Genes, Development, and Evolution (Back to the beginning)

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Genes, Development, and Evolution(Back to the
beginning)
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Genes, Development, and Evolution

• Key Concepts
• Development Involves Distinct but Overlapping
  Processes
• Changes in Gene Expression Underlie Cell
  Differentiation in Development
• Spatial Differences in Gene Expression Lead to
  Morphogenesis
• Gene Expression Pathways Underlie the Evolution
  of Development
• Developmental Genes Contribute to Species
  Evolution but Also Pose Constraints

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Development in Multicellular Organisms

• Multicellular Organisms made of differentiated
  cells undergo development after fertilization.
• Fertilization may occur in a variety of ways.
• For many Fungi, once the hyphae of two strains
  come in contact, their cells fuse, creating the
  zygote (2n). Generally this develops into the
  diploid fruiting body that releases spores. Few
  cells differentiate to produce the
  spore-producing cells. (2n?n)
• When spores germinate, hyphae (collectively known
  as mycelium) radiate out in a circular pattern of
  undifferentiated haploid cells.

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Development in Multicellular Organisms

• More complex organisms such as plants and animals
  have a much more complex development.

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Development Involves Distinct but Overlapping
Processes

• As a zygote develops, the cell fate of each
  undifferentiated cell drives it to become part of
  a particular type of tissue.
• Experiments in which specific cells of an early
  embryo are grafted to new positions on another
  embryo show that cell fate is determined during
  development.
• Determination is influenced by changes in gene
  expression as well as the external environment.
• Determination is a commitment the final
  realization of that commitment is
  differentiation.
• Differentiation is the actual changes in
  biochemistry, structure, and function that result
  in cells of different types.

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Development Involves Distinct but Overlapping
Processes

• Developmentthe process by which a multicellular
  organism undergoes a series of changes, taking on
  forms that characterize its life cycle.
• After the egg is fertilized, it is called a
  zygote.
• In its earliest stages, a plant or animal is
  called an embryo.
• The embryo can be protected in a seed, an egg
  shell, or a uterus.
• Four processes of development
• Determination sets the fate of the cell
• Differentiation is the process by which different
  types of cells arise
• Morphogenesis is the organization and spatial
  distribution of differentiated cells
• Growth is an increase in body size by cell
  division and cell expansion

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Figure 14.1 Development (Part 1)
Predict the point of each of the processes of
development
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Fertilization occurs-A wave of Ca2 release
during the cortical reaction- part of the process
that prevents polyspermy, the zygote is formed.
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Figure 47.8x Cleavage in a frog embryo- the
resulting mass of cells (bottom right) is called
the Morula.
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Figure 47.8d Cross section of a frog blastula
essentially the morula with a cavity known as the
blastocoel
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Figure 47.20 Fate maps for two chordates
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Table 47.1 Derivatives of the Three Embryonic
Germ Layers in Vertebrates
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Development Involves Distinct but Overlapping
Processes
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Development Involves Distinct but Overlapping
Processes

• Determination is followed by differentiationunder
  certain conditions a cell can become
  undetermined again.
• It may become totipotentable to become any type
  of cell, including extraembryonic cells
  (placental). Most plant cells are totipotent.
  Differentiated animal cells can be manipulated to
  be totipotent (used in cloning).
• Pluripotent - cells in the blastocyst embryonic
  stage retain the ability to form all of the cells
  in the body.
• Multipotentthey produce cells that differentiate
  into a few cell types. Multipotent stem cells
  differentiate on demand.
• Stem cells in the bone marrow differentiate in
  response to certain signals, which can be from
  adjacent cells or from the circulation.

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Figure 14.4 Cloning a Mammal (Part 1)
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Figure 14.6 Two Ways to Obtain Pluripotent Stem
Cells
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Changes in Gene Expression Underlie Cell
Differentiation in Development

• Major controls of gene expression in
  differentiation are transcriptional controls.
• While all cells in an organism have the same DNA,
  it can be demonstrated with nucleic acid
  hybridization that differentiated cells have
  different mRNAs.
• Two ways to make a cell transcribe different
  genes
• Asymmetrical factors that are unequally
  distributed in the cytoplasm may end up in
  different amounts in progeny cells
• Differential exposure of cells to an external
  inducer

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Changes in Gene Expression Underlie Cell
Differentiation in Development

• Polarityhaving a top and a bottom may
  develop in the embryo.
• The animal pole is the top, the vegetal pole is
  the bottom.
• Polarity can lead to determination of cell fates
  early in development.
• Polarity was demonstrated using sea urchin
  embryos.
• If an eight-cell embryo is cut vertically, it
  develops into two normal but small embryos.
• If the eight-cell embryo is cut horizontally, the
  bottom develops into a small embryo, the top does
  not develop.

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Changes in Gene Expression Underlie Cell
Differentiation in Development

• In sea urchin eggs, a protein binds to the
  growing end () of a microfilament and to an mRNA
  encoding a cytoplasmic determinant (RNA or
  protein).
• As the microfilament grows toward one end of the
  cell, it pulls the mRNA along.
• The unequal distribution of mRNA results in
  unequal distribution of the protein it encodes.
• This results in cells with different fates.

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Concept 14.2 Changes in Gene Expression Underlie
Cell Differentiation in Development

• Induction refers to the signaling events in a
  developing embryo.
• Cells influence one anothers developmental fate
  via chemical signals and signal transduction
  mechanisms.
• Exposure to different amounts of inductive
  signals can lead to differences in gene
  expression.

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Figure 14.9 Induction during Vulval Development
in Caenorhabditis elegans
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Concept 14.2 Changes in Gene Expression Underlie
Cell Differentiation in Development

• Induction involves the activation or inactivation
  of specific genes through signal transduction
  cascades in the responding cells.
• Example from nematode development
• Much of development is controlled by the
  molecular switches that allow a cell to proceed
  down one of two alternative tracks.

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• Pattern formationthe process that results in the
  spatial organization of tissueslinked with
  morphogenesis, creation of body form
• Spatial differences in gene expression depend on
• Cells in body must know where they are in
  relation to the body.
• Cells must activate appropriate pattern of gene
  expression.

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• Positional information comes in the form an
  inducer, a morphogen, which diffuses from one
  group of cells to another, setting up a
  concentration gradient.
• To be a morphogen
• It must directly affect target cells
• Different concentrations of the morphogen result
  in different effects

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• The French flag model explains morphogens and
  can be applied to differentiation of the vulva in
  C. elegans and to development of vertebrate
  limbs.
• Vertebrate limbs develop from paddle-shaped limb
  budscells must receive positional information.
• Cells of the zone of polarizing activity (ZPA)
  secrete a morphogen called Sonic hedgehog (Shh).
  It forms a gradient that determines the
  posterioranterior axis.

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Figure 14.12 The French Flag Model
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Spatial Differences in Gene Expression Lead to
Morphogenesis

• Programmed cell deathapoptosisis also
  important.
• Many cells and structures form and then disappear
  during development.
• Sequential expression of two genes called ced-3
  and ced-4 (for cell death) are essential for
  apoptosis.
• Their expression in the human embryo guides
  development of fingers and toes.

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• The fruit fly Drosophila melanogaster has a body
  made of different segments.
• The head, thorax, and abdomen are each made of
  several segments.
• 24 hours after fertilization a larva appears,
  with recognizable segments that look similar.
• The fates of the cells to become different adult
  segments are already determined.

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• Several types of genes are expressed sequentially
  to define the segments
• Maternal effect genes set up anteriorposterior
  and dorsalventral axes in the egg. (Uneven
  production distribution lead to polarity.)

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• Segmentation genes determine properties of the
  larval segments determine boundaries and
  polarity.
• Three classes of genes act in sequence
• Gap genes organize broad areas along the axis
• Pair rule genes divide embryo into units of two
  segments each
• Segment polarity genes determine boundaries and
  anteriorposterior organization in individual
  segments

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• Hox genes are expressed in different combinations
  along the length of the embryo determine what
  organ will be made at a given location
• They determine cell fates within each segment and
  direct cells to become certain structures, such
  as eyes or wings.
• Hox genes are homeotic genes that are shared by
  all animals.

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Spatial Differences in Gene Expression Lead to
Morphogenesis

• Clues to hox gene function came from homeotic
  mutants.
• Antennapedia mutationlegs grow in place of
  antennae.
• Bithorax mutationan extra pair of wings grow.

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Gene Expression Pathways Underlie the Evolution
of Development

• Discovery of developmental genes allowed study of
  other organisms.
• The homeobox is also present in many genes in
  other organisms, showing a similarity in the
  molecular events of morphogenesis.
• Evolutionary developmental biology (evo-devo) is
  the study of evolution and developmental
  processes.

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Gene Expression Pathways Underlie the Evolution
of Development

• Principles of evo-devo
• Many groups of animals and plants share similar
  molecular mechanisms for morphogenesis and
  pattern formation.
• The molecular pathways that determine different
  developmental processes operate independently
  from one another called modularity.

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Gene Expression Pathways Underlie the Evolution
of Development

• Changes in location and timing of expression of
  particular genes are important in the evolution
  of new body forms and structures.
• Development produces morphology, and
  morphological evolution occurs by modification of
  existing developmental pathwaysnot through new
  mechanisms.

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Gene Expression Pathways Underlie the Evolution
of Development

• Through hybridization, sequencing, and
  comparative genomics, it is known that diverse
  animals share molecular pathways for gene
  expression in development.
• Fruit fly genes have mouse and human
  orthologs(genes traced to a common ancestor) for
  developmental genes.
• These genes are arranged on the chromosome in the
  same order as they are expressed along the
  anteriorposterior axis of their embryosthe
  positional information has been conserved.

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Figure 14.15 Regulatory Genes Show Similar
Expression Patterns
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Concept 14.4 Gene Expression Pathways Underlie
the Evolution of Development

• Certain developmental mechanisms, controlled by
  specific DNA sequences, have been conserved over
  long periods during the evolution of
  multicellular organisms.
• These sequences comprise the genetic toolkit,
  which has been modified over the course of
  evolution to produce the diversity of organisms
  in the world today.

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Gene Expression Pathways Underlie the Evolution
of Development

• In an embryo, genetic switches integrate
  positional information and play a key role in
  making different modules develop differently.
• Genetic switches control the activity of Hox
  genes by activating each Hox gene in different
  zones of the body.
• The same switch can have different effects on
  target genes in different species, important in
  evolution.

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Figure 14.16 Segments Differentiate under
Control of Genetic Switches (Part 1)
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Figure 14.16 Segments Differentiate under
Control of Genetic Switches (Part 2)
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Gene Expression Pathways Underlie the Evolution
of Development

• Modularity also allows the timing of
  developmental processes to be independentheteroch
  rony.
• Example The giraffes neck has the same number
  of vertebrae as other mammals, but the bones grow
  for a longer period.
• The signaling process for stopping growth is
  delayedchanges in the timing of gene expression
  led to longer necks.

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Figure 14.17 Heterochrony in the Development of
a Longer Neck
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Developmental Genes Contribute to Species
Evolution but Also Pose Constraints

• Evolution of form has not been a result of
  radically new genes but has resulted from
  modifications of existing genes.
• Developmental genes constrain evolution in two
  ways
• Nearly all evolutionary innovations are
  modifications of existing structures.
• Genes that control development are highly
  conserved.

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Developmental Genes Contribute to Species
Evolution but Also Pose Constraints

• Genetic switches that determine where and when
  genes are expressed underlie both development and
  the evolution of differences among species.
• Among arthropods, the Hox gene Ubx produces
  different effects.
• In centipedes, Ubx protein activates the Dll gene
  to promote the formation of legs.
• In insects, a change in the Ubx gene results in a
  protein that represses Dll expression, so leg
  formation is inhibited.

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Figure 14.19 A Mutation in a Hox Gene Changed
the Number of Legs in Insects
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Developmental Genes Contribute to Species
Evolution but Also Pose Constraints

• Wings arose as modifications of existing
  structures.
• In vertebrates, wings are modified limbs.
• Organisms also lose structures.
• Ancestors of snakes lost their forelimbs as a
  result of changes in expression of Hox genes.
• Then hindlimbs were lost by the loss of
  expression of the Sonic hedgehog gene in limb bud
  tissue.

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Figure 14.20 Wings Evolved Three Times in
Vertebrates
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Developmental Genes Contribute to Species
Evolution but Also Pose Constraints

• Many developmental genes exist in similar form
  across a wide range of species.
• Highly conserved developmental genes make it
  likely that similar traits will evolve
  repeatedly Parallel phenotypic evolution.