Chapter 21

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

3. ARABIDOPSIS THAMANA (COMMON WALL CRESS) MUS MUSCULUS (MOUSE) DROSOPHILA MELANOGASTER (FRUIT FLY) DANIO RERIO (ZEBRAFISH) CAENORHABDITIS ELEGANS (NEMATODE) 0.25 mm Researchers select model organisms that are representative of a larger group, suitable for the questions under investigation, and easy to grow in the lab

4. 21.1: Embryonic development involves cell division, cell differentiation, and morphogenesis • Embryonic development of most organisms begins with a single-celled zygote giving rise to cells of many different types - structure & function

5. (b) Tadpole hatching from egg (a) Fertilized eggs of a frog • Cell division -Zygote gives rise to large # of cells • Cell differentiation - Cells become specialized in structure & function • Morphogenesis - processes that give shape to organism & its various parts • The transformation from a zygote into an organism has 3 interrelated processes:

6. (a) Animal development. Most animals go through some variation of the blastula and gastrula stages. The blastula is a sphere of cells surrounding a fluid-filled cavity. The gastrula forms when a region of the blastula folds inward, creating a tube—a rudimentary gut. Once the animal is mature, differentiation occurs in only a limited way—for the replacement of damaged or lost cells. Gut Cell movement Zygote (fertilized egg) Eight cells Blastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell division Morphogenesis Observable cell differentiation (b) Plant development. In plants with seeds, a complete embryo develops within the seed. Morphogenesis, which involves cell division and cell wall expansion rather than cell or tissue movement, occurs throughout the plant’s lifetime. Apical meristems (purple) continuously arise and develop into the various plant organs as the plant grows to an indeterminate size. Seed leaves Shoot apical meristem Root apical meristem Zygote (fertilized egg) Two cells Embryo inside seed Plant Figure 21.4a, b • 3 processes of development overlap in time

7. 21.2: Different cell types result from differential gene expression in cells with the same DNA • Differences between cells in multicellular organism come almost entirely from differences in geneexpression (not differences in cells’ genomes) • Many experiments support that nearly all cells of an organism have genomicequivalence (they have the same genes)

8. Transverse section of carrot root CONCLUSION EXPERIMENT 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. RESULTS Adult plant At least some differentiated (somatic) cells in plants are toipotent, able to reverse their differentiation and then give rise to all the cell types in a mature plant. Figure 21.5 Totipotency One experimental approach for testing genomic equivalenceis to see whether a differentiated cell can generate a whole organism

9. Totipotent cell • One capable of generating a complete new organism • Cloning • Using one or more somatic cells from a multicellular organism to make another genetically identical individual

10. Somatic Cell Nuclear Transplantation (SCNT) • Nucleus of an unfertilized egg cell or zygote is replaced with nucleus of a differentiated cell

11. Reproductive Cloning of Mammals • In 1997, Scottish researchers cloned a lamb from an adult sheep by nuclear transplantation

12. Egg cell donor Mammary cell donor APPLICATION This method is used to produce cloned animals whose nuclear genes are identical to the donor animal supplying the nucleus. 1 2 Egg cell from ovary Nucleus removed Nucleus removed Cells fused Cultured mammary cells are semistarved, arresting the cell cycle and causing dedifferentiation 3 TECHNIQUE Shown here is the procedure used to produce Dolly, the first reported case of a mammal cloned using the nucleus of a differentiated cell. Nucleus from mammary cell Grown in culture 4 RESULTS The cloned animal is identical in appearance and genetic makeup to the donor animal supplying the nucleus, but differs from the egg cell donor and surrogate mother. Early embryo Implanted in uterus of a third sheep 5 Surrogate mother Embryonic development 6 Lamb (“Dolly”) genetically identical to mammary cell donor Figure 21.7

13. Figure 21.8 • “Copy Cat” • Was the first cat ever cloned - 2002

14. Problems Associated with Animal Cloning • In most nuclear transplantation studies performed thus far • Only small percentage of cloned embryos develop normally to birth • Many health problems

15. Reproductive vs. Therapeutic Cloning • Reproductive Cloning – making an entire organism • Therapeutic Cloning – making tissues for organ/tissue replacement (Clone organism and remove stem cells)

16. The Stem Cells of Animals • Stem cell - • Relatively unspecialized cell • Can reproduce itself indefinitely • Can differentiate into specialized cells of one or more types, given appropriate conditions

17. Stem cells can be isolated from early embryos at blastocyst stage Totipotent – can become all cell types Pluripotent – can become most cell types (embryonic stem cells)

18. Embryonic stem cells Adult stem cells Early human embryo at blastocyst stage (mammalian equiva- lent of blastula) From bone marrow in this example Totipotent cells Pluripotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Liver cells Blood cells Nerve cells Figure 21.9 Multipotent – Can become some cell types (adult stem cells) http://www.youtube.com/watch?v=fqwEiSnQcGE

20. Embryonic Stem Cells without the Controversy • iPS Cells – Induced pluripotent stem cells • Somatic cell “reprogrammed” back into embryonic-like stem cell • http://www.pbs.org/wgbh/nova/body/stem-cells-breakthrough.html

21. Transcriptional Regulation of Gene Expression During Development • Cell determination • Precedes differentiation - involves expression of master control genes for tissue-specific proteins (ex: MyoD) • Master control genes turn on other tissue-specific proteins • Enable differentiated cells to carry out their specific tasks

22. 2 1 Nucleus Master control gene myoD Other muscle-specific genes DNA OFF OFF Embryonicprecursor cell Determination. Signals from othercells lead to activation of a masterregulatory gene called myoD, andthe cell makes MyoD protein, atranscription factor. The cell, nowcalled a myoblast, is irreversiblycommitted to becoming a skeletalmuscle cell. OFF mRNA MyoD protein(transcriptionfactor) Myoblast (determined) Differentiation. MyoD protein stimulatesthe myoD gene further, and activatesgenes encoding other muscle-specifictranscription factors, which in turn activate genes for muscle proteins. MyoD also turns on genes that block the cell cycle, thus stopping cell division. The nondividing myoblasts fuse to become mature multinucleate muscle cells, alsocalled muscle fibers. mRNA mRNA mRNA mRNA Myosin, othermuscle proteins,and cell-cycleblocking proteins MyoD Anothertranscriptionfactor Muscle cell(fully differentiated) • Determination and differentiation of muscle cells Figure 21.10

23. Unfertilized egg cell Molecules of a a cytoplasmic determinant Fertilization Nucleus Zygote (fertilized egg) Mitotic cell division Two-celled embryo (a) Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its cytoplasm, encoded by the mother’s genes, that influence development. Many of these cytoplasmic determinants, like the two shown here, are unevenly distributed in the egg. After fertilization and mitotic division, the cell nuclei of the embryo are exposed to different sets of cytoplasmic determinants and, as a result, express different genes. Cytoplasmic Determinants Sperm • Cytoplasmic determinants in cytoplasm of unfertilized egg regulate expression of genes in zygote that affect developmental fate of embryonic cells Molecules of another cyto- plasmic deter- minant

24. (b) Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing chemicals that signal nearby cells to change their gene expression. Early embryo (32 cells) • In process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells NUCLEUS Signal transduction pathway Signal receptor Signal molecule (inducer)

25. 21.3: Pattern formation in animals and plants results from similar genetic and cellular mechanisms • Pattern formation • Development of a spatial organization of tissues and organs • Occurs continually in plants • Mostly limited to embryos and juveniles in animals • Positional information • Consists of molecular cues that control pattern formation • Tells a cell its location relative to body’s axes and to other cells

26. Eye Antenna Leg Wild type Mutant Genetic Analysis of Early Development: Scientific Inquiry • Study of developmental mutants • Laid groundwork for understanding mechanisms of development Figure 21.13

27. Follicle cell Nucleus Egg cell developing within ovarian follicle Egg cell Nurse cell Fertilization Laying of egg Fertilized egg Egg shell Nucleus Embryo Multinucleate single cell 3 4 6 7 1 5 2 Early blastoderm Plasma membrane formation Yolk Late blastoderm Cells of embryo Body segments Segmented embryo 0.1 mm Hatching Larval stages (3) Pupa Metamorphosis Head Abdomen Thorax Adult fly 0.5 mm Dorsal Anterior BODY AXES Posterior Figure 21.12 Ventral Key developmental events in the life cycle of Drosophila 1st - Maternal effects genes (egg polarity genes) 2nd – Segmentation Genes 3rd – Homeotic Genes

28. Axis Establishment • Maternal effect genes • Encode for cytoplasmic determinants that initially establish axes of the body of Drosophila

29. Tail Head T1 A8 T2 A7 T3 A6 A1 A5 A2 A4 A3 Wild-type larva Tail Tail A8 A8 A7 A6 A7 Mutant larva (bicoid) (a) Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation in the mother’s bicoid gene leads to tail structures at both ends (bottom larva). The numbers refer to the thoracic and abdominal segments that are present. • Flies with the bicoidmutation do not develop a body axis correctly

30. Segmentation Pattern • Segmentation genes • Produce proteins that direct formation of segments after the embryo’s major body axes are formed

31. Identity of Body Parts • Anatomical identity of Drosophilasegments is set by master regulatory genes called homeotic genes

32. Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse Figure 21.23 Identical or very similar nucleotide sequencehas been discovered in homeotic genes of both vertebrates & invertebrates Aka HOX genes in vertebrates

33. Hierarchy of Gene Activity in Early Drosophila Development Maternal effect genes (egg-polarity genes) Gap genes Segmentation genes of the embryo Pair-rule genes Segment polarity genes Homeotic genes of the embryo Other genes of the embryo • A summary of gene activity during Drosophila development

34. 2 µm Figure 21.17 Programmed Cell Death (Apoptosis) Apoptosis - Cell signaling is involved in programmed cell death

35. Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Ced-4 Ced-3 Death signal receptor Inactive proteins Cell forms blebs Ced-9 (inactive) (a) No death signal Death signal Other proteases Active Ced-3 Active Ced-4 Nucleases Activation cascade (b) Death signal Figure 21.18a, b In C. elegans, a protein in outer mitochondrial membrane serves as master regulator of apoptosis

36. Interdigital tissue 1 mm Figure 21.19 • In vertebrates, apoptosis is essential for normal morphogenesis of hands & feet in humans & paws in other animals

37. Plant Development: Cell Signaling and Transcriptional Regulation • Thanks to DNA technology and clues from animal research, plant research is now progressing rapidly • In general, cell lineage is much less important for pattern formation in plants than in animals • The embryonic development of most plants occurs inside the seed

38. Carpel Stamen Petal L1 L2 Cell layers L3 Sepal Floral meristem Anatomy of a flower Tomato flower Figure 21.20 Pattern Formation in Flowers • Floral meristems- Contain 3 cell types that affect flower development

39. Wild type Mutant • Organ identity genes • Determine type of structure that will grow from a meristem • Analogous to homeotic genes in animals

40. 21.4: Comparative studies explain how evolution leads to morphological diversity • Biologists in field of evolutionary developmental biology (“evo-devo”) compare developmental processes of different multicellular organisms • There is widespread conservation of developmental genes among animals • Molecular analysis of homeotic genes in Drosophila has shown that they all include a sequence called a homeobox

41. Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse Figure 21.23 Identical or very similar nucleotide sequencehas been discovered in homeotic genes of both vertebrates & invertebrates Aka HOX genes in vertebrates

42. Related genetic sequences have been found in regulatory genes of yeasts, plants, and even prokaryotes • In addition to developmental genes, many other genes involved in development are highly conserved from species to species

43. Genital segments Abdomen Thorax Thorax Abdomen Figure 21.24 • In some cases, small changes in regulatory sequences of particular genes can lead to major changes in body form, as in crustaceans and insects

44. In other cases, genes with conserved sequences play different roles in development of different species • In plants, homeobox-containing genes do not function in pattern formation as they do in animals

45. Comparison of Animal and Plant Development • Both plants & animals • Development relies on cascade of transcriptional regulators turning genes on or off in a finely tuned series • But genes that direct analogous developmental processes • Differ considerably in sequence in plants and animals, as a result of their remote ancestry