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 Fate Determination and Differentiation • 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis

3. Chapter 14 Genes, Development, and Evolution • Key Concepts • 14.4 Changes in 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 How do gene products control the development of the eye?

5. Concept 14.1 Development Involves Distinct but Overlapping Processes • Development—changes a multicellular organism undergoes as it progresses from a single cell to an embryo and to mature adulthood • Zygote—fertilized egg • Embryo—earliest developmental stages • Many organisms continue to develop throughout their lives, with development ceasing only at death.

6. Figure 14.1 Development

7. Concept 14.1 Development Involves Distinct but Overlapping Processes • Processes that underlie development: • Determination—sets fate of a cell • Differentiation—different types of cells arise from less specialized cells • Morphogenesis—organization and spatial distribution of differentiated cells • Growth—increase in body size by cell division and expansion • All involve differential gene expression and signaling between cells.

8. Concept 14.1 Development Involves Distinct but Overlapping Processes • As a zygote develops, each undifferentiated cell is destined to become part of a particular tissue—the cell fate. • Experiments in which specific cells of an early embryo are transplanted to new positions on another embryo show when cell fate is determined. • Determination is influenced by changes in gene expression and by the external environment.

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

10. Concept 14.1 Development Involves Distinct but Overlapping Processes • During animal development, cell fate becomes progressively restricted. • Cell potency—a cell’s potential to differentiate: • Early embryo cells are totipotent—they can become any type of cell. • In later stages, some cells may be pluripotent—can develop into most other cell types, but cannot form new embryos.

11. Concept 14.1 Development Involves Distinct but Overlapping Processes • Some cells remain multipotent through adult stages—can differentiate into several related cell types. • Many cells in a mature organism are unipotent—produce only the same cell type.

12. Concept 14.1 Development Involves Distinct but Overlapping Processes • Under certain conditions, a determined or differentiated cell can become undetermined again, even totipotent. • Some plant cells can be grown in culture and induced to dedifferentiate. • If given the right chemical cues, these cells can develop into new plants (clones of the original plants).

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 • This cloning technique is used extensively in agriculture and forestry. • The ability to produce clones is evidence for the genomic equivalence of somatic cells: • All somatic cells in a plant have a complete genome and thus all the genetic information needed to become any cell in the plant.

16. Concept 14.1 Development Involves Distinct but Overlapping Processes • In animals, nuclear transfer experiments show 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.

17. Figure 14.4 Cloning a Mammal (Part 1)

18. Figure 14.4 Cloning a Mammal (Part 2)

19. Figure 14.4 Cloning a Mammal (Part 3)

20. Concept 14.1 Development Involves Distinct but Overlapping Processes • Many animals have now been cloned; their differentiated cells also have genomic equivalence. • Practical applications: • Increase the numbers of transgenic animals with valuable phenotypes (e.g., cows that produce human growth hormone) • Preservation of endangered species with low reproduction rates

21. Concept 14.1 Development Involves Distinct but Overlapping Processes • Resurrection of extinct species from intact fossil DNA • The genomes of some extinct species have been sequenced, including wooly mammoths and Neanderthals.

22. Concept 14.1 Development Involves Distinct but Overlapping Processes • In adult plants, growing regions at the tips of roots and stems contain meristems—clusters of undifferentiated, rapidly dividing stem cells. • The plant body undergoes constant growth and renewal, with new organs forming often.

23. Concept 14.1 Development Involves Distinct but Overlapping Processes • In adult mammals, stem cells occur in many tissues, especially those that require frequent replacement—skin, blood, intestinal lining. • There are about 300 cell types in mammals.

24. Concept 14.1 Development Involves Distinct but Overlapping Processes • Stem cells in some mammalian tissues are multipotent. • In bone marrow: • Hematopoietic stem cells produce red and white blood cells. • Mesenchymal stem cells produce bone and surrounding tissue, including muscle.

25. Concept 14.1 Development Involves Distinct but Overlapping Processes • Multipotent stem cells differentiate “on demand.” • Hematopoietic cells differentiate in response to signals from adjacent cells or from the circulating blood. • This is the basis of a cancer therapy called hematopoietic stem cell transplantation (HSCT).

26. 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, or they may come from a donor. • After treatment, the stem cells, which retain their ability to differentiate, are injected back into the patient.

27. Figure 14.5 Multipotent Stem Cells

28. Concept 14.1 Development Involves Distinct but Overlapping Processes • In mammals, some pluripotent cells in the blastocyst embryonic stage retain the ability to form all of the cells in the body—called embryonic stem cells (ESCs). • They can be removed from a blastocyst and grown in laboratory culture almost indefinitely; they can differentiate when proper signals are provided.

29. Concept 14.1 Development Involves Distinct but Overlapping Processes • ESCs might be useful in repairing damage caused by diseases such as diabetes and Parkinson’s disease. • ESCs can be harvested from human embryos conceived by in vitro fertilization. Problems with this approach include: • Objections to the destruction of human embryos for this purpose • The stem cells could provoke an immune response in a recipient

30. Figure 14.6 Two Ways to Obtain Pluripotent Stem Cells

31. Concept 14.1 Development Involves Distinct but Overlapping Processes • An alternative was developed by Yamanaka and coworkers: • Induced pluripotent stem cells (iPS cells) can be made from skin cells. • Microarrays were used to find genes uniquely expressed in ESCs. They encode transcription factors essential to stem cell function.

32. Concept 14.1 Development Involves Distinct but Overlapping Processes • The genes were inserted into a vector for genetic transformation of skin cells. • The transformed cells were pluripotent and could be induced to differentiate into many tissue types.

33. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • Genes that determine cell fate and trigger differentiation usually encode transcription factors. • Cell fate can be determined by: • Asymmetrical distribution of cytoplasmic factors in a cell, so the progeny cells receive unequal amounts of the factors • Differential exposure of two cells to an external signal (an inducer)

34. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • Axes that relate to the body plan of the organism are established early in development. • Polarity—having a “top” (animal pole) and a “bottom” (vegetal pole) • Polarity can lead to determination of cell fates early in development.

35. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • Sea urchin embryos can be bisected at the eight-cell stage in two different ways:

36. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • If the two halves are allowed to develop: • In an embryo cut into top and bottom halves, the bottom develops into a small sea urchin and the top half does not develop at all. • In an embryo cut into two side halves, both halves develop normally, though smaller. • Indicates that the top and bottom halves have already developed distinct fates.

37. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • The model of cytoplasmic segregation states that cytoplasmic determinants are distributed unequally in the egg. • Cytoplasmic determinants include transcription factors that promote differential gene expression in the two daughter cells. • Small regulatory RNAs and mRNAs also contribute to differential gene expression.

38. Figure 14.7 The Concept of Cytoplasmic Segregation

39. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • The cytoskeleton contributes to the distribution of cytoplasmic determinants: • Microtubules and microfilaments have polarity. • Cytoskeletal elements can bind certain proteins that can be used in the transport of mRNA.

40. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • Induction: signaling events by which cells in a developing embryo communicate and influence one another’s developmental fate • Cells influence one another’s developmental fate via chemical signals (inducers) and signal transduction mechanisms.

41. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • Development of C. elegans is easily observed under a microscope. • All of the cell divisions from zygote to the 959 cells in the adult can be followed. • Nematodes are hermaphroditic, containing both male and female reproductive organs. • Eggs are laid through a pore, the vulva.

42. Figure 14.8 Induction during Vulval Development in Caenorhabditis elegans (Part 1)

43. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • During development, an anchor cell induces the vulva to form from 6 cells on the ventral surface. • Anchor cell secretes LIN-3 (primary inducer). • Concentration of LIN-3 is key—it diffuses out of the anchor cell and forms a concentration gradient. • The primary precursor cell gets the most LIN-3, and it secretes a secondary inducer.

44. Figure 14.8 Induction during Vulval Development in Caenorhabditis elegans (Part 2)

45. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • Induction involves the activation or inactivation of specific genes through signal transduction cascades in the responding cells. • Much of development is controlled by the molecular switches that allow a cell to proceed down one of two alternative paths.

46. Figure 14.9 The Concept of Embryonic Induction

47. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • An important mechanism for cell differentiation is differential gene transcription. • A well-studied example is the conversion of undifferentiated muscle precursor cells (myoblasts) into muscle fiber cells. • A key event in commitment is that the cells stop dividing. • In many parts of the embryo, cell division and cell differentiation are mutually exclusive.

48. Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation • 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, and other transcription factors trigger differentiation.

49. Figure 14.10 Transcription and Differentiation in the Formation of Muscle Cells

50. Concept 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis • Pattern formation: processes that result in the spatial organization of a tissue or organism; results from spatial differences in gene expression • Cells in body must “know” where they are in relation to the rest of the body. • Cells must activate the appropriate pattern of gene expression.