Genes, Development, and Evolution

Genes, Development, and Evolution
14 Genes, Development, and Evolution 3fr2f4r13f

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?

Draw a picture of an anima that you can draw well
Draw a picture of an anima that you can draw well. (side profile) Draw as much detail as you can and don’t show anyone. Pin it on the board ----- Meeting Notes (1/11/17 10:44) ----- Look at all the animals drawn. brainstorm commonalities. simple things like a head end and a tail end, paired appendages, body segments, sensory organs etc. Talk about characteristics that have been conserved in many animals, despite millions of years of evolution tinkring and modifying lineages. perhaps try to arrange them into some kind of evolutionary sequence.

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. VIDEO 14.1 From egg to tadpole: Embryonic development in a frog, Xenopus VIDEO 14.2 Embryonic development in two zebrafish :

Figure 14.1 Development (Part 1)

Figure 14.1 Development (Part 2)

http://www. highschool. bfwpub
- /launchpad/item/hillis2e_activities_34?mode=Preview&includeDiscussion=False&renderFNE=True&renderIn=fne Activity 14.1 stages of development

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.

http://www. highschool. bfwpub
- /launchpad/item/bsi__6E83E1FF__6488__445B__9FCF__54DF ?mode=Preview&includeDiscussion=False&renderFNE=True&renderIn=fne Transplanting cells in early frog embryos. Cell fates

Figure 14.2 A Cell’s Fate Is Determined in the Embryo
1st image Frog embryo. 2nd Green end develops into head if cells from the rear of a donor blastula 3rd if thre donated cells come from the rear an older embryo, the cells fate Tan end develops into tail are donated to the head end of a recipent has already been determined. Tail cells will form at the head region. embryo early in devel, the donated cells take on the fate of their new environment This shows that cell fate is determined as the embryo develops.The further an embryo has progressed through development, the more restricted each cells fate becomes.

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. INTERACTIVE TUTORIAL 14.1 Cell Fates: Genetic vs. Environmental Influences

Figure 14.3 Cloning a Plant (Part 1)

Figure 14.3 Cloning a Plant (Part 2)

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. Agriculture take cells from plants with desireable traits (remember all plant cells exhibit totipotency) and regenerate clones of those plants with desireabl etraits.

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. I

https://www.youtube.com/watch?v=K7D6iA7 bZG0
What can stem cells do???

http://learn.genetics.utah.edu/content/clonin g/clickandclone/

Figure 14.4 Cloning a Mammal (Part 1)
How dolly was cloned. Egg removed from a scottish blackface ewe Cells removed from the udder of a dorset sheep. Udder cells are deprived of nutrients, halting the cell cycle before the S phase. The nucleus is removed from the blackface ewe egg and replaced with the nucleus from an udder cell. Mitosis is stimulated causing the cell to divide, creating an early embryo, which is inserted into a receptive ewe. Lamb is a clone of the dorset sheep that donated the nucleus.

As in plants, no genetic information is lost as the cell passes through developmental stages—genomic equivalence.ie, No genetic info is lost. Practical applications for cloning: Expansion of numbers of valuable animals Preservation of endangered species Preservation of pets See Chapter 13

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.These stem cells are used to replace cells lost due to wear and tear. There are about 300 cell types in mammals. See Chapter 25

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. Both types are found in the bone marrow. Multipotent cells are differentiated on an as needed basis. No mammalian cells, other than embryonic stem cells are completely totipotent. This means they can diff into any cell in the body, including the tissue from the placenta

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).

Figure 14.5 Multipotent Stem Cells

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 (Blastula) embryonic stage retain the ability to form all of the cells in the body, but they cannot form an entire embryo. 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. 4 days after fertilization, totipotent cells divide, and mature into pluripotent cells. Pluripotent cells can give rise to any cell in the body but they cannot give rise to the tissue that becomes the placenta for the embryo, therefore they cannot produce an entire organism.

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 See Chapter 31

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. See Chapter 13

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. See Chapter 11 Most fundamental decisions in cell development and diff are controlled at the level of transcription. Genes that determine cell fate typically encode transcription factors.

Two ways to make a cell transcribe different genes:
Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development 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 THIS IS A BIG SLIDE INTRO TO EVO DEVO.

https://www.blendspace.com/lessons/crRmc dXVSZg-yw/evo-devo

Polarity—having a “top” and a “bottom” may develop in the embryo.
Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development Polarity—having a “top” and a “bottom” may develop in the embryo. An early event in embryonic development is often the establishment of an axis that relates to the body plan of an organism. The animal pole is the top, the vegetal pole is the bottom. Polarity can lead to determination of cell fates early in development. ANIMATED TUTORIAL 14.2 Early Asymmetry in the Embryo

2 normal, small sea urchins
In-Text Art, Ch. 14, p. 270 Polarity was demonstrated using sea urchin embryos at the 8 cell stage.. 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. This demonstrates that the top and bottom halves of the embryo have already developed distinct fates. This results from “cytoplasmic segregation,” the idea that certain “cytoplasmic determinants” are distributed unevenly in the cytoplasm. 2 normal, small sea urchins Bottom becomes a normal urchin, top half nothing at all.

Polarity was demonstrated using sea urchin embryos at the 8 cell stage.. 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.

Microtubules and microfilaments have polarity.
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. See Chapter 4

Figure 14.8 The Concept of Cytoplasmic Segregation (Part 1)
Shows unequal distribution of cytop Plasmic determinants in the egg, leading to the creation of Animal and Vegetal poles. After division,uneven distribution is maintained. These 2 cell still have the same fate at this point. After the next division, the cells at the vegetal poles contain the cytoplasmic matrerial while the top have none. At this point, the upper cells and lower cells have Different fates.

http://bcs.whfreeman.com/pol2e/default.asp - 940573__943472__

In sea urchin eggs, a protein binds to the growing end (+) of a microfilament and to an mRNA encoding a cytoplasmic determinant. 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.

Induction refers to the signaling events in a developing embryo.
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 called “INDUCERS” and signal transduction mechanisms. Exposure to different amounts of inductive signals can lead to differences in gene expression. Thus, proximity of cell to inducer and concentration of the inducers is critical. Not essential but cool

Eggs are laid through a pore, the vulva.
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. See Chapter 12 VIDEO 14.3 Embryogenesis of a nematode worm, C. elegans The fully developed adult C. elegans has 959 cells and reaches adulthood in 3.5 days.

Figure 14.9 Induction during Vulval Development in Caenorhabditis elegans (Part 1)
It is the formation of the cells that become the vulval that we are concerned.

The anchor cell secretes the primary inducer, LIN 3protein, which diffuses out of the anchor cell and creates a concentration gradient relative to the other adjacent cells. The cell closest to the anchor cell receives the highest amounts of LIN3. It becomes the primary vulval precursor cell. The primary precursor cell that received the most LIN-3 then secretes a secondary inducer (lateral signal) that acts on its neighbors. These primary and secondary inducers activate or deactivate select gene sets LINK For more on signal transduction cascades, see Concepts 5.5 and 5.6 The anchor cell secretes the primary inducer, LIN 3, which diffuses out of the anchor cell and creates a concentration gradient relatve to the other adjacent cells. The 3 cells closest to the anchor cells receive the highest amounts of LIN3. They become vulvall precursor cells

The neighboring 2 cells that receive smaller amounts of the primary inducer LIN3, as well as the secondary inducer that was released by the primary vulval precursor cell become secondary vulval precursor cells. Of the 6 original cells, 3 cells receive no primary or secondary inducers. They become epidermal cells. The gene expression patterns triggered by these molecular switches determine cell fates.

Figure 14.9 Induction during Vulval Development in Caenorhabditis elegans (Part 2)
The primary inducer LIN 3 activates gene set 1 in each of the recipient cells. Those that receive secondary inducer have gene set 2 activated and gene set 1 deactivated. Those cells receiving no primary inducer or secondary inducer express only gene set 3.

Example from nematode development:
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.

Figure 14.10 The Concept of Embryonic Induction
Cell releases inducer molecules Thi cell binds higher Concentration of inducers This cell binds fewer inducers The binding of inducers to Receptors causes the transcription Factors to move into the nucleus. No transcription This shows how concentration and proximity can impact transcription of a gene. The first cell releases induce A transcription factor binds A promoter, activating gene transcription Theresulting protein stimulates Cell differentiation, determining The cell’s fate.

Spatial differences in gene expression depend on:
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 so arm buds appear where they should. Cells must activate appropriate pattern of gene expression so arms actually grow from the arm buds, not something else. This means arms grow where arms should grow, etc.

Fate of a cell is often determined by where the cell is.
Concept 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis 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

Ohhhh Cute! Is there anything cuter than Baby fingers.
But how does a thumb know to become a thumb, and a pinky a pinky?

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).(Yeah, that’s right). It forms a gradient that determines the posterior–anterior axis. APPLY THE CONCEPT Spatial differences in gene expression lead to morphogenesis

Figure 14.12 The French Flag Model (Part 1)
Each cell has the potential to become red, white or blue. Their ultimate fate will depend on concentration of morphogen Exposure. This kind of gene regulation and expression is thought to be an explanation for the intricate spiralling patterns os sea shells and snails.

Figure 14.12 The French Flag Model (Part 2)
Front limb bud Rear limb bud The cells that develop into digits need to receive positional information, so we have the correct digits forming in the correct locations. But how do the cells at the top of the limb bud know to form a thumb, and those at the bottom into a pinky????? At the base of a limb bud is a group of cells called the Zone of Polarizing Activity (ZPA). ZPA cells release the morphogen sonic hedgehog, creating a concentration gradient of SHH to the limb bud cells. The cells closest to the ZPA receive the highest dose of SHH and develop into the little finger, while the cells furthest away receive the least SHH and develop into the thumb. TheZPA cells release the Morphogen sonic hedge hog, which forms a concentration gradient. Lowest dose of SHH Thumb Highest dose of SHH pinky

Programmed cell death—apoptosis—is also important.
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. LINK Concept 7.5 describes some of the cellular events of apoptosis

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

The head, thorax, and abdomen are each made of several segments.
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.

Experimental genetics were used:
Concept 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis Cytokinesis does not occur in the early Drosophila mitoses after fertilization. The zygote undergoes 12 mitotic divisions with no cytokinesis. The embryo until then is multinucleate, (1 cell with lots of nuclei )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. The Z For ex. A certain mutation by be 2 heads. They id the gene that causes 2 heads when mutated. Genes were isolated and mutant strains of the protein were injected into embryos, which expressed the mutant form.

In-Text Art, Ch. 14, p. 276 (1)

Hox genes determine what organ will be made at a given location.
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 between each segment and polarity of segments. Hox genes determine what organ will be made at a given location.

Two genes—bicoid and nanos—determine the anterior–posterior axis.
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. After fertilization, the bicoid mRNA is translated and the Bicoid protein diffuses away from the anterior end, establishing a gradient.

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.The result is no head at the posterior end.  After the anterior and posterior ends are established, the next step is determination of segment number and locations.

In-Text Art, Ch. 14, p. 276 (2)

Segmentation genes determine properties of the larval segments.
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

Figure 14.13 A Gene Cascade Controls Pattern Formation in the Drosophila Embryo
Maternal effect genes determine head and butt Gap genes organize segemnts into broad regions. Pair rules refine the segments By the end of this cascade, nuclei “Know” which segment they will be part of in the adult embryo. Next, the HOX benes will determine the identity, form and function of each segment. Together, these 3 genes control expression of the HOX genes, which will define the identity of each segment.

Hox genes are homeotic genes that are shared by all animals.
Concept 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis Hox genes encode a family of transcription factors that are expressed in different combinations in the segments 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. They are “big picture” genes. They direct where to put a wing, but not how to make the wing. Hox genes are homeotic genes that are shared by all animals.

Clues to hox gene function came from homeotic mutants.
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 on a segment of the thorax where wings do not typically occur. So the normal function of the Hox genes is to tell a segment what organ to grow. ANIMATED TUTORIAL 14.3 Pattern Formation in the Drosophila Embryo

Figure 14.14 A Homeotic Mutation in Drosophila (Part 1)
A wild type fruitfly

Figure 14.14 A Homeotic Mutation in Drosophila (Part 2)
Antennapedia mutation. A mutation to the Hox gene anntennapedia causes legs to grow in place of antennae. A mutation t

Antennapedia and bithorax genes 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. LINK To review mechanisms of transcriptional regulation, see Concept 11.3

http://bcs.whfreeman.com/pol2e/default.asp - 940573__943473__

Discovery of developmental genes allowed study of other organisms.
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. See Figure 10.8

Principles of evo-devo:
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, including a toolkit of regulatory molecules that control gene expression. The molecular pathways that determine different developmental processes are able to operate independently in different tissues and body regions— called modularity. This means that evolutionry change can occur in independendent “modules”. Incremental changes may be dramatic on certain modules, with little to no change elsewhere.

Sme very clear similarities in general body plan in these 4 species that share a common ancestor. Perhaps the only significant change visible is to body covering, with all else remaining remarkably similar.

Concept 14.4 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.

Mapping genomes from throughout the animal world has revealed 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 Head end 1st—the positional information has been conserved. See Concept 12.1

Figure 14.15 Regulatory Genes Show Similar Expression Patterns
Similar genes encoding similar transcription factors are expressed in similar patterns along the anterior posterior axis of both insects and vertebrates. Even in the millions of years since mice and fruitflies diverged from a common ancestor, these genes have been conserved, suggesting that their function is essential for animal 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.

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. APPLY THE CONCEPT Gene expression pathways underlie the evolution of development

Figure 14.16 Segments Differentiate under Control of Genetic Switches (Part 1)
Drosophila belong to a group of insects called Dipthera, means 2 wings (1 pair). Most insects have 4 wings. In fruit flies the wings develop on the second thoracic segment. A pair of balancing organs called Halteres develop on the third thoracic segment. Comparison between cells in segments 2 and 3 show that the hox gene UBX is expressed in segment 3 but not in segment 2. The UBX represses the wing development on the third segment. Inducing a mutation to inactivate the UBX gene causes a 2nd pair of wings to develop, on the third segment.

Figure 14.16 Segments Differentiate under Control of Genetic Switches (Part 2)
So you see UBX inhibits transcription of the wing gene in the 3rd segment .In segment 2 there is no UBX protein to inhibit transcription of the wing gene.

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. ANIMATED TUTORIAL 14.4 Modularity

Figure 14.17 Heterochrony in the Development of a Longer Neck
Same number of vertebrae, but giraffe vertebrae experience a longer growth period

Bones grow due to the proliferation of cartilage producing cells called chondrocytes. Bone growth stops when a signal causes chondrocyte death.. In giraffes, the signaling is delayed, extending the growth period in the cervical vertebrae. So long necks evolved through changes in the timing of gene expression period, not through the acquisition of additional vertebrae.

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

Developmental genes constrain evolution in two ways:
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, that is, the regulatory genes usually change very slowly over the course of evolution.

Among arthropods, the Hox gene Ubx produces different effects.
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. LINK Arthropod evolution and diversity are discussed in Concept 23.3

Mutation to UBX gene prevents leg growth on the abdominal segments
Figure A Mutation in a Hox Gene Changed the Number of Legs in Insects Mutation to UBX gene prevents leg growth on the abdominal segments

Wings arose as modifications of existing structures.
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
The wing evolved 3 times in vertebrates. New “wing genes” did not appear. The wing resulted from mutations to a pre existing Limb gene.

Example: Three-spined sticklebacks (Gasterosteus aculeatus)
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)

These are greatly reduced in freshwater populations.
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. VIDEO 14.4 An example of phenotypic plasticity: Predator-induced development pathways in tadpoles

Figure 14.21 Parallel Phenotypic Evolution in Sticklebacks

Concept 14.5 Developmental Genes Contribute to Species Evolution but Also Pose Constraints
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. It is therefore a good example of parallel phenotypic evolution.

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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