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14
Genes, Development, and
Evolution
The Process of Evolution Drives the Diversity &
Unity of Life
BIG IDEA #1
Concept 14.1 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.
Figure 14.1 Development (Part 2)
Figure 14.1 Development (Part 1)
Concept 14.2 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.
Concept 14.2 Changes in Gene Expression Underlie Cell
Differentiation 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.
In-Text Art, Ch. 14, p. 270
Figure 14.8 The Concept of Cytoplasmic Segregation
Concept 14.3 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.
In-Text Art, Ch. 14, p. 276 (2)
In-Text Art, Ch. 14, p. 276 (1)
Concept 14.2 Changes in Gene Expression Underlie Cell
Differentiation in Development
Model of cytoplasmic segregation states that
cytoplasmic determinants are distributed
unequally in the egg.
The cytoskeleton contributes to distribution of
cytoplasmic determinants:
• Microtubules and microfilaments have polarity.
• Cytoskeletal elements can bind certain proteins.
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
Cytokinesis does not occur in the early Drosophila
mitoses after fertilization.
The embryo until then is multinucleate, allowing for
easy diffusion of morphogens.
Experimental genetics were used:
•
Developmental mutant strains were identified.
•
Genes for mutations were identified.
•
Transgenic flies were produced to confirm the
developmental pathway.
Concept 14.3 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.
•
Segmentation genes determine boundaries and
polarity.
•
Hox genes determine what organ will be made at
a given location.
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
Maternal effect genes produce cytoplasmic
determinants in unequal distributions in the egg.
Two genes—bicoid and nanos—determine the
anterior–posterior axis.
Their mRNAs diffuse to the anterior end of the egg.
Bicoid protein diffuses away from the anterior end,
establishing a gradient.
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
At sufficient concentration, bicoid stimulates
transcription of the Hunchback gene. A gradient
of that protein establishes the head.
Nanos mRNA is transported to the posterior end.
Nanos protein inhibits translation of Hunchback.
After the anterior and posterior ends are established,
the next step is determination of segment number
and locations.
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
Segmentation genes determine properties of the
larval segments.
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
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
Hox genes are expressed in different combinations
along the length of the embryo.
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.
Concept 14.3 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.
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
Antennapedia and bithorax have a common 180-bp
sequence—the homeobox, that encodes a 60amino acid sequence called the homeodomain.
The homeodomain binds to a specific DNA
sequence in promoters of target genes.
Figure 14.13 A Gene Cascade Controls Pattern Formation in the
Drosophila Embryo
Concept 14.4 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 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.
Concept 14.4 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.
Concept 14.4 Gene Expression Pathways Underlie the Evolution
of Development
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.
Concept 14.4 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.
Concept 14.5 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 14.15 Regulatory Genes Show Similar Expression Patterns
Figure 14.16 Segments Differentiate under Control of Genetic Switches (Part 2)
Figure 14.16 Segments Differentiate under Control of Genetic Switches
Concept 14.1 Development Involves Distinct but Overlapping
Processes
As 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.
Figure 14.2 A Cell’s Fate Is Determined in the Embryo
Concept 14.1 Development Involves Distinct but Overlapping
Processes
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
Concept 14.2 Changes in Gene Expression Underlie Cell
Differentiation in Development
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 inducer
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.
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.
Concept 14.2 Changes in Gene Expression Underlie Cell
Differentiation in Development
In C. elegans, the cell divisions from the fertilized
egg to the 959 adult cells can be followed.
Nematodes are hermaphroditic and contain male
and female reproductive organs.
Eggs are laid through a pore, the vulva.
During development, a single anchor cell induces the
vulva to form from six cells on the ventral surface
of the worm.
Concept 14.2 Changes in Gene Expression Underlie Cell
Differentiation in Development
LIN-3, a protein secreted by the anchor cell acts as
the primary inducer.
The primary precursor cell that received the most
LIN-3 then secretes a secondary inducer (lateral
signal) that acts on its neighbors.
The gene expression patterns triggered by these
molecular switches determine cell fates.
Figure 14.9 Induction during Vulval Development in
Caenorhabditis elegans
Figure 14.10 The Concept of Embryonic Induction
Concept 14.3 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.
Worksheet 201 Positive and
negative control of apoptosis
Concept 14.4 Gene Expression Pathways Underlie the Evolution
of Development
Webbed feet in ducks result from an altered spatial
expression pattern of a developmental gene.
Duck and chicken embryos both have webbing, and
both express BMP4, a protein that instructs cells
in the webbing to undergo apoptosis.
Concept 14.4 Gene Expression Pathways Underlie the Evolution
of Development
In ducks, a gene called Gremlin, which encodes a
BMP inhibitor protein, is expressed in webbing
cells.
In chickens, Gremlin is not expressed, and BMP4
signals apoptosis of the webbing cells.
Experimental application of Gremlin to chicken feet
results in a webbed foot.
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
Flowers are composed of four organ types (sepals,
petals, stamens, carpels) arranged around a
central axis in whorls.
In Arabidopsis thaliana, flowers develop from a
meristem at the growing point on the stem.
The identity of each whorl is determined by organ
identity genes.
Figure 14.11 Gene Expression and Morphogenesis in
Arabidopsis Flowers
Concept 14.3 Spatial Differences in Gene Expression Lead to
Morphogenesis
Fate of a cell is determined by where the cell is.
Positional information comes in the form an
inducer, a morphogen, which diffuses from one
group of cells to another, setting up a
concentration gradient.
•
directly affect target cells
•
Different concentrations of the morphogen result
in different effects
Concept 14.3 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
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
Concept 14.5 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.
Figure 14.19 A Mutation in a Hox Gene Changed the Number of
Legs in Insects
Concept 14.5 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
Concept 14.5 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.
Example: Three-spined sticklebacks (Gasterosteus
aculeatus)
Concept 14.5 Developmental Genes Contribute to Species
Evolution but Also Pose Constraints
Marine populations of sticklebacks return to freshwater to
breed. Freshwater populations never go into saltwater
environments.
Freshwater populations have arisen many times from adjacent
marine populations.
Marine populations have pelvic spines and bony plates that
protect them from predation.
These are greatly reduced in freshwater populations.
One gene, Pitx1, is not expressed in freshwater sticklebacks,
and spines do not develop.
This same gene has evolved to produce similar phenotypic
changes in several independent populations.
Figure 14.21 Parallel Phenotypic Evolution in Sticklebacks
Stem Cells
Stem cells are valuable because they are
not differentiated and can develop into
several kinds of cells.
When fat stem cells are injected into a
damaged area they respond to the
environment of that tissue.
Inducers in the environment determine
the products of cell differentiation.
Figure 14.22 Differentiation Potential of Stem Cells from Fat
Concept 14.1 Development Involves Distinct but Overlapping
Processes
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.
Concept 14.1 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.
Plant cells are usually totipotent but can be induced
to dedifferentiate into masses of calli, which can
be cultured into clones.
Genomic equivalence—all cells in a plant have the
complete genome for that plant.
Concept 14.1 Development Involves Distinct but Overlapping
Processes
Pluripotent cells in the blastocyst embryonic stage
retain the ability to form all of the cells in the body.
In mice, embryonic stem cells (ESCs) can be
removed from the blastocyst and grown in
laboratory culture almost indefinitely.
ESCs in the laboratory can also be induced to
differentiate by specific signals, such as Vitamin A
to form neurons or growth factors to form blood
cells.
Figure 14.6 Two Ways to Obtain Pluripotent Stem Cells
Concept 14.1 Development Involves Distinct but Overlapping
Processes
Induced pluripotent stem cells (iPS cells) can be
made from skin cells:
• Microarrays are used to find genes uniquely
expressed at high levels in ESCs.
• The genes are inserted into a vector for genetic
transformation of skin cells—skin cells express
added genes at high levels.
• The transformed cells become iPS cells and can
be induced to differentiate into many tissues.
Concept 14.1 Development Involves Distinct but Overlapping
Processes
Stem cells in some mammalian tissues are
multipotent—they produce cells that differentiate
into a few cell types.
Hematopoietic stem cells produce red and white
blood cells.
Mesenchymal stem cells produce bone and
connective tissue cells.
Concept 14.1 Development Involves Distinct but Overlapping
Processes
In animals, nuclear transfer experiments have
shown that genetic material from a cell can be
used to create cloned animals.
The nucleus is removed from an unfertilized egg,
forming an enucleated egg.
A donor nucleus from a differentiated cell is then
injected into the enucleated egg.
The egg divides and develops into a clone of the
nuclear donor.
Figure 14.4 Cloning a Mammal (Part 1)
Concept 14.1 Development Involves Distinct but Overlapping
Processes
As in plants, no genetic information is lost as the
cell passes through developmental stages—
genomic equivalence.
Practical applications for cloning:
• Expansion of numbers of valuable animals
• Preservation of endangered species
• Preservation of pets
Concept 14.1 Development Involves Distinct but Overlapping
Processes
Therapies that kill cancer cells can also kill other
rapidly dividing cells such as bone marrow stem
cells.
The stem cells are removed and stored during the
therapy, and then returned to the bone marrow.
The stored stem cells retain their ability to
differentiate.
Concept 14.1 Development Involves Distinct but Overlapping
Processes
ESC cultures may be sources of differentiated cells
to repair damaged tissues, as in diabetes or
Parkinson’s disease.
ESCs can be harvested from human embryos
conceived by in vitro fertilization, with consent of
the donors. However:
• Some people object to the destruction of human
embryos for this purpose
• The stem cells could provoke an immune
response in a recipient
Concept 14.2 Changes in Gene Expression Underlie Cell
Differentiation in Development
In the vertebrate embryo, muscle precursor cells
come from a tissue layer called the mesoderm.
• When these cells commit to becoming muscle
cells, they stop dividing—in many parts of the
embryo, cell division and cell differentiation are
mutually exclusive.
Concept 14.2 Changes in Gene Expression Underlie Cell
Differentiation in Development
• Cell signaling activates the gene for a
transcription factor called MyoD.
• MyoD activates the gene for p21, an inhibitor of
cyclin-dependent kinases that normally stimulate
the cell cycle at G1.
• The cell cycle stops so that differentiation can
begin.
Figure 14.7 Transcription and Differentiation in the Formation of
Muscle Cells