embryonic development

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Types of Genes Associated with Cancer

• Cancer can be caused by mutations to genes that
  regulate cell growth and division
• Tumor viruses can cause cancer in animals
  including humans
• Oncogenes are cancer-causing genes
• Proto-oncogenes are the corresponding normal
  cellular genes that are responsible for normal
  cell growth and division
• Conversion of a proto-oncogene to an oncogene can
  lead to abnormal stimulation of the cell cycle

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Fig. 18-20
Proto-oncogene
DNA
Point mutation
Gene amplification
Translocation or transposition
within the gene
within a control element
New promoter
Oncogene
Oncogene
Normal growth- stimulating protein in excess
Normal growth-stimulating protein in excess
Normal growth- stimulating protein in excess
Hyperactive or degradation- resistant protein
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• Proto-oncogenes can be converted to oncogenes by
• Movement of DNA within the genome if it ends up
  near an active promoter, transcription may
  increase
• Amplification of a proto-oncogene increases the
  number of copies of the gene
• Point mutations in the proto-oncogene or its
  control elements causes an increase in gene
  expression

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Tumor-Suppressor Genes

• Tumor-suppressor genes help prevent uncontrolled
  cell growth
• Mutations that decrease protein products of
  tumor-suppressor genes may contribute to cancer
  onset
• Tumor-suppressor proteins
• Repair damaged DNA
• Control cell adhesion
• Inhibit the cell cycle in the cell-signaling
  pathway

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Interference with Normal Cell-Signaling Pathways

• Mutations in the ras proto-oncogene and p53
  tumor-suppressor gene are common in human cancers
• Mutations in the ras gene can lead to production
  of a hyperactive Ras protein and increased cell
  division

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Fig. 18-21a
1
1
Growth factor
MUTATION
Hyperactive Ras protein (product
of oncogene) issues signals on its own
Ras
G protein
3
GTP
Ras
GTP
Protein kinases (phosphorylation cascade)
2
Receptor
4
NUCLEUS
Transcription factor (activator)
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DNA
Gene expression
Protein that stimulates the cell cycle
(a) Cell cyclestimulating pathway
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Fig. 18-21b
Protein kinases
2
MUTATION
Defective or missing transcription factor,
such as p53, cannot activate transcription
Active form of p53
3
UV light
DNA damage in genome
1
DNA
Protein that inhibits the cell cycle
(b) Cell cycleinhibiting pathway
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Fig. 18-21c
EFFECTS OF MUTATIONS
Protein overexpressed
Protein absent
Cell cycle not inhibited
Increased cell division
Cell cycle overstimulated
(c) Effects of mutations
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• Suppression of the cell cycle can be important in
  the case of damage to a cells DNA p53 prevents
  a cell from passing on mutations due to DNA
  damage
• Mutations in the p53 gene prevent suppression of
  the cell cycle

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The Multistep Model of Cancer Development

• Multiple mutations are generally needed for
  full-fledged cancer thus the incidence increases
  with age
• At the DNA level, a cancerous cell is usually
  characterized by at least one active oncogene and
  the mutation of several tumor-suppressor genes

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Hedgehog Signaling Pathway
http//www.youtube.com/watch?vFCNJp6Y901M
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• The Hedgehog Pathway is very important in
  development but after adult state is reached it
  is used in maintenance of stem cells.

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Development in AnimalsChap 47 (part of 2, 3)

• Timing and coordination to produce stages
• After fertilization, embryonic development
  proceeds through cleavage, gastrulation, and
  organogenesis
• The sperms contact with the eggs surface
  initiates metabolic reactions in the egg that
  trigger the onset of embryonic development

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• Important events regulating development occur
  during fertilization and the three stages that
  build the animals body
• Cleavage cell division creates a hollow ball of
  cells called a blastula
• Gastrulation cells are rearranged into a
  three-layered gastrula
• Organogenesis the three layers interact and move
  to give rise to organs

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Fig. 47-6
(a) Fertilized egg
(b) Four-cell stage
(c) Early blastula
(d) Later blastula
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Gastrulation

• Gastrulation rearranges the cells of a blastula
  into a three-layered embryo, called a gastrula,
  which has a primitive gut
• The three layers produced by gastrulation are
  called embryonic germ layers
• The ectoderm forms the outer layer
• The endoderm lines the digestive tract
• The mesoderm partly fills the space between the
  endoderm and ectoderm

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These germ layers become

• Ectoderm skin and nervous system
• Mesoderm skeleton, muscles, circulatory, lining
  of body cavity
• Endoderm lining of digestive and respiratory
  tract, liver, many glands (pancreas, thymus,
  thyroid, parathyroid)

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Fig. 47-14
ECTODERM
MESODERM
ENDODERM
NotochordSkeletal systemMuscular
systemMuscular layer ofstomach and
intestineExcretory systemCirculatory and
lymphaticsystems Reproductive system(except
germ cells) Dermis of skinLining of body
cavityAdrenal cortex
Epidermis of skin and itsderivatives (including
sweatglands, hair follicles)Epithelial lining
of mouthand anusCornea and lens of eyeNervous
systemSensory receptors inepidermisAdrenal
medullaTooth enamelEpithelium of pineal
andpituitary glands
Epithelial lining ofdigestive tractEpithelial
lining ofrespiratory systemLining of urethra,
urinarybladder, and reproductivesystemLiverPan
creasThymusThyroid and parathyroidglands
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Fig. 47-9-6
Key
Future ectoderm
Future mesoderm
Future endoderm
Archenteron
Blastocoel
Filopodiapullingarchenterontip
Animalpole
Blastocoel
Archenteron
Blastocoel
Blastopore
Mesenchymecells
Ectoderm
Vegetalplate
Vegetalpole
Mouth
Mesenchymecells
Mesenchyme(mesodermforms futureskeleton)
Digestive tube (endoderm)
Blastopore
50 µm
Anus (from blastopore)
http//www.gastrulation.org/Movie9_3.mov
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Gastrulation in the frog
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• Early in vertebrate organogenesis, the notochord
  forms from mesoderm, and the neural plate (which
  will becomes the nervous system) forms from
  ectoderm

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Fig. 47-12
Tail bud
Somites
Eye
Neural folds
Neural plate
Neuralfold
SEM
1 mm
1 mm
Neural crestcells
Neural tube
Neuralfold
Neural plate
Notochord
Coelom
Neural crestcells
Somite
Notochord
Ectoderm
Archenteron(digestivecavity)
Outer layerof ectoderm
Mesoderm
Endoderm
Neural crestcells
(c) Somites
Archenteron
(a) Neural plate formation
Neural tube
(b) Neural tube formation
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Fig. 47-21a
Epidermis
Centralnervoussystem
Epidermis
Notochord
Mesoderm
Endoderm
Neural tube stage(transverse section)
Blastula
(a) Fate map of a frog embryo
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In humans,

• At completion of cleavage, the blastocyst forms
• A group of cells called the inner cell mass
  develops into the embryo
• The trophoblast, the outer epithelium of the
  blastocyst, initiates implantation in the uterus.
• As implantation is completed, gastrulation begins

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Fig. 47-16-1
Endometrialepithelium(uterine lining)
Uterus
Inner cell mass
Trophoblast
Blastocoel
will become embryo
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Restriction of the Developmental Potential of
Cells

• In many species that have cytoplasmic
  determinants, only the very early stages of the
  embryo are totipotent.
• That is, only the zygote can develop into all the
  cell types in the adult
• As embryonic development proceeds, potency of
  cells becomes more limited

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Stem Cells of Animals

• A stem cell is a relatively unspecialized cell
  that can reproduce itself indefinitely and
  differentiate into specialized cells of one or
  more types
• Stem cells isolated from early embryos at the
  blastocyst stage are called embryonic stem cells
  these are able to differentiate into all cell
  types
• The adult body also has stem cells, which replace
  nonreproducing specialized cells

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Spemann Experiment
Gray crescent not bisected equally
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• The gray crescent acts as an organizer by
  inducing cells to become certain parts of the
  embryo.
• A signaling protein perhaps?

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Importance of Apoptosis in development

• Elimination of transitory organs and tissues.
  Examples include tadpole tails and gills.
• Tissue remodeling.
• Vertebrate limb bud development, removal of
  interdigital skin.
• Nutrients are reused!

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When the grim and reaper genes work together,
they help guide cells in flies through their
death process, apoptosismuch like that spectre
of 15th century folklore, the Grim Reaper.
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Comparing plant and animal development

• Since plants have rigid cell walls, there is no
  morphogenetic movement of cells.
• plant development depends upon differential rates
  of cell division then directed enlargement of
  cells.

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• All postembryonic growth occur at meristems which
  give rise to all adult structures (shoots, roots,
  stems, leaves and flowers) and have the capacity
  to divide repeatedly and give rise to a number of
  tissues (like stem cells).
• Two meristems are established in the embryo, one
  at the root tip and one at the tip of the shoot.
  
• The developmental patterning of organs therefore
  continues throughout the life of the plant.

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• Their fate is determined largely by their
  position but they do have signaling.
• Homeotic genes control organ identity (ABC model)
  but genes are called Mad-box genes instead of Hox
  genes

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Similarities in development of plants and animals

• Both involve a cascade of transcription factors
• But differences in regulatory genes as stated in
  previous slide

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Evo-DevoComparing developmental processes of
different multicellular organisms

• Many groups of animals and plants, even distantly
  related ones, share similar molecular mechanisms
  for morphogenesis and pattern formation.
• These mechanisms can be thought of as genetic
  toolkits.

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• Development produces morphology and much of
  morphological evolution occurs by modifications
  of existing development genes and pathways rather
  than the introduction of radically new
  developmental mechanisms.

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Our common ancestor