Shared Developmental Processes in Multicellular Organisms

1. Ch 21 Introduction • How does a single fertilized egg cell develop into an embryo and then into a baby and eventually an adult? • A few fundamental principles are common to all developmental sequences observed in multicellular organisms.

2. Shared Developmental Processes

3. Shared Developmental Processes location, timing, extent cell divisions tightly controlled by regulation

4. Meristems and Stem Cells • Most cells stop proliferating at maturity. However, there are some specialized, undifferentiated cells that continue proliferating throughout the organism’s life. • In plants, these specialized cells are called meristems. • In animals, they are called stem cells.

5. Shared Developmental Processes abnormal apoptosis can lead to disease or deformation

7. Shared Developmental Processes Plant cells don’t move; they change the orientation of cell division. Animal cells move: in gastrulation, cells in different parts of an early embryo rearrange themselves into three distinctive types of embryonic tissues

8. Shared Developmental Processes Differentiation is a progressive process.

9. Cell Differentiation • Many plant cells are totipotent – capable of de-differentiating even after they have specialized. • Animal cells are unable to de-differentiate. • Plant meristems and animal stem cells do not become specialized adult cells, but instead remain undifferentiated. • Stem cells retain the ability to divide and give rise to an array of specialized cell types. • Meristems can give rise to various structures that develop throughout life.

10. Shared Developmental Processes change patterns of gene expression and are essential for changing cell activity during development

11. The Role of Differential Gene Expression in Development • Differentialgeneexpression, the expression of different genes in different cell types, is key to cell differentiation during development. • An important question that scientists had to answer was whether cells express different genes because they contain different genes or whether they all contain the same genes but express different subsets.

12. Are Differentiated Cells Genetically Equivalent? • Plant cells can de-differentiate to form other plant parts, thus each cell must contain the genes required by all different types of plant cells. • Early experiments showed that transplanting nuclei from diploid frog cells into unfertilized eggs without nuclei resulted in development of normal tadpoles. • Nuclear transfer experiments in sheep reinforced these results.

13. Animal Cloning • Mammary gland cells from an adult sheep were fused with enucleated eggs. • The resulting embryos were implanted into surrogate mothers. • A fertile, genetically identical clone of the parent sheep was born (“Dolly”). • Mouse, cow, horse, monkey, and other species have now also been successfully cloned.

14. How Does Differential Gene Expression Occur? • A gene can be regulated at multiple levels: • transcription, RNA processing, translation, and post-translation. • In eukaryotes, transcription is controlled primarily by the presence of proteins called regulatory transcription factors. • One such factor sets up the primary body axes

15. Cell-Cell Signals Trigger Differential Gene Expression • The fate of a cell depends on timing (the current stage of development of the organism) and its spatial location (where it is in the body of the organism). • Spatial location in early development is determined by three major body axes: • Anterior-posterior • Ventral-dorsal • Left-right • Cell-cell signals tell cells where they are in time and space. This information activates transcription factors that turn specific genes on or off, resulting in differentiation.

16. Master Regulators Set Up the Major Body Axes • Pattern formation is the series of events that determine the spatial organization of an embryo. • Certain early signals act as master regulators, setting up the major body axes of the embryo. • These master regulators activate a network of genes that sends signals with more specific information about the spatial location of cells. • As development proceeds, a series of signals arrive and activate genes that specify finer and finer control over what a cell becomes.

17. The Discovery of Pattern Formation Mutants • In the 1970s, Christiane Nüsslein-Volhard and Eric Wieschaus applied a genetic approach to studying development in the fruit fly (Drosophilamelanogaster), eventually identifying more than 100 genes that play fundamental roles in pattern formation. • They did this by producing mutant embryos.

18. The bicoid Gene • One of the most dramatic mutations resulted in structures on the anterior end being replaced with posterior structures. They named the gene responsible bicoid, for “two-tailed.” • Nüsslein-Volhard and Wieschaus suspected that the bicoid gene’s product provides positional information. In other words, they hypothesized that the bicoid gene coded for a signal that tells cells where they are located along the anterior-posterior body axis.

19. Where Is the bicoid Product Found? • To determine the location of bicoid mRNA in the egg, researchers used a technique called in situ hybridization. • They found that bicoid mRNA was highly localized in the anterior of the egg. • Bicoid protein is made from mRNA in the anterior end and diffuses away from that end of the embryo. • This produces a steep concentration gradient from the anterior to the posterior end.

20. How Does Bicoid Work? • Bicoid forms a concentration gradient which provides cells with information about their position along the anterior-posterior axis. • Bicoid also turns on genes responsible for forming anterior structures. The absence of Bicoid contributes to the formation of posterior structures.

21. Auxin’s Role in Plant Development • The master regulator in plants is not a transcription factor but rather a hormone. • In plant embryos, the cell-cell signal called auxin enters cells and triggers the production of transcription factors that affect differentiation. • Like Bicoid, auxin also works by forming concentration gradients. • In both plants and animals, molecules that provide spatial information during early embryonic development, via a concentration gradient, are called morphogens.

22. Segmentation Genes • Regulatory genes provide increasingly specific positional information. • A segment is a distinct region of an animal body that contains a distinct set of structures and is repeated along its length. • In the fruit fly and other animals, segmentation genes organize cells and tissues into distinct segments.

23. What Do Segmentation Genes Do? • Three general classes of segmentation genes have been identified in Drosophila: • Gap genes define the general position of segments in the anterior, middle, or posterior of the body. • Pair-rule genes demarcate the boundaries of individual segments. • Segment polarity genes delineate boundaries within individual segments. • These segmentation gene sets are expressed in sequence and in increasingly restricted regions.

24. Homeotic Genes (trigger devt. of structures) • After the segmentation genes have established the identity of each segment along the anterior-posterior axis, development continues with activation of the homeotic genes. • Homeotic gene products identify each segment’s structural role. • Specifically, homeotic genes trigger the development of structures that are appropriate to each type of segment.

25. Hox Genes (homeotic genes in Drosophila) • The eight homeotic genes in Drosophila are called Hox genes. • The Hox genes are expressed in a distinctive pattern along the anterior-posterior axis, after segments are established. • These genes code for regulatory transcription factors that trigger the production of segment-specific structures. • Some Drosophila mutants have a segment that has been transformed into another segment, with its associated structures. • This homeosis occurs when cells get incorrect information about where they are in the body.

27. Regulatory Genes Form Cascades • The interactions among bicoid and the segmentation genes form a regulatory cascade. • Master regulators trigger the production of other regulatory signals and transcription factors, which trigger production of another set of signals and regulatory proteins, and so on. • The bicoid gene, gap genes, pair-rule genes, segment polarity genes, and homeotic genes each define a level in the cascade.

28. The Overall Function of Regulatory Genes • Regulatory genes act in a sequence, triggering gene cascades that provide progressively detailed information about where cells are located in time and space. • Cells receive unique positional information because the identity and concentration of signals and transcription factors vary along the three major body axes. • Each level in a regulatory cascade provides a more specific level of information about where a cell is. • As regulatory cascades proceed, a cell's fate becomes more and more finely determined.

29. Evolutionary Conservation of Hox Genes • Clusters of Hox genes occur in virtually every animal examined to date. • The number of Hox genes varies widely among species, but their chromosomal organization is similar. • Biologists hypothesize that the genes in Hox complexes of animals are homologous. • At least some of the molecular mechanisms of pattern formation have been highly conserved during animal evolution. • Although animal bodies are spectacularly diverse in size and shape, the underlying mechanisms responsible for their development are similar.

31. Developmental Pathways and Evolutionary Change • Once biologists began working out regulatory signals and cascades, they realized that the genetic changes altering these developmental processes must be the foundation of evolutionary change. • Evo-devo is the research field of evolutionary-developmental biology. It focuses on understanding how changes in developmentally important genes have led to the evolution of new phenotypes.

32. Results of Changes in Homeotic Gene Expression • Changes in regulation of where the homeotic genes Hoxc6 and Hoxc8 are expressed led to the evolutionary loss of the forelimb in snakes. • Normally, Hoxc6 is expressed without Hoxc8 in the region that gives rise to forelimbs in vertebrates. • In snakes, Hoxc6 and Hoxc8 are always expressed together, so no forelimb is formed.