Genetic Basis of Development: From Single Cell to Multicellular Organism

1. Chapter 21 The Genetic Basis of Development

2. Overview: From Single Cell to Multicellular Organism • Genetic analysis and DNA technology have revolutionized the study of development • Researchers use mutations to deduce developmental pathways • They apply concepts and tools of molecular genetics to the study of developmental biology

4. When the primary research goal is to understand broad biological principles, the organism chosen for study is called a model organism • Researchers select model organisms that are representative of a larger group, suitable for the questions under investigation, and easy to grow in the lab Video: C. elegans Crawling

9. Concept 21.1: Embryonic development involves cell division, cell differentiation, and morphogenesis • In embryonic development of most organisms, a single-celled zygote gives rise to cells of many different types, each with a different structure and corresponding function • Development involves three processes: cell division, cell differentiation, and morphogenesis (“creation of form”)

10. LE 21-3 Fertilized egg of a frog Tadpole hatching from egg

11. Through a succession of mitotic cell divisions, the zygote gives rise to a large number of cells • In cell differentiation, cells become specialized in structure and function • Morphogenesis encompasses the processes that give shape to the organism and its various parts

12. LE 21-4 Animal development Gut Cell movement Zygote (fertilized egg) Eight cells Blastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell division Morphogenesis Observable cell differentiation Seed leaves Plant development Shoot apical meristem Root apical meristem Two cells Zygote (fertilized egg) Embryo inside seed Plant

13. Concept 21.2: Different cell types result from differential gene expression in cells with the same DNA • Differences between cells in a multicellular organism come almost entirely from gene expression, not differences in the cells’ genomes • These differences arise during development, as regulatory mechanisms turn genes off and on

14. Evidence for Genomic Equivalence • Many experiments support the conclusion that nearly all cells of an organism have genomic equivalence (the same genes) • A key question that emerges is whether genes are irreversibly inactivated during differentiation

15. Totipotency in Plants • One experimental approach for testing genomic equivalence is to see whether a differentiated cell can generate a whole organism • A totipotent cell is one that can generate a complete new organism • Cloning is using one or more somatic cells from a multicellular organism to make a genetically identical individual

16. LE 21-5 Transverse section of carrot root 2-mg fragments Fragments cul- tured in nutrient medium; stir- ring causes single cells to shear off into liquid. Single cells free in suspension begin to divide. Embryonic plant develops from a cultured single cell. Plantlet is cul- tured on agar medium. Later it is planted in soil. A single somatic (nonreproductive) carrot cell developed into a mature carrot plant. The new plant was a genetic duplicate (clone) of the parent plant. Adult plant

17. Nuclear Transplantation in Animals • In nuclear transplantation, the nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell • Experiments with frog embryos have shown that a transplanted nucleus can often support normal development of the egg

18. LE 21-6 Frog embryo Frog egg cell Frog tadpole UV Fully differ- entiated (intestinal) cell Less differ- entiated cell Donor nucleus trans- planted Donor nucleus trans- planted Enucleated egg cell Most develop into tadpoles <2% develop into tadpoles

19. Reproductive Cloning of Mammals • In 1997, Scottish researchers announced the birth of Dolly, a lamb cloned from an adult sheep by nuclear transplantation from a differentiated mammary cell • Dolly’s premature death in 2003, as well as her arthritis, led to speculation that her cells were “older” than those of a normal sheep, possibly reflecting incomplete reprogramming of the original transplanted nucleus

20. LE 21-7 Mammary cell donor Egg cell donor Egg cell from ovary Nucleus removed Cells fused Cultured mammary cells are semistarved, arresting the cell cycle and causing dedifferentiation Nucleus from mammary cell Grown in culture Early embryo Implanted in uterus of a third sheep Surrogate mother Embryonic development Lamb (“Dolly”) genetically identical to mammary cell donor

21. Since 1997, cloning has been demonstrated in many mammals, including mice, cats, cows, horses, and pigs • “Copy Cat” was the first cat cloned

23. Problems Associated with Animal Cloning • In most nuclear transplantation studies, only a small percentage of cloned embryos have developed normally to birth • Many epigenetic changes, such as acetylation of histones or methylation of DNA, must be reversed in the nucleus from a donor animal in order for genes to be expressed or repressed appropriately for early stages of development

24. The Stem Cells of Animals • A stem cell is a relatively unspecialized cell that can reproduce itself indefinitely and differentiate into specialized cells of one or more types • Stem cells isolated from early embryos at the blastocyst stage are called embryonic stem cells • The adult body also has stem cells, which replace nonreproducing specialized cells • Embryonic stem cells are totipotent, able to differentiate into all cell types • Adult stem cells are pluripotent, able to give rise to multiple but not all cell types

25. LE 21-9 Embryonic stem cells Adult stem cells Totipotent cells Pluripotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Liver cells Nerve cells Blood cells

26. Transcriptional Regulation of Gene Expression During Development • Cell determination precedes differentiation and involves expression of genes for tissue-specific proteins • Tissue-specific proteins enable differentiated cells to carry out their specific tasks

27. LE 21-10_1 Nucleus Master control gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF

28. LE 21-10_2 Nucleus Master control gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF Determination mRNA OFF MyoD protein (transcription factor) Myoblast (determined)

29. LE 21-10_3 Nucleus Master control gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF Determination mRNA OFF MyoD protein (transcription factor) Myoblast (determined) Differentiation mRNA mRNA mRNA mRNA Myosin, other muscle proteins, and cell-cycle blocking proteins MyoD Another transcription factor Muscle cell (fully differentiated)

30. Cytoplasmic Determinants and Cell-Cell Signals in Cell Differentiation • Maternal substances that influence early development are called cytoplasmic determinants • These substances regulate expression of genes that affect the cell’s developmental fate Animation: Cell Signaling

31. LE 21-11a Unfertilized egg cell Sperm Molecules of another cytoplasmic determinant Molecules of a cytoplasmic determinant Nucleus Fertilization Zygote (fertilized egg) Mitotic cell division Two-celled embryo Cytoplasmic determinants in the egg

32. The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells • In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells

33. LE 21-11b Early embryo (32 cells) Signal transduction pathway NUCLEUS Signal receptor Signal molecule (inducer) Induction by nearby cells

34. Concept 21.3: Pattern formation in animals and plants results from similar genetic and cellular mechanisms • Pattern formation is the development of a spatial organization of tissues and organs • It occurs continually in plants, but it is mostly limited to embryos and juveniles in animals • Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells

35. Drosophila Development: A Cascade of Gene Activations • Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster • Combining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans

36. The Life Cycle of Drosophila • After fertilization, positional information specifies the body segments in Drosophila • Positional information triggers the formation of each segment’s characteristic structures • Sequential gene expression produces regional differences in the formation of the segments

37. LE 21-12 Follicle cell Egg cell developing within ovarian follicle Nucleus Egg cell Nurse cell Fertilization Laying of egg Fertilized egg Egg shell Nucleus Embryo Multinucleate single cell Early blastoderm Plasma membrane formation Yolk Late blastoderm Body segments Cells of embryo Segmented embryo 0.1 mm Hatching Larval stages (3) Pupa Metamorphosis Adult fly Thorax Abdomen Head 0.5 mm Dorsal BODY AXES Anterior Posterior Ventral

38. Genetic Analysis of Early Development: Scientific Inquiry • Study of developmental mutants laid the groundwork for understanding the mechanisms of development • Mutations that affect segmentation are likely to be embryonic lethals, leading to death at the embryonic or larval stage

39. LE 21-13 Eye Leg Antenna Wild type Mutant

40. Axis Establishment • Maternal effect genes encode for cytoplasmic determinants that initially establish the axes of the body of Drosophila • These maternal effect genes are also called egg-polarity genes because they control orientation of the egg and consequently the fly

41. One maternal effect gene, the bicoid gene, affects the front half of the body • An embryo whose mother has a mutant bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends • This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end • This hypothesis is an example of the gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features

42. LE 21-14a Tail Head Wild-type larva Tail Tail Mutant larva (bicoid) Drosophila larvae with wild-type and bicoid mutant phenotypes

43. LE 21-14b Egg cell Nurse cells Developing egg cell bicoid mRNA Bicoid mRNA in mature unfertilized egg Fertilization Translation of bicoid mRNA 100 m m Bicoid protein in early embryo Anterior end Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo

44. Animation: Development of Head-Tail Axis in Fruit Flies

45. The bicoid research is important for three reasons: 1. It identified a specific protein required for some early steps in pattern formation 2. It increased understanding of the mother’s role in embryo development 3. It demonstrated a key developmental principle that a gradient of molecules can determine polarity and position in the embryo

46. Segmentation Pattern • Segmentation genes produce proteins that direct formation of segments after the embryo’s major body axes are formed • Positional information is provided by sequential activation of three sets of segmentation genes: gap genes, pair-rule genes, and segment-polarity genes

47. Identity of Body Parts • The anatomical identity of Drosophila segments is set by master regulatory genes called homeotic genes • Mutations to homeotic genes produce flies with strange traits, such as legs growing from the head in place of antennae

49. C. elegans: The Role of Cell Signaling • The nematode C. elegans is a very useful model organism for investigating the roles of cell signaling, induction, and programmed cell death in development • Researchers know the entire ancestry of every cell of an adult C. elegans—the organism’s complete cell lineage Video: C. elegans Embryo Development (time lapse)

50. LE 21-15 Zygote 0 First cell division Germ line (future gametes) Nervous system, outer skin, mus- culature Musculature, gonads Outer skin, nervous system Time after fertilization (hours) Musculature 10 Hatching Intestine Intestine Eggs Vulva ANTERIOR POSTERIOR 1.2 mm