Download Segmentation Genes

Ch 22 Introduction
• How does a single fertilized egg cell develop into an embryo and
then into a baby and eventually an adult?
• A few fundamental principles are common to all developmental
sequences observed in multicellular organisms.
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Shared Developmental Processes
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Shared Developmental Processes
location, timing,
extent cell
divisions tightly
controlled by
regulation
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Meristems and Stem Cells
• Most cells stop proliferating at maturity. However, there are some
specialized, undifferentiated cells that continue proliferating
throughout the organism’s life.
• In plants, these specialized cells are called meristems.
• In animals, they are called stem cells.
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Shared Developmental Processes
abnormal apoptosis
can lead to disease
or deformation
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Shared Developmental Processes
Plant cells don’t
move; they change
the orientation of
cell division. Animal
cells move: in
gastrulation, cells in
different parts of an
early embryo
rearrange themselves
into three distinctive
types of embryonic
tissues
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Shared Developmental Processes
Differentiation is a
progressive process.
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Cell Differentiation
• Many plant cells are totipotent – capable of de-differentiating even
after they have specialized.
– Animal cells are unable to de-differentiate.
• Plant meristems and animal stem cells do not become specialized
adult cells, but instead remain undifferentiated.
– Stem cells retain the ability to divide and give rise to an array
of specialized cell types.
– Meristems can give rise to various structures that develop
throughout life.
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Shared Developmental Processes
change patterns of
gene expression and
are essential for
changing cell
activity during
development
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The Role of Differential Gene Expression in Development
• Differential gene expression, the expression of different genes in
different cell types, is key to cell differentiation during
development.
• An important question that scientists had to answer was whether
cells express different genes because they contain different genes or
whether they all contain the same genes but express different
subsets.
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Are Differentiated Cells Genetically Equivalent?
• Plant cells can de-differentiate to form other plant parts, thus each
cell must contain the genes required by all different types of plant
cells.
• Early experiments showed that transplanting nuclei from diploid
frog cells into unfertilized eggs without nuclei resulted in
development of normal tadpoles.
– Nuclear transfer experiments in sheep reinforced these results.
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Animal Cloning
• Mammary gland cells from an adult
sheep were fused with enucleated eggs.
– The resulting embryos were
implanted into surrogate mothers.
– A fertile, genetically identical
clone of the parent sheep was born
(“Dolly”).
• Mouse, cow, horse, monkey, and other
species have now also been
successfully cloned.
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How Does Differential Gene Expression Occur?
• A gene can be regulated at multiple levels:
– transcription, RNA processing, translation, and posttranslation.
• In eukaryotes, transcription is controlled primarily by the presence
of proteins called regulatory transcription factors.
– One such factor sets up the primary body axes
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Cell-Cell Signals Trigger Differential Gene Expression
• The fate of a cell depends on timing
(the current stage of development of
the organism) and its spatial location
(where it is in the body of the
organism).
• Spatial location in early development is
determined by three major body axes:
– Anterior-posterior
– Ventral-dorsal
– Left-right
Cell-cell signals tell cells where they
are in time and space. This information
activates transcription factors that turn
specific genes on or off, resulting in
differentiation.
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Master Regulators Set Up the Major Body Axes
• Pattern formation is the series of events that determine the spatial
organization of an embryo.
• Certain early signals act as master regulators, setting up the major
body axes of the embryo.
– These master regulators activate a network of genes that sends
signals with more specific information about the spatial
location of cells.
• As development proceeds, a series of signals arrive and activate
genes that specify finer and finer control over what a cell becomes.
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The Discovery of Pattern Formation Mutants
• In the 1970s, Christiane Nüsslein-Volhard and Eric Wieschaus
applied a genetic approach to studying development in the fruit fly
(Drosophila melanogaster), eventually identifying more than 100
genes that play fundamental roles in pattern formation.
– They did this by producing mutant embryos.
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The bicoid Gene
• One of the most dramatic mutations
resulted in structures on the anterior end
being replaced with posterior structures.
They named the gene responsible bicoid,
for “two-tailed.”
• Nüsslein-Volhard and Wieschaus
suspected that the bicoid gene’s product
provides positional information. In other
words, they hypothesized that the bicoid
gene coded for a signal that tells cells
where they are located along the anteriorposterior body axis.
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Where Is the bicoid Product Found?
• To determine the location of bicoid
mRNA in the embryo, researchers used a
technique called in situ hybridization.
• They found that bicoid mRNA was highly
localized in the anterior of the embryo.
– Bicoid protein is made from mRNA
in the anterior end and diffuses away
from that end of the embryo.
• This produces a steep concentration
gradient from the anterior to the posterior
end.
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How Does Bicoid Work?
• Bicoid forms a concentration gradient which provides cells with information
about their position along the anterior-posterior axis.
• Bicoid also turns on genes responsible for forming anterior structures. The
absence of Bicoid contributes to the formation of posterior structures.
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Auxin’s Role in Plant Development
• The master regulator in plants is not a transcription factor but rather
a hormone.
• In plant embryos, the cell-cell signal called auxin enters cells and
triggers the production of transcription factors that affect
differentiation.
• Like Bicoid, auxin also works by forming concentration gradients.
• In both plants and animals, molecules that provide spatial
information during early embryonic development, via a
concentration gradient, are called morphogens.
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Segmentation Genes
• Regulatory genes provide
increasingly specific positional
information.
• A segment is a distinct region of
an animal body that contains a
distinct set of structures and is
repeated along its length.
• In the fruit fly and other animals,
segmentation genes organize
cells and tissues into distinct
segments.
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What Do Segmentation Genes Do?
• Three general classes of segmentation
genes have been identified in
Drosophila:
1. Gap genes define the general
position of segments in the
anterior, middle, or posterior of
the body.
2. Pair-rule genes demarcate the
boundaries of individual
segments.
3. Segment polarity genes delineate
boundaries within individual
segments.
• These segmentation gene sets are
expressed in sequence and in
increasingly restricted regions.
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Homeotic Genes (trigger devt. of structures)
• After the segmentation genes have established the identity of each
segment along the anterior-posterior axis, development continues
with activation of the homeotic genes.
• Homeotic gene products identify each segment’s structural role.
– Specifically, homeotic genes trigger the development of
structures that are appropriate to each type of segment.
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Hox Genes (homeotic genes in Drosophila)
• The eight homeotic genes in Drosophila are called Hox genes.
• The Hox genes are expressed in a distinctive pattern along the
anterior-posterior axis, after segments are established.
• These genes code for regulatory transcription factors that trigger the
production of segment-specific structures.
• Some Drosophila mutants have a segment that has been
transformed into another segment, with its associated structures.
– This homeosis occurs when cells get incorrect information
about where they are in the body.
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Regulatory Genes Form Cascades
• The interactions among bicoid and the
segmentation genes form a regulatory
cascade.
• Master regulators trigger the production
of other regulatory signals and
transcription factors, which trigger
production of another set of signals and
regulatory proteins, and so on.
• The bicoid gene, gap genes, pair-rule
genes, segment polarity genes, and
homeotic genes each define a level in the
cascade.
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The Overall Function of Regulatory Genes
Regulatory genes act in a sequence, triggering gene cascades that
provide progressively detailed information about where cells are
located in time and space.
– Cells receive unique positional information because the
identity and concentration of signals and transcription factors
vary along the three major body axes.
– Each level in a regulatory cascade provides a more specific
level of information about where a cell is.
– As regulatory cascades proceed, a cell's fate becomes more and
more finely determined.
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Evolutionary Conservation of Hox Genes
• Clusters of Hox genes occur in virtually every animal examined to
date.
– The number of Hox genes varies widely among species, but
their chromosomal organization is similar.
• Biologists hypothesize that the genes in Hox complexes of animals
are homologous.
– At least some of the molecular mechanisms of pattern
formation have been highly conserved during animal evolution.
• Although animal bodies are spectacularly diverse in size and shape,
the underlying mechanisms responsible for their development are
similar.
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Developmental Pathways and Evolutionary Change
Once biologists began working out regulatory signals and
cascades, they realized that the genetic changes altering these
developmental processes must be the foundation of evolutionary
change.
•
Evo-devo is the research field of evolutionary-developmental
biology. It focuses on understanding how changes in
developmentally important genes have led to the evolution of new
phenotypes.
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Results of Changes in Homeotic Gene Expression
• Changes in regulation of where the homeotic genes Hoxc6 and Hoxc8 are expressed led to
the evolutionary loss of the forelimb in snakes.
• Normally, Hoxc6 is expressed without Hoxc8 in the region that gives rise to forelimbs in
vertebrates.
– In snakes, Hoxc6 and Hoxc8 are always expressed together, so no forelimb is
formed.
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