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Development and Genes Part II Pattern Formation and Morphogenesis Using Drosophila as a Model Organism for Devlopment 2 Using Drosophila as a Model Organism for Devlopment 3 Using Drosophila as a Model Organism for Devlopment 4 Example: Drosophila Pattern Formation for the Posterior-Anterior Axes Christane Nüsslein-Volhard and Eric Wieschaus undertook a project to discover the genetic mechanisms of how a fertilized Drosophila egg became a segmented embryo. Drosophila larvae begin segmentation shortly after fertilization, as the cells organize into fourteen distinct body segments. To investigate this process, Nüsslein-Volhard and Wieschaus exposed fly embryos to mutagens, and systematically characterized their phenotypic mutations. They screened mutated embryos that exhibited abnormal development of the body axis or segmentation to identify which genes had gone awry. 5 Example: After Segmentation, Segments are Specialized In normal flies, structures like legs, wings, and antennae develop on particular segments, and this process requires the action of homeotic genes. Ed Lewis (1940’s), who discovered homeotic mutants - mutant flies in which structures characteristic of one segment of the embryo are found at some other segment. In normal flies, structures like legs, wings, and antennae develop on particular segments, and this process requires the action of homeotic genes. 6 Example: Drosophila development of body structures Edward Lewis (1918-2004) discovered that the genes that provided the code for the fly's body were segmented and ordered, even in the embryo stage. These genes dictated the development of each segment of the body. By causing mutations in certain genes, he found that he could cause flies to grow extra body parts or other abnormal features. Homeotic genes are genes which regulate the development of anatomical structures in various organisms such as insects, mammals, and plants. 7 The homeotic genes encode transcription factors that control the expression of genes responsible for particular anatomical structures, such as wings, legs, and antennae. The homeotic genes has a subsection which includes a 180 nucleotide sequence called the homeobox (also called hox genes), which is translated into a 60 amino acid domain, called the homeodomain. The homeodomain is involved in DNA binding. 8 A homeoprotein or HOX protein contains a homoedomain (in red). It is the homeodomain (60 amino acids long) that interacts with the DNA as a transcription factor for a particular gene needed in development. 9 Hox genes are switched on in different segments. Patterns of Hox gene activity give each segment an identity, telling it where it is in the body and what structures it should grow. For instance, genes that are active in the head direct the growth of mouth parts and antennae, while genes that are active in the thorax direct the growth of legs and wings. Drosophila, like all insects, has eight Hox genes. These are clustered into two complexes, both of which are located on chromosome 3. 10 Interestingly, Hox genes are arranged in clusters. Typically, their order on the chromosome is the same as the order in which they appear along the body. In other words, the genes on the left control patterning in the head, and the genes on the right control patterning in the tail. 11 Changes to Hox gene expression change a segment’s identity. For example the first segment of the thorax normally grows legs, the second grows legs and wings, and the third grows legs and halteres. When the Hox gene activity in the third segment is made the same as that in the second, both segments grow legs and wings. 12 On the right is the normal development, and on the left is the mutant. 13 Evolution and Importance of Hox Genes 14 Example of Hox Gene 15 Example of Hox Gene Developmental Mutations Evolution of the Hox genes 16 Pattern Formation and Morphogenesis These are the hox genes that control certain vertebrae and their development. Once the body pattern is established, and then morphogenesis can occur, which is the formation of various organs and systems 17 18 Morphogenesis Can Involve Cell Death Occurs Apoptosis occurs in the development of the digits of a hand or paw. Shown is the development of a mouse paw. The genes that regulate apoptosis are similar in both vertebrates and invertebrates such as nematodes. Fungi including yeast also have genes that regulate apoptosis indicating these are ancient genes. 19 During Development Cell Death Occurs Apoptosis- Is a programed cell death. Occurs in • Infected cells • Development • Cells that are aged and no longer functional The cell below is a normal leukocyte and the cell above is a cell undergoing apoptosis 20 Heterotrophy Shown is the development of the hind limb of chicken versus a duck. The duck retains the webbing between the digits. 21 Heterochrony Heterochrony is the regulation of developmental stages by changing the duration of the developmental process 22 The one of the differences between the plain zebra and Grevy zebra is that the Grevy zebra has more narrow stripes. It is thought that the genes responsible for the stripes are delayed in the plain zebra, resulting in wider stripes. Timing in development is very important. 23 Neighboring Tissues Can Effect Morphogenesis 24 Genes on the Y chromosome can influence morphogenesis There is primordial tissue that is destined to become gonads. In mammals the sex-determining region Y (SRY) gene on the Y chromosome will cause the development of testis. Without the presence of the SRY protein the tissue will develop into ovaries no matter the chromosomal condition. 25 Genes on the Y chromosome can influence morphogenesis 26 Effect of Testosterone on Development 27 Cloning using adult differentiated cells While it was thought that differentiated adult cells could not be used to make a clone because genes had been permanently inactivated. In 1997 researchers at the Roslin Institute were able to clone a lamb from an adult differentiated cell. 28