genes, development, and evolution 14. concept 14.1 development involves distinct but overlapping...

Genes, Development, and Evolution

14

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 1)

Figure 14.1 Development (Part 2)


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.


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.


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 4)


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


In plants, growing regions contain meristems—clusters of undifferentiated, rapidly dividing stem cells.

Plants have fewer cell types (15–20) than animals (as many as 200).

In mammals, stem cells occur in most tissues, especially those that require frequent replacement—skin, blood, intestinal lining.

There are about 300 cell types in mammals.


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.


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.

This is the basis of a cancer therapy called hematopoietic stem cell transplantation (HSCP).


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.


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.


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

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.


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.


• 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 (Part 1)

Figure 14.7 Transcription and Differentiation in the Formation of Muscle Cells (Part 2)


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


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.


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.

Figure 14.8 The Concept of Cytoplasmic Segregation (Part 1)

Figure 14.8 The Concept of Cytoplasmic Segregation (Part 2)


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 14.3 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


Fate of a cell is often 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.

To be a morphogen:

• It must directly affect target cells

• Different concentrations of the morphogen result in different effects

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.

Figure 14.15 Regulatory Genes Show Similar Expression Patterns


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.

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

Figure 14.21 Parallel Phenotypic Evolution in Sticklebacks

genes, development, and evolution 14. concept 14.1 development involves distinct but overlapping...

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