19 Differential Gene Expression in Development

19 Differential Gene Expression in Development 19.1 What Are the Processes of Development? 19.2 Is Cell Differentiation Irreversible? 19.3 What Is the Role of Gene Expression in Cell Differentiation? 19.4 How Is Cell Fate Determined? 19.5 How Does Gene Expression Determine Pattern Formation?

19.1 What Are the Processes of Development? Development: the process in which a multicellular organism undergoes a series of progressive changes that characterizes its life cycle. 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 19.1 From Fertilized Egg to Adult (Part 1)

Figure 19.1 From Fertilized Egg to Adult (Part 2)

19.1 What Are the Processes 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 shapes differentiated cells into organs, etc. Growth is an increase in body size by cell division and cell expansion.

19.1 What Are the Processes of Development? Cells in a multicellular organisms are genetically identical; they differ from one another because of differential gene expression. In early embryos, every cell has potential to develop in many different ways.

19.1 What Are the Processes of Development? Morphogenesis in plant cells results from organized division and expansion of cells. In animals, cell movements are important in morphogenesis. Apoptosis (programmed cell death) is also important in orderly development.

19.1 What Are the Processes of Development? A cell’s fate, the type of cell it will ultimately become, is a function of differential gene expression and morphogenesis. Experiments in which specific cells of an early embryo are grafted to new positions on another embryo show the role of morphogenesis.

Figure 19.2 Developmental Potential in Early Frog Embryos (Part 1)

Figure 19.2 Developmental Potential in Early Frog Embryos (Part 2)

19.1 What Are the Processes of Development? Early embryonic cells have a range of possible fates, but possibilities become more restricted as development proceeds. The extracellular environment, as well as the cell contents, influence the genome and differentiation.

19.2 Is Cell Differentiation Irreversible? A zygote is totipotent, it can give rise to every cell type in the adult body. Later in development, the cells lose totipotency and become determined. Determination is followed by differentiation. But most cells retain the entire genome, and have the capacity for totipotency.

19.2 Is Cell Differentiation Irreversible? Plant cells are usually totipotent. Differentiated cells can be removed from a plant and grown in a culture, and eventually form a genetically identical plant—a clone. This ability is exploited in agricultural biotechnology.

Figure 19.3 Cloning a Plant

19.2 Is Cell Differentiation Irreversible? Animal somatic cells can also retain their totipotency. Experimental fusion of later embryo cells or nuclei with enucleated eggs stimulates cell division and development into normal adults.

19.2 Is Cell Differentiation Irreversible? These experiments indicate that: No genetic information is lost as the cell passes through developmental stages— called genomic equivalence. The cytoplasmic environment can modify the cell’s fate.

19.2 Is Cell Differentiation Irreversible? Totipotency of early embryonic cells is used in assisted reproductive technologies. The 8-cell embryo can be isolated, and one cell removed to test for harmful genetic conditions. The cells are then stimulated to divide to form an embryo and are implanted into the mother’s uterus.

19.2 Is Cell Differentiation Irreversible? Adult somatic cells also retain totipotency. The cell fusion technique was used to clone a sheep in the 1990s. The cells used in the experiments were starved for one week to arrest them in the G1 phase of the cell cycle.

Figure 19.4 Cloning a Mammal (Part 1)

Figure 19.4 Cloning a Mammal (Part 2)

19.2 Is Cell Differentiation Irreversible? One goal of the sheep cloning was to develop ways to produce transgenic sheep—for example in “pharming.” Many mammals have now been cloned— mice, goats, cattle, horses. Cloning may help to preserve some endangered animal species.

Figure 19.5 Cloned Mice

19.2 Is Cell Differentiation Irreversible? Differentiated cells stay differentiated because of their environment and developmental history. In normal development, a complex series of timed signals results in patterns of differentiation that result in the mature organism.

19.2 Is Cell Differentiation Irreversible? In plants, growing regions contain meristems—clusters of undifferentiated cells that can give rise to specialized structures such as roots and stems. Plants have fewer cell types than animals, and differ mostly in the structure of the cell walls.

19.2 Is Cell Differentiation Irreversible? In mammals, stem cells occur in tissues that require frequent replacement—skin, blood, intestinal lining. Stem cells produce daughter cells that differentiate into several cell types. Not totipotent, but pluripotent. Differentiation of pluripotent stem cells occurs as needed.

19.2 Is Cell Differentiation Irreversible? Bone marrow transplantation is used in cancer therapies. Therapies that kill cancer cells can also kill other rapidly dividing cells such as bone marrow stem cells. The stem cells are removed during the therapy, and then returned to the bone marrow.

19.2 Is Cell Differentiation Irreversible? Adjacent cells can influence stem cell differentiation. If bone marrow stem cells that can form muscle are transplanted to the heart, they form muscle. This has been used in animals to repair a damaged heart.

Figure 19.6 Repairing a Damaged Heart

19.2 Is Cell Differentiation Irreversible? Totipotent stem cells are found only in early embryos. Cells can be removed from embryos and grown indefinitely. These cells can be stimulated to differentiate with appropriate signals. For example, a derivative of vitamin A causes them to form neurons.

Figure 19.7 The Potential Use of Embryonic Stem Cells in Medicine (Part 1)

Figure 19.7 The Potential Use of Embryonic Stem Cells in Medicine (Part 2)

19.2 Is Cell Differentiation Irreversible? There is a potential to use human embryonic stem cells in medical applications. Human embryos are produced by in vitro fertilization, and only a few are implanted into the mother’s uterus.

19.2 Is Cell Differentiation Irreversible? Tissues from embryonic stem cells could be rejected by recipients because T cells would recognize them as nonself. Therapeutic cloning would involve nuclear transplantation and stem cell implantation combined. Stem cells would be derived from an embryo after being implanted with the patient’s own nuclei.

Figure 19.8 Therapeutic Cloning (Part 1)

Figure 19.8 Therapeutic Cloning (Part 2)

19.3 What Is the Role of Gene Expression in Cell Differentiation? 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.

19.3 What Is the Role of Gene Expression in Cell Differentiation? Myoblasts are undifferentiated precursors to muscle cells. Expression of a gene called MyoD produces a transcription factor MyoD. MyoD binds to promoters of muscle- determining genes and acts as its own promoter to keep levels high.

19.3 What Is the Role of Gene Expression in Cell Differentiation? If MyoD is transfected into other cell precursors, they also become muscle cells. Genes such as MyoD that encode for transcription factors fundamental to development are called developmental genes.

19.3 What Is the Role of Gene Expression in Cell Differentiation? Determination and differentiation are carried out by complex interactions between many genes and their products. Researchers using the sea urchin estimate that 1/3 of the eukaryotic genome is used only during development.

19.4 How Is Cell Fate Determined? Transcriptional controls that lead to differentiation are stimulated by chemical signals. Two mechanisms to produce the signals: Cytoplasmic segregation Induction

19.4 How Is Cell Fate Determined? Cytoplasmic segregation: Some patterns of gene expression are under cytoplasmic control. Polarity: having a “top” and a “bottom.” It can develop even in the zygote: the animal pole is the top, the vegetal pole is the bottom. Yolk and other factors can be distributed asymmetrically.

19.4 How Is Cell Fate Determined? Polarity was demonstrated using sea urchin embryos. If an 8-cell embryo is cut vertically, it develops into two small larvae. If the 8-cell embryo is cut horizontally, the bottom develops into a larva, the top remains embryonic.

Figure 19.9 Asymmetry in the Early Sea Urchin Embryo (Part 1)

Figure 19.9 Asymmetry in the Early Sea Urchin Embryo (Part 2)

19.4 How Is Cell Fate Determined? Cytoplasmic determinants are distributed unequally in the egg cytoplasm. These materials play a role in development of many animals.

Figure The Principle of Cytoplasmic Segregation

19.4 How Is Cell Fate Determined? The cytoskeleton contributes to distribution of cytoplasmic determinants. Microtubules and microfilaments have polarity, and cytoskeletal elements can bind certain proteins. In sea urchin eggs, a protein binds to the growing end (+) of a microfilament and to an mRNA encoding a cytoplasmic determinant.

19.4 How Is Cell Fate Determined? As 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.

19.4 How Is Cell Fate Determined? Induction: Fates of particular cells and tissues are sometimes determined by interactions with other tissues. Mediated by chemical signals and signal transduction pathways.

19.4 How Is Cell Fate Determined? Development of the lens in the vertebrate eye: The forebrain bulges out to form optic vesicles, which come in contact with cells at the surface of the head. These surface cells ultimately become the lens. The optic vesicle must contact the surface cells, or the lens won’t develop.

19.4 How Is Cell Fate Determined? The surface cells receive a signal, or inducer, from the optic vesicles. The developing lens also induces surface cells covering it to develop into the cornea.

Figure Embryonic Inducers in the Vertebrate Eye

19.4 How Is Cell Fate Determined? Vulval development in Caenorhabditis elegans: Adult C. elegans has 959 somatic cells; the source of each cell has been determined. Adults are hermaphroditic; eggs are laid through a ventral pore called the vulva.

Figure Induction during Vulval Development in Caenorhabditis elegans (A)

19.4 How Is Cell Fate Determined? During development, a single cell, the anchor cell, induces the vulva to form. If the anchor cell is destroyed, the vulva does not form. Anchor cell controls fate of six cells on the ventral surface by two signals—the primary and secondary inducers.

Figure Induction during Vulval Development in Caenorhabditis elegans (B)

19.4 How Is Cell Fate Determined? Anchor cell produces primary inducer— cells that receive it become vulval precursor cells. Other cells become epidermis. Cell closest to anchor cell becomes the primary vulval precursor—produces the secondary inducer. The inducers control activation or inactivation of genes through signal transduction cascades.

19.4 How Is Cell Fate Determined? Much of development is controlled by such molecular switches, that allow a cell to follow one of two alternative tracks. Primary inducer released by the anchor cell is homologous to a human growth factor called EGF (epidermal growth factor).

Figure Embryonic Induction

19.5 How Does Gene Expression Determine Pattern Formation? Pattern formation: the process that results in the spatial organization of tissues. Linked with morphogenesis. Programmed cell death—apoptosis—is also important.

19.5 How Does Gene Expression Determine Pattern Formation? Apoptosis can “sculpt” organs such as the hands during development. Connective tissue links fingers in early human embryo. The connective cells die later, freeing the fingers.

Figure Apoptosis Removes the Tissue between Human Fingers

19.5 How Does Gene Expression Determine Pattern Formation? C. elegans produces exactly 1,090 somatic cells as it develops, but 131 of those cells die. The sequential expression of two genes control this cell death. A third gene codes for an inhibitor of apoptosis.

19.5 How Does Gene Expression Determine Pattern Formation? A similar system acts in humans: Caspases that stimulate apoptosis, are similar to proteins encoded by the nematode genes, as is the inhibitor of apoptosis.

19.5 How Does Gene Expression Determine Pattern Formation? Flowers are composed of organs (sepals, petals, stamens, carpels) arranged around a central axis in whorls. The whorls develop from meristems (undifferentiated cells)—organ identity is determined by organ identity genes.

19.5 How Does Gene Expression Determine Pattern Formation? Organ identity genes have been studied in Arabidopsis. Three classes of organ identity genes: Class A, expressed in sepals and petals. Class B, expressed in petals and stamens. Class C, expressed in stamens and carpels.

Figure Organ Identity Genes in Arabidopsis Flowers (A)

19.5 How Does Gene Expression Determine Pattern Formation? Two lines of experimental evidence support this model: Loss-of-function mutations—mutation in A results in no sepals or petals. Gain-of-function mutations—promoter for C can be coupled to A—result is only sepals and petals.

Figure Organ Identity Genes in Arabidopsis Flowers (B)

19.5 How Does Gene Expression Determine Pattern Formation? Gene classes A, B, and C code for subunits of transcription factors, which are active as dimers. Gene regulation is combinatorial. A common feature of the transcription factors is a DNA-binding domain called the MADS box. They also have domains that can bind to other proteins in a transcription initiation complex.

19.5 How Does Gene Expression Determine Pattern Formation? A gene called leafy codes for a protein that controls transcription of organ identity genes. Plants with a mutation that causes underexpression of leafy do not produce flowers. Protein product of this gene acts as a transcription factor to stimulate gene classes A, B, and C.

Figure A Nonflowering Mutant

19.5 How Does Gene Expression Determine Pattern Formation? Fate of a cell is often determined by where the cell is. Positional information comes in the form a signal, a morphogen, that diffuses down a body axis, setting up a concentration gradient.

19.5 How Does Gene Expression Determine Pattern Formation? A morphogen must directly affect target cells, and different concentrations of the morphogen result in different effects. Example: development of a vertebrate limb. Cells in the developing limb bud that become bone and muscle must receive positional information.

19.5 How Does Gene Expression Determine Pattern Formation? Cells at the posterior base of the limb bud, called the zone of polarizing activity, make a morphogen called BMP2. The gradient of BMP2 determines the posterior-anterior axis of the developing limb. Cells getting the highest dose make the little finger, those getting the lowest dose make the thumb.

19.5 How Does Gene Expression Determine Pattern Formation? The fruit fly Drosophila melanogaster has a segmented body: head, thorax, and abdomen, each made of several segments. Several types of genes are expressed sequentially to define these segments. Genes in each step code for transcription factors that in turn control synthesis of other transcription factors—a transcriptional cascade.

19.5 How Does Gene Expression Determine Pattern Formation? Maternal effect genes are transcribed in the cells of the ovary that surround the anterior part of the egg. Bicoid and nanos determine the anterior- posterior axis. The mRNAs diffuse to the anterior end of egg. Bicoid mRNA stays in the anterior end, and bicoid protein diffuses out, creating a gradient.

19.5 How Does Gene Expression Determine Pattern Formation? At high 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.

Figure Bicoid Protein Provides Positional Information

19.5 How Does Gene Expression Determine Pattern Formation? Actions of these genes have been determined by causing mutations in the genes and from experiments in which cytoplasm was transferred from one egg to another. After egg is fertilized, nuclear division produce a multinucleate cell called a syncytium. Bicoid and nanos mRNAs are translated and establish gradients.

19.5 How Does Gene Expression Determine Pattern Formation? Segmentation genes determine properties of the larval segments. Three classes of genes act in sequence: Gap genes organize broad areas. Pair rule genes divide embryo into units of two segments each. Segment polarity genes determine boundaries and anterior-posterior organization in individual segments.

Figure A Gene Cascade Controls Pattern Formation in the Drosophila Embryo

19.5 How Does Gene Expression Determine Pattern Formation? Hox genes are expressed in different combinations along the length of the embryo; they determine what each segment will become. Hox genes map on chromosome 3, in two clusters, in the same order as the segments whose functions they determine.

Figure Hox Genes in Drosophila

19.5 How Does Gene Expression Determine Pattern Formation? 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 A Homeotic Mutation in Drosophila

19.5 How Does Gene Expression Determine Pattern Formation? All the hox genes have a common DNA sequence and probably arose from a single gene in an unsegmented ancestor. The common 180-base pair sequence is called the homeobox. It encodes a transcription factor that binds DNA— called the homeodomain.

19.5 How Does Gene Expression Determine Pattern Formation? Genes containing the homeobox are found in many animals, including humans. Their role is similar to MADS in plants.