Genes, Development, and Evolution

Genes, Development, and Evolution

14

Chapter 14 Genes, Development, and Evolution

Key Concepts

• 14.1 Development Involves Distinct but Overlapping Processes

• 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development

• 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis

Chapter 14 Genes, Development, and Evolution

Key Concepts

• 14.4 Gene Expression Pathways Underlie the Evolution of Development

• 14.5 Developmental Genes Contribute to Species Evolution but Also Pose Constraints

Chapter 14 Opening Question

Why are stem cells so useful?

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.


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


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


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 14.2 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 inducer


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.


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.

Figure 14.10 The Concept of Embryonic Induction

Concept 14.3 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.


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.

In-Text Art, Ch. 14, p. 273


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 (Part 1)


Three classes of organ identity genes in Arabidopsis:

• Class A, expressed in sepals and petals

• Class B, expressed in petals and stamens

• Class C, expressed in stamens and carpels

Gene regulation is combinatorial—the composition of active dimers depends on the location of the cell and determines which genes will be activated.



Figure 14.12 The French Flag Model (Part 2)


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 (1)


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.


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.

Figure 14.14 A Homeotic Mutation in Drosophila (Part 1)

Figure 14.14 A Homeotic Mutation in Drosophila (Part 2)


Antennapedia and bithorax have a common 180-bp sequence—the homeobox, that encodes a 60-amino acid sequence called the homeodomain.

The homeodomain binds to a specific DNA sequence in promoters of target genes.

Concept 14.4 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.


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.


• 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.


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.

Figure 14.15 Regulatory Genes Show Similar Expression Patterns


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.


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)


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


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.


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.

Figure 14.18 Changes in Gremlin Expression Correlate with Changes in Hindlimb Structure

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.


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


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


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)


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.

Figure 14.21 Parallel Phenotypic Evolution in Sticklebacks


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.

Answer to Opening Question

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

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Genes, Development, and Evolution