1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: From Single Cell to Multicellular Organism 1. In this chapter, you will learn how a complex, multicellular organism develops from a single cell. This is called __________________________________. You have already studied all of the processes involved in development: cell signaling (Chapter 11), how genes code for proteins (Chapter 17), and the regulation/control of gene expression (Chapter 19), so there is really nothing new in this chapter. If you can understand the processes described in this chapter, it means that you understand the previous chapters and you can apply your knowledge. Go back and review often!

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2. What happened?!

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Video: C. elegans Crawling 3.

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4. Drosophila melanogaster

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4. Mus musculus

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4. Danio (zebrafish)

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4. Arabidopsis

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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”)

9. Figure 21-3 Fertilized egg of a frogTadpole hatching from egg

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 (“creation of form”) encompasses the processes that give shape to the organism and its various parts 5.

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 6. Major differences between the developmental mechanisms of animals and plants In animals, but not in plants, movements of cells and tissues are necessary to transform the early embryo into the characteristic three-dimensional form of the organism. In plants, but not in animals, morphogenesis and growth in overall size are not limited to the embryonic and juvenile periods, but occur throughout the life of the plant.

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

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 7. A zygote is a fertilized egg. A blastula is a sphere of cells surrounding a fluid- filled cavity. A gastrula forms when a region of the blastula folds inward, creating a tube. (This tube becomes the digestive system.) An apical meristem is a perpetually embryonic region in the tips of the shoots and roots of plants.

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 8. 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

16. 9. Figure 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 10-11. Totipotency in Plants Steward’s experimental approach for testing genomic equivalence was to see whether a differentiated cell could 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

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 12-13. A clone is a new individual produced by cloning. If you have ever grown a new plant from a cutting, you have practiced cloning. NO, you cannot clone animals (or yourself) in this way.

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 14-15. But, you can do 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

20. 15-16. Figure 21-6 Frog embryoFrog egg cellFrog tadpole UV Less differ- entiated cell Donor nucleus trans- planted Enucleated egg cell Most develop into tadpoles <2% develop into tadpoles Donor nucleus trans- planted Fully differ- entiated (intestinal) cell

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 17. 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

22. 18. Figure 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 Early embryo Grown in culture Implanted in uterus of a third sheep Surrogate mother Embryonic development Lamb (“Dolly”) genetically identical to mammary cell donor

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Since 1997, cloning has been demonstrated in many mammals, including mice, cats, cows, horses, and pigs “Copy Cat” was the first cat cloned

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 19. Copy Cat had a calico coat, like her single female parent, but the color and pattern were different, because of random X inactivation. Environmental influences and random phenomena play a significant role during development.

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 20-21. 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

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 22-24. 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 (adult stem 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

27. Figure 21-9 Embryonic stem cellsAdult stem cells Pluripotent cells Totipotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Liver cellsNerve cellsBlood cells

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 25. Medical applications of stem cells The ultimate goal is to supply cells for the repair of damaged or diseased organs Example: insulin-producing pancreatic cells for people with diabetes Example: brain cells for people with Parkinson’s disease or Huntington’s disease This is called therapeutic cloning

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 26. Ethical concerns What are the major arguments?

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 27-30. Transcriptional Regulation of Gene Expression During Development Cell determination refers to the events that lead to the observable differentiation of a cell. Determination precedes differentiation and involves expression of genes for tissue-specific proteins A committed cell is one that is irreversibly on the path to become a specific type of cell Tissue-specific proteins are found only in a specific cell type and give the cell its characteristic structure and function. They enable differentiated cells to carry out their specific tasks

31. Figure 21-10_1 Nucleus Embryonic precursor cell DNA OFF Master control gene myoDOther muscle-specific genes

32. Figure 21-10_2 Nucleus Embryonic precursor cell DNA OFF Master control gene myoDOther muscle-specific genes mRNAOFF Determination Myoblast (determined) MyoD protein (transcription factor)

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

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 31. Note that DNA codes for _____________, which codes for _________________ (Chapter 17!). Notice also that genes are often named with a short letter (and sometimes number) code that starts with a lowercase letter. The protein the gene codes for often has the same name, with an uppercase letter. Furthermore, as you study this Figure, you’ll notice that MyoD is a transcription factor, so it does what transcription factors do, which is bind to the ______________________ of a gene and turn it __________. If you study this figure even more closely, you’ll notice that the MyoD protein stimulates the myoD gene. This is an example of ___________________ ______________________.

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 32. The MyoD protein is powerful. Researchers have used it to change some kinds of fully differentiated nonmuscle cells, such as fat cells and liver cells, into muscle cells. This doesn’t work on all cells. One likely explanation is that activation of the muscle-specific genes is not solely dependent on MyoD, but requires a particular combination of regulatory proteins, some of which may be lacking in cells that do not respond to MyoD.

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 33. a. Cytoplasmic Determinants Maternal substances that influence early development are called cytoplasmic determinants These substances, which are unevenly distributed in the unfertilized egg, regulate expression of genes that affect the cell’s developmental fate Animation: Cell Signaling Animation: Cell Signaling

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

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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.b. Induction

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

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 34-35. Pattern formation in animals and plants results from similar genetic and cellular mechanisms An organism’s body plan is its overall three- dimensional arrangment (Ex. Vertebrate body plan) Pattern formation is the development of a spatial organization in which the tissues and organs of an organisms are all in their characteristic places It occurs continually in the apical meristems of plants, but it is mostly limited to embryos and juveniles in animals

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 36. Positional information are the molecular cues that control pattern formation These molecules may be cytoplasmic determinants or inductive signals (we just covered these in #33.a-b!) Positional information tells a cell its location relative to the body axes and to neighboring cells

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 37. 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 (BTW – rarely, pattern formation mutations occur in humans!)

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

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 37. The Life Cycle of Drosophila GENES and POSITIONAL INFORMATION (cytoplasmic determinants and inductive signals) determine Drosophila’s body pattern 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

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 37.a. Genetic Analysis of Early Development: Scientific Inquiry Lewis’s study of developmental mutants laid the groundwork for understanding the mechanisms of development He studied bizarre mutant flies (see next slide) He located the mutations on the fly’s genetic map, providing concrete evidence that developmental processes are under genetic control

46. Figure 21-13 Eye Antenna Leg Wild typeMutant

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 37.b. Nusslein-Volhard and Wieschaus set out to identify all the genes that affect segment formation in Drosophila Problems: 1. looking for a needle in a haystack, 2. embryonic lethals – can’t be bred for study, 3. have to study maternal genes as well (cytoplasmic determinants play a role in axis formation) So, they studied recessive mutations They eventually identified 1,200 genes essential for embryonic development (120 of which were needed for normal segmentation). They mapped them and cloned them. Lots of hard work, but they won the 1995 Nobel Prize (Lewis, too)

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 38. Axis Establishment-answers on next slide… BUT, before the 1,200 genes discovered by the researchers above can direct the development of Drosophila, other substances must establish the body plan in the egg. These substances are cytoplasmic determinants coded for by genes of the mother fly, therefore they are called ____________________ ____________________ _________________, also known as ______________________________ ____________________. These genes set up the anterior-posterior and dorsal-ventral axes of the embryo.

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings BUT, before the 1,200 genes discovered by the researchers above can direct the development of Drosophila, other substances must establish the body plan in the egg. These substances are cytoplasmic determinants coded for by genes of the mother fly, therefore they are called maternal effect genes, also known as egg-polarity genes. These genes set up the anterior-posterior and dorsal- ventral axes of the embryo.

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Hypothesis: This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end This is an example of the gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features 39.a.

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

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 39.b-c. Scientists injected pure bicoid mRNA into various regions of early embryos. The protein that resulted from its translation caused anterior structures to form at the injection sites. (BTW – they had to clone the bicoid gene and use a nucleic acid probe derived from the gene to find the location of the bicoid mRNA in the eggs to do this) In Drosophila, gradients of specific proteins determine the posterior end as well as the anterior and also are responsible for establishing the dorsal- ventral axis.

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

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Conclusion:

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 40-41. 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 Working together, the products of egg-polarity genes like bicoid regulate the regional expression of gap genes, which control the localized expression of pair- rule genes, which in turn activate specific segment polarity genes in different parts of each segment. The boundaries and axes of the segments are now set.

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 42. 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 (review Figure 21-13)

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 43. Like many of the egg-polarity and segmentation genes, the homeotic genes code for specific transcription factors. These regulatory proteins turn genes on or off, regulating expression of genes responsible for particular anatomical structures. For example, a homeotic protein made in the cells of a particular head segment specifies antenna development. In contrast, a homeotic protein active in a certain thoracic segment selectively activates genes that bring about leg development.

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 44. A mutant version of the gene encoding the thoracic homeotic protein causes the protein to be expressed also in the head segment. There, it overrides the normal antennal gene-activating protein, labeling the segments as “thoracic” instead of “head” and causing legs to develop instead of antennae.

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

60. 45. 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 All adult C. elegans have exactly 959 cells Video: C. elegans Embryo Development (time lapse) Video: C. elegans Embryo Development (time lapse)

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

62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Induction As early as the four-cell stage in C. elegans, cell signaling helps direct daughter cells down the appropriate pathways, a process called induction

63. Figure 21-16a Anterior EMBRYO Posterior Receptor Signal protein 1 2 3 4 4 3 Signal Posterior daughter cell of 3 Anterior daughter cell of 3 Will go on to form muscle and gonads Will go on to form adult intestine Induction of the intestinal precursor cell at the four-cell stage

64. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 46. A signal from cell 4 acts on cell 3 so that one of the daughter cells of cell 3 eventually gives rise to the intestine. If cell 4 is experimentally removed early in the four-cell stage, no intestine is formed. If an isolated cell 3 and cell 4 are recombined, the intestine develops as normal. This is induction!

65. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Induction is also critical later in nematode development, as the embryo passes through three larval stages prior to becoming an adult 47.

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 47.a-c. The vulva is the tiny opening through which a worm lays its eggs The vulva arises from six cells that are present on the ventral surface at the second larval stage of development The anchor cell starts a cascade of inductive signals that establishes the fates of the six vulval precursor cells If an experimenter destroys the anchor cell, the vulva will not form

67. Figure 21-16b Epidermis LARVA ADULT Gonad Anchor cell Signal protein Vulval precursor cells Inner vulvaOuter vulva Induction of vulval cell types during larval development

68. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Four important concepts apply to C. elegans and many other animals: – In the developing embryo, sequential inductions drive organ formation – The effect of an inducer can depend on its concentration – Inducers produce their effects via signal transduction pathways, as in adult cells – The induced cell often responds by activating genes that establish a pattern of gene activity characteristic of a particular kind of cell 47.d.

69. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 48. Programmed Cell Death (Apoptosis) Apoptosis is programmed cell death It is caused by cell signaling which activates a cascade of “suicide” proteins in the cells destined to die During apoptosis, a cell shrinks and becomes lobed (called “blebbing”); the nucleus condenses; and the DNA is fragmented

70. Figure 21-17 – Human white blood cell blebbing 2  m

71. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In C. elegans, there are two key apoptosis genes: ced-3 (codes for Ced-3 protein) and ced-4 (codes for Ced-4 protein) (ced stands for cell death) Another gene, ced-9, codes for Ced-9, which is a protein in the outer mitochondrial membrane and is the master regulator of apoptosis 49.

72. 50. Figure 21-18 – Here’s how it works Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Ced-4Ced-3 Inactive proteins Death signal receptor No death signal Death signal Ced-9 (inactive) Cell forms blebs Active Ced-4 Active Ced-3 Other proteases Nucleases Activation cascade Death signal

73. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 51. Research on mammals has revealed a prominent role for mitochondria in apoptosis A built-in cell suicide mechanism is essential to development in all animals

74. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The timely activation of apoptosis proteins in some cells functions during normal development and growth in both embryos and adult In vertebrates, apoptosis is part of normal development of the nervous system, operation of the immune system, and morphogenesis of hands and feet in humans and paws in other mammals 51.

75. 51. Figure 21-19 Interdigital tissue 1 mm

76. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 51. Webbed fingers or toes! Syndactyly 1/2000 – 2500 humans Ashton Kutcher, Joseph Stalin, Dan Ackroyd

77. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant Development: Cell Signaling and Transcriptional Regulation Genetic analysis of plant development, using model organisms such as Arabidopsis, has lagged behind that of animal models because fewer researchers work on plants Plant research is now progressing rapidly, thanks to DNA technology and clues from animal research

78. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanisms of Plant Development 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, and thus is relatively inaccessible to study However, other important aspects of plant development are observable in plant meristems, particularly apical meristems at the tips of shoots In meristems, cell division, morphogenesis, and differentiation give rise to new organs

79. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 52. Environmental signals, such as day length or temperature, trigger signal transduction pathways that convert ordinary shoot meristems to floral meristems.

80. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 53. Pattern Formation in Flowers A floral meristem has three layers of cells (L1-L3), all of which participate in forming a flower with four types of organs: carpels (containing egg cells) stamens (containing sperm-bearing pollen) petals sepals (leaflike structures outside the petals)

81. 53.a. Figure 21-20 Cell layers L1 L2 L3 Floral meristemAnatomy of a flower Stamen Carpel Petal Sepal Tomato flower

82. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 53.b-c. Tomato plants homozygous for the mutant allele fasciated (f), produce flowers with abnormally large numbers of organs. Researchers grafted stems from fasciated plants onto those of wild-type plants, then grew new plants from the shoots that grew near the graft sites. Many of the new plants were chimeras, organisms with a mixture of genetically different cells. (BTW-very rarely, humans can be chimeras, too!)

83. 53.c. Figure 21-21 Sepal Petal Carpel Stamen Wild-type, normal Fasciated (ff), extra organs Graft Chimeras Key Wild-type (FF) Fasciated (ff) Floral meristem L1 L2 L3 PlantFlowerPhenotypeFloral Meristem Wild-type parent Fasciated (ff) parent Wild-type Fasciated Wild-type Chimera 1 Chimera 2 Chimera 3

84. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The number of organs per flower depended on genes of the L3 (innermost) cell layer Cells in the L3 layer induce the L1 and L2 layers to form flowers with a particular number of organs. 53.c. Conclusions

85. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In contrast to genes controlling organ number in flowers, genes controlling organ identity (organ identity genes) determine the types of structures that will grow from a meristem Organ identity genes are analogous to homeotic genes in animals Mutations cause plant structures to grow in unusual places, such as carpels in place of sepals 54. Organ identity genes

86. 54. Figure 21-22 Wild typeMutant

87. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 55-56. Comparative studies help explain how the evolution of development leads to morphological diversity Evolutionary developmental biology (“evo-devo”) compares developmental processes of different multicellular organisms The aim of evo-devo is to understand how developmental processes have evolved and how changes in these processes can modify existing organismal features or lead to new ones

88. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 57-59. Widespread Conservation of Developmental Genes Among Animals Homeotic genes are master regulatory genes that, when mutated, cause major structural abnormalities (called Hox genes in animals) Molecular analysis of the homeotic genes in Drosophila has shown that they all include 180-nucleotide sequence called a homeobox, which codes for a 60-amino-acid homeodomain in the protein An identical or very similar nucleotide sequence has been discovered in the homeotic genes of both vertebrates and invertebrates. In fact, the vertebrate genes homologous to the homeotic genes of fruit flies have even kept their chromosomal arrangement (see next slide)

89. Figure 21-23 Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse

90. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 60. Proteins with homeodomains probably regulate development by coordinating the transcription of batteries of developmental genes, switching them on or off.

91. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Related genetic sequences have been found in regulatory genes of yeasts, plants, and even prokaryotes In addition to developmental genes, many other genes are highly conserved from species to species The bottom line of this section is that __________________ changes in regulatory sequences of particular genes can lead to ____________________ changes in body form. This provides evidence that all living things are ___________________ to each other and these changes ____________________ over time, leading to the great diversity of living things on Earth. 61.

92. Figure 21-24 –Evolutionary changes in four Hox genes accounts for the different body plans of crustaceans and insects ThoraxAbdomen Genital segments ThoraxAbdomen

93. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 62-63. Comparison of Animal and Plant Development The last common ancestor of animals and plants was probably a single-celled microbe that lived hundreds of millions of years ago. In both plants and animals, development relies on a 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 ancestry