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Genes, Development, and
Evolution
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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
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 sporeproducing cells. (2nn)
When spores germinate, hyphae (collectively
known as mycelium) radiate out in a circular
pattern of undifferentiated haploid cells.
Development in Multicellular Organisms
More complex organisms such as
plants and animals have a much
more complex development.
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.
Development Involves Distinct but Overlapping Processes
Development—the 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
Figure 14.1 Development (Part 1)
Predict the point of each of the processes of development:
Fertilization occursA wave of Ca2+ release during the cortical reaction- part of the process that prevents
polyspermy, the zygote is formed.
Figure 47.8x Cleavage in a frog embryo- the resulting mass of cells (bottom right) is called the Morula.
Figure 47.8d Cross section of a frog blastula – essentially the morula with a cavity known as the blastocoel
Figure 47.20 Fate maps for two chordates
Table 47.1 Derivatives of the Three Embryonic Germ Layers in Vertebrates
Development Involves Distinct but Overlapping Processes
Development Involves Distinct but Overlapping Processes
Determination is followed by differentiation—under certain
conditions a cell can become undetermined again.
It may become totipotent—able 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.
Multipotent—they 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.
Figure 14.4 Cloning a Mammal (Part 1)
Figure 14.6 Two Ways to Obtain Pluripotent Stem Cells
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
Changes in Gene Expression Underlie Cell Differentiation in
Development
Polarity—having 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.
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.
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 another’s developmental
fate via chemical signals and signal
transduction mechanisms.
Exposure to different amounts of inductive
signals can lead to differences in gene
expression.
Figure 14.9 Induction during Vulval Development in Caenorhabditis elegans
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.
Spatial Differences in Gene Expression Lead to Morphogenesis
Pattern formation—the process that results
in the spatial organization of tissues—
linked 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.
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
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 paddleshaped limb buds—cells 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 posterior–anterior axis.
Figure 14.12 The French Flag Model
Spatial Differences in Gene Expression Lead to Morphogenesis
Programmed cell death—apoptosis—is 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.
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.
Spatial Differences in Gene Expression Lead to Morphogenesis
Several types of genes are expressed sequentially to
define the segments:
•
Maternal effect genes set up anterior–posterior
and dorsal–ventral axes in the egg. (Uneven
production & distribution lead to polarity.)
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 anterior–posterior
organization in individual segments
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.
Spatial Differences in Gene Expression Lead to Morphogenesis
Clues to hox gene function came from homeotic mutants.
Antennapedia mutation—legs grow in place of antennae.
Bithorax mutation—an extra pair of wings grow.
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 (evodevo) is the study of evolution and
developmental processes.
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.
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
pathways—not through new mechanisms.
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 anterior–
posterior axis of their embryos—the
positional information has been
conserved.
Figure 14.15 Regulatory Genes Show Similar Expression Patterns
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.
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.
Figure 14.16 Segments Differentiate under Control of Genetic Switches (Part 1)
Figure 14.16 Segments Differentiate under Control of Genetic Switches (Part 2)
Gene Expression Pathways Underlie the Evolution of
Development
Modularity also allows the timing of
developmental processes to be
independent—heterochrony.
Example: The giraffe’s 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
delayed—changes in the timing of gene
expression led to longer necks.
Figure 14.17 Heterochrony in the Development of a Longer Neck
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.
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.
Figure 14.19 A Mutation in a Hox Gene Changed the Number of Legs in Insects
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.
Figure 14.20 Wings Evolved Three Times in Vertebrates
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.