Genes, Development, and Evolution

1. 14 Genes, Development, and Evolution

2. 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

3. 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

4. Chapter 14 Opening Question Why are stem cells so useful?

5. 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.

6. Figure 14.1 Development (Part 1)

7. Figure 14.1 Development (Part 2)

8. Concept 14.1 Development Involves Distinct but Overlapping Processes • Four processes of development: • Determination sets the fate of the cell • Differentiation is theprocess by which different types of cells arise • Morphogenesis is the organization and spatial distribution of differentiated cells • Growth is anincrease in body size by cell division and cell expansion

9. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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.

10. Figure 14.2 A Cell’s Fate Is Determined in the Embryo

11. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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.

12. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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.

13. Figure 14.3 Cloning a Plant (Part 1)

14. Figure 14.3 Cloning a Plant (Part 2)

15. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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.

16. Figure 14.4 Cloning a Mammal (Part 1)

17. Figure 14.4 Cloning a Mammal (Part 2)

18. Figure 14.4 Cloning a Mammal (Part 3)

19. Figure 14.4 Cloning a Mammal (Part 4)

20. Concept 14.1 Development Involves Distinct but Overlapping Processes • As in plants, no genetic information is lost as the cell passes through developmental stages—genomic equivalence. • Practical applications for cloning: • Expansion of numbers of valuable animals • Preservation of endangered species • Preservation of pets

21. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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. • There are about 300 cell types in mammals.

22. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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.

23. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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).

24. Figure 14.5 Multipotent Stem Cells

25. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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.

26. Concept 14.1 Development Involves Distinct but Overlapping Processes • Pluripotent cells in the blastocyst embryonic stage retain the ability to form all of the cells in the body. • 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.

27. Figure 14.6 Two Ways to Obtain Pluripotent Stem Cells

28. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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

29. Concept 14.1 Development Involves Distinct but Overlapping Processes • 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.

30. Figure 14.6 Two Ways to Obtain Pluripotent Stem Cells

31. 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.

32. Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development • In the vertebrate embryo, muscle precursor cells come from a tissue layer called the mesoderm. • When these cells commit to becoming muscle cells, they stop dividing—in many parts of the embryo, cell division and cell differentiation are mutually exclusive.

33. Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development • Cell signaling activates the gene for a transcription factor called MyoD. • MyoD activates the gene for p21, an inhibitor of cyclin-dependent kinases that normally stimulate the cell cycle at G1. • The cell cycle stops so that differentiation can begin.

34. Figure 14.7 Transcription and Differentiation in the Formation of Muscle Cells (Part 1)

35. Figure 14.7 Transcription and Differentiation in the Formation of Muscle Cells (Part 2)

36. Concept 14.2 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

37. Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development • Polarity—having a “top” and a “bottom” may develop in the embryo. • The animal pole is the top, the vegetal pole is the bottom. • Polarity can lead to determination of cell fates early in development.

38. Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development • Polarity was demonstrated using sea urchin embryos. • 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.

39. In-Text Art, Ch. 14, p. 270

40. 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.

41. Figure 14.8 The Concept of Cytoplasmic Segregation (Part 1)

42. Figure 14.8 The Concept of Cytoplasmic Segregation (Part 2)

43. Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development • 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.

44. 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 and signal transduction mechanisms. • Exposure to different amounts of inductive signals can lead to differences in gene expression.

45. 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.

46. Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development • LIN-3, a protein secreted by the anchor cell acts as the primary inducer. • The primary precursor cell that received the most LIN-3 then secretes a secondary inducer (lateral signal) that acts on its neighbors. • The gene expression patterns triggered by these molecular switches determine cell fates.

47. Figure 14.9 Induction during Vulval Development in Caenorhabditis elegans (Part 1)

48. Figure 14.9 Induction during Vulval Development in Caenorhabditis elegans (Part 2)

49. 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.

50. Figure 14.10 The Concept of Embryonic Induction