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

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 occurs-
A wave of Ca 2+ 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 S onic h edge h og (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 ce ll d eath) 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 .