Developmental Genetics

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Developmental Genetics

• Chapter 17

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The Model Organisms of Developmental Biology

• Which group of organisms does each model
  represent?
• What features of Drosophila melanogaster,
  Caenorhabditis elegans, Mus musculus, and
  Arabidopsis thaliana have made these organisms
  valuable models in developmental genetics?

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Fruit Fly

• D. melanogaster
• Small chromosome (2N4)
• Mutants have been around for a long time
• Developmental mutations identified
• Genes that determine the body plan of an organism
  
• It turns out that
• Most body building and organ forming genes in the
  fruit fly have a direct counterpart in mammals
  (including humans)
• All animals share a common toolkit of body
  building genes

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Drosophila Life Cycle

• Egg is fertilized
• Embryogenesis results in a sexually immature
  larva
• The periods between molts are called instars.
• Pupae metamorphosize into sexually mature adults

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Positional information during development

• Each cell in the body must become the appropriate
  cell type based on its relative position
• Each cell receives positional information that
  tells it where to go and what to become
• Cells may respond by
• Cell division,
• cell migration,
• cell differentiation or
• cell death (apoptosis)

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Position or Spatial Organization is Everything

• 2 main mechanisms used to communicate positional
  information
• Morphogens
• Cell adhesion

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Morphogens

• Give positional information and promote cellular
  changes
• Act in a concentration dependent manner with a
  critical threshold concentration
• Distributed asymmetrically
• In the oocyte or egg precursor
• In the embryo by secretion and transport

Cell migration in C. elegans
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Cell adhesion

• Each cell makes its own cell adhesion molecules
  (CAMs)
• Positioning of a cell within a multicellular
  organism is strongly influenced by the
  combination of contacts it makes with other cells
  and with the extracellular matrix

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Animal development

• Drosophila model
• Oocyte establishes pattern for adult
• Elongated cell with positional information
• After fertilization, zygote develops into
  blastoderm
• Series of nuclear divisions without cytoplasmic
  division (produces many free nuclei) synctial
  blastoderm
• Individual cells are created after nuclei line up
  along cell membrane (cellular blastoderm)

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• Gastrulation involves cells migrating to the
  interior
• 3 cell layers formed- ectoderm, mesoderm and
  endoderm
• Segmented body plan develops
• Head, thorax and abdomen
• Larva free living
• Pupa undergoes metamorphosis
• Adult
• Egg to adult in 10 days

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Gastrulation and in Drosophila

• Three tissue layers of the embryo
• Mesoderm invaginates
• In insects, the nerve cord lies ventrally
• Segmentation begins

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Hierarchy of transcription factors

• Four general phases for body formation
• Organize body along major axes
• Organize into smaller regions (organs, legs)
• Cells organize to produce body parts
• Cells themselves change morphologies and become
  differentiated
• Differential gene regulation certain genes
  expressed at specific phase of development in a
  particular cell type
• Parallel between phases and expression of
  specific transcription factors

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Four general phases for body formation

• Organize body along major axes
• Organize into smaller regions (organs, legs)
• Cells organize to produce body parts
• Cells themselves change morphologies and become
  differentiated

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Phase 1 Pattern developmentMaternal Effect Genes

• Genes that are transcribed in maternal tissues
  and transported to the egg
• Morphogens are distributed prior to fertilization
• The genes that organize the structure of the egg
• First phase is establishment of body axes

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Phase 1 Pattern development

• Bicoid, example morphogen
• Mutation results in larva with 2 posterior ends
• The spiracle is the tracheal opening

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• Nurse cells are located near anterior end of
  oocyte
• Bicoid gene transcribed in nurse cells and mRNA
  transported into anterior end of oocyte

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Maternal Effect Genes

• Bicoid and Nanos mRNA attach to the cytoskeleton
  of the egg and travel to their respective
  positions forming a gradient
• Translation of Bicoid and Nanos transcription
  factors occurs in the embryo
• Bicoid and Nanos transcription factors activate
  particular genes at specific times
• Asymmetrical distribution means they will
  activate other genes only in certain regions

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Phase 2 Segmentation genes act sequentially to
divide the embryo into segments

• Normal Drosophila embryo divided into 15 segments
• 3 head, 3 thoracic and 9 abdominal
• Each will give rise to unique morphological
  features in adult

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Segmentation Genes

• Generate repeating pattern of body segments in
  embryo
• Gap genes
• organize anterior, middle, posterior regions
• Pair-rule genes
• affect all segments
• Segment polarity genes
• affect all segments

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Gap Genes

• Are the first set of segmentation genes to act
• Act on small groups of body segments
• A mutation in a gap gene causes the absence of
  one or more body segments in an embryo

• Proteins of these genes are expressed in normal
  embryos
• The same region is absent in embryos where the
  gene is mutated

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Maternal Effect and Gap Genes
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Gap Genes

• hunchback
• krüppel
• giant
• tailless
• knirpe
• All encode transcription factors
• Cause the absence of one or more body segments in
  an embryo

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Pair-Rule Genes

• Affect all segments
• Mutations in pair-rule genes delete every other
  segment
• All are transcription factors
• hairy
• even-skipped
• runt
• fushi-tarazu

• Proteins of these genes are expressed in normal
  embryos
• The same region is absent in embryos where the
  gene is mutated

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Pair-Rule Genes
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Segment Polarity Genes

• Produce segments in which one part is missing and
  the other is duplicated as a mirror image
• Many segment polarity genes, unlike gap and
  pair-rule genes, remain active throughout
  development the segment polarity network
  remembers the pattern imprinted upon it, then
  provides positional read-outs for subsequent
  developmental processes.

• Proteins of these genes are expressed in normal
  embryos
• The same region is absent in embryos where the
  gene is mutated

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Segment polarity genes at work in a Fruit fly

• The pattern of activity of the Engrailed gene. It
  marks the back-end of each developing segment of
  the insects body
• Engrailed is a transcription factor

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Segment Polarity Gene Networks

• Hedgehog HH
• Wingless WG

• Engrailed EN
• Patched PTC

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Hierarchy of transcription factors

• To create a segment in phase 2, a group of genes
  acts sequentially to govern the fate of a given
  body region
• Maternal effect genes, which promote phase 1
  pattern development, activate gap genes
• Seen as broad bands of gap gene expression in the
  embryo
• Gap genes and maternal effect genes then activate
  the pair-rule genes in alternating stripes in the
  embryo
• Once the pair-rule genes are activated, their
  gene products then regulate the segment-polarity
  genes
• Expression of a segment-polarity gene corresponds
  to portions of segments in the adult fly

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Phase 3 Homeotic Genes Control the development
of segment characteristics

• Role of homeotic genes to determine identity of
  particular segments
• Genes that alter how each segment develops
• Homeobox genes (HOX genes)
• One body part is transformed into another
• A leg grows instead of antennae
• Specify identity of each segment

Antennapedia complex
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Homeotic Genes in Fruit Flies

• As early as 1915 two fruit fly mutants were
  discovered
• By 1983, two clusters of genes residing on
  chromosome 3 were isolated
• Bithorax complex 3 genes that affect the
  posterior end of the fly
• Antennapedia complex 5 genes that affect the
  anterior part of the fly

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• Bithorax gene complex
• Normal wings on 2nd thoracic segment and 2
  halteres on 3rd thoracic segment (far left photo,
  halteres in white)
• Mutant 3rd segment has wings so 2 sets of wings
  and no halteres

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• Antennapedia complex
• 5 genes that affect the anterior part of the fly
• When mutated, legs grow instead of antennae

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Homeobox Genes

• Fruit flies have 8 homeobox genes.
• All 8 of the homeotic genes have a short stretch
  of 180 bases that are similar in sequence called
  a Homeobox
• The homeobox is also found in other genes that
  are not homeotic genes such as bicoid.

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• 2 domains in regulatory transcription factors
• Site where protein binds to DNA (homeodomain)
• Site for small effector molecule

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• Homeotic genes encode homeotic proteins that
  functionas transcription factors
• Activate transcription of specific genes that
  promote developmental changes
• Homeobox coding sequence of homeotic genes
  contains 180-bp sequence
• Encodes homeodomain for DNA binding

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A Homologous Group of Homeotic Genes Is Found in
All Animals

• Vertebrate Hox genes are homologous to those that
  control development in simpler organisms such as
  Drosophila
• Homologous genes are evolutionarily derived from
  the same ancestral gene and have similar DNA
  sequences
• Hox genes in mice
• Follow colinearity rule
• Key role in patterning anteroposterior axis

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• Fruit flies have only one Antennepedia-bithorax
  complex
• Humans and many other vertebrates have 4 similar
  Hox gene clusters
• They probably arose through gene duplication
• Hox genes shape the number and appearance of body
  segments (repeated structures) along the main
  body axes of both vertebrates and invertebrates

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Homeotic genes in Mus

• The mouse has Hox genes on 4 different
  chromosomes
• Five antennapedia genes (1,2, 4,5,6)
• Three bithorax genes

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Hox genes

• Found in all animals
• Genetic variation may have been critical event in
  the formation of new body plans
• Number and arrangement of Hox genes varies among
  different types of animals
• Increases in the number of Hox genes may have led
  to greater complexity in body structure

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Hox genes

• Three lines of evidence support the idea that Hox
  gene complexity has been instrumental in the
  evolution and speciation of animals with
  different body patterns
• Hox genes are known to control body development
• General trend for simpler animals to have fewer
  Hox genes and Hox gene clusters
• Comparison of Hox gene evolution and animal
  evolution bear striking parallel

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Hox genes in the Animal Kingdom
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Hox Genes and EvoDevo
A anterior Group 3 C Central PPosterior
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Hox genes determine the number and types of
vertebrae in animals

• Hoxc-6 determines that in the chicken the 7
  vertebrae will develop into ribs
• Snake Hoxc-6 is expanded dramatically toward the
  head and toward the rear.

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Hox Genes and the Arthropods

• Differential gene expression of the same Hox
  genes explains the diversity of body plans in the
  arthropods.
• of segments is the same but,
• the expression of Hox genes is shifted

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Phase 4 Cell Differentiation

• Emphasis shifts to cell differentiation
• Studied in mammalian cell culture lines
• Differential gene expression underlies cell
  differentiation
• Stem cell characteristics
• Capacity to divide
• Daughter cells can differentiate into 1 or more
  cell types

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• Stem cell characteristics
• Capacity to divide
• Daughter cells can differentiate into 1 or more
  cell types

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Stem cell categories

• Pluripotent
• Embryonic stem cells (ES cells)
• Embryonic germ cells (EG cells)
• Can differentiate into almost any cell but a
  single cell has lost the ability to produce an
  entire individual
• Multipotent
• Adult stem cells
• Can differentiate far fewer types of cells
• Hematopoietic stem cells (HScs)

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The Fates of Hematopoietic Stem Cells
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• Advantages
• Can be used to cure some diseases
• Directed differentiation possible
• If isolated from the same patient, no tissue
  rejection issues
• No ethical issues
• Disadvantages
• No predictable location in adult tissues
• Limited tools for identifying them
• Limited regeneration of certain cell types)

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The Promise of ESC

• In the Future
• insulin producing islet cells (pancreas)
• dopamine producing cells
• cardiac tissue for heart patients
• skin tissue for burn victims
• bone tissue for osteoporosis

• ESs can become specialized cells when scientists
  use
• growth factors
• hormones

Cardiomyocyte differentiated from human embryonic
stem cells.
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Master Genes and Body PlansGenerating the
Diversity of Life

• Many genes have since been discovered that
  determine development in animals as distant as
  vertebrates, invertebrates and other animals
• Each of these proteins contains a homeodomain,
  this means that they are all DNA binding proteins
  (but are not Hox genes)

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Master Genes and Body Plans

• Systems of master genes specifying organisms
  body plan
• Segmentation genes orientation of segments
• Hox development of body plan
• Pax-6 development of eyes
• Dll (distal-less) controls the development of
  limbs
• Tinman development of the heart
• BMP and Gremlin
• Pitx1 (Stickleback fish)

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The Study of the Pax6 Gene Indicates That
Different Types of Eyes Evolved from a Simpler
Form

• Explaining how a complex organ comes into
  existence is a major challenge
• Researchers have discovered many different types
  of eyes
• Thought that eyes may have independently arisen
  many different times during evolution
• Pax6 is a master control gene that controls the
  expression of many other genes and influences eye
  development

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Eyeless and Pax-6

• Eyes of Drosophila and mammals are evolutionarily
  derived from a modification of an eye that arose
  once during evolution
• If Drosophila and mammalian eyes had arisen
  independently, the Pax6 gene from mice would not
  be expected to induce the formation of eyes in
  Drosophila
• Hypothesized that the eyes from many different
  species all evolved from a common ancestral form
  consisting of, as proposed by Darwin, one
  photoreceptor cell and one pigment cell

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Evolution of Complex Eyes

• The ancestor possessed two light sensitive organs
• Simple eyes
• Photoreceptor R-opsin
• Brain photolock
• Part of the animal brain that processes light
• Contains the light sensitive protein C-opsin

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• All three eyes incorporated the photoreceptor
  r-opsin
• The vertebrate camera eye also incorporated the
  brain photolock and c-opsin

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Larvae have 2-celled eyes
Adult eyes are made of layers of larval eyes

• Such a simple 2-celled eye is found in the larvae
  of the marine ragworm Platynereis dumerilii
• The development of the larger, adult shaped eye
  begins near the larval eyes and is assembled with
  many more photoreceptors and pigment cells
• Pax-6 and at least 2 other eye building genes are
  involved

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• Complexity in the case of eyes is a matter of
  arranging larger numbers of the same type of eye
  cells in three dimensional space
• The same genes and building materials..a
  different oranization

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BMP4 and Gremlin

• Changes in developmental genes affect traits that
  can be acted on by natural selection
• Compare chicken and duck foot
• Due to differences in expression of 2
  cell-signaling proteins
• BMP4 causes cells to undergo apoptosis and die
• Gremlin inhibits the function of BMP4 and
  allows cell to survive

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• Mutations on the expression of BMP4 and gremlin
  provided variation in phenotype of feet
• In terrestrial settings, non-webbed feet are an
  advantage
• Natural selection maintains non-webbed feet
• In aquatic environments, webbed feet are an
  advantage
• Natural selection would have favored webbed feet
• Speciation may have been promoted by geographical
  isolation of habitats