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Developmental Biology of Drosophila

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Developmental Biology of Drosophila

  1. 1. DROSOPHILA DEVELOPMENT Arun Kumar Pradhan ONA Odisha NET Academy ONA 09337727724
  2. 2. DEVELOPMENT  An organism arises from a fertilized egg as the result of three related processes • Cell division • Cell differentiation • Morphogenesis Odisha NET Academy ONA 09337727724
  3. 3. DIFFERENTIATION  Cells may initially remain undifferentiated • Embryonic stem cells  Cells ultimately differentiate • Become specialized in structure and function Odisha NET Academy ONA 09337727724
  4. 4. DIFFERENTIATION  Virtually all cells within a multicellular organism are genetically identical  Differences between cells are due to differences in gene expression • Different subsets of genes are “on” and “off” • Different cell types make different proteins Odisha NET Academy ONA 09337727724
  5. 5. GENE EXPRESSION  Much of the regulation of gene expression occurs at the level of transcription  Transcriptional regulation of gene expression is directed by • Maternal molecules in the cell’s cytoplasm • Signals from other cells Odisha NET Academy ONA 09337727724
  6. 6. PATTERN FORMATION  The development of a spatial organization in which an organism’s tissues and organs are all in their characteristic places  In animals, it begins in early embryo • Basic body plan is established • Major axes are established early  Studied most extensively in Drosophila melanogaster Odisha NET Academy ONA 09337727724
  7. 7. Drosophila melanogaster  Bilaterally symmetric segmented body • Head • Thorax • Abdomen Odisha NET Academy ONA 09337727724
  8. 8. Drosophila melanogaster  Cytoplasmic determinants are present in the unfertilized egg • Provide positional information for placement of axes prior to fertilization • Establishes number and orientation of segments • Ultimately trigger formation of specific structures within each segment Odisha NET Academy ONA 09337727724
  9. 9. Drosophila melanogaster  Egg develops in mother’s ovary • Surrounding cells with nutrients, etc.  Mitosis begins following fertilization • First ten divisions include no growth, cytokinesis • Single multinucleate cell results Odisha NET Academy ONA 09337727724
  10. 10. Drosophila melanogaster  Nuclei migrate to periphery of embryo at tenth division  Plasma membranes finally partition ~6,000 nuclei into separate cells at the thirteenth division • Basic body plan already determined at this time  Body axes and segment boundaries Odisha NET Academy ONA 09337727724
  11. 11. Drosophila melanogaster  Subsequent embryonic events create clearly visible segments • Initially look very similar  Some cells move to new positions • Organs form  Wormlike larva hatches • Eats, grows, & molts Odisha NET Academy ONA 09337727724
  12. 12. Drosophila melanogaster  Larva eventually forms a pupa • Enclosed in a case  Metamorphosis occurs • Change from larva to adult fly  Adult fly emerges from case • Each segment is anatomically distinct Odisha NET Academy ONA 09337727724
  13. 13. Drosophila melanogaster  Each segment in the adult fly is anatomically distinct • Characteristic appendages Odisha NET Academy ONA 09337727724
  14. 14. Drosophila melanogaster  Gradients of maternal molecules in the early embryo control axis formation • Cytoplasmic determinants already present in unfertilized egg • Encoded by mother’s maternal effect genes  a.k.a., “Egg-polarity genes”  Encode proteins or mRNAs that are placed into the egg while still in the mother’s ovary Odisha NET Academy ONA 09337727724
  15. 15. Drosophila melanogaster  One group of maternal effect genes establishes the anterior-posterior axis of the embryo  Another set of maternal effect genes establishes the dorsal-ventral axis  Female flies possessing mutations in maternal effect genes appear phenotypically normal, but produce offspring with mutant phenotypes Odisha NET Academy ONA 09337727724
  16. 16. Drosophila melanogaster  Bicoid is an egg-polarity gene • “Two-tailed”  Mothers defective in bicoid produce embryos lacking the front half of their body • Duplicate posterior structure at both ends Odisha NET Academy ONA 09337727724
  17. 17. Drosophila melanogaster  Bicoid gene product is concentrated at anterior end of fly embryo • Gradient of gene product • Essential for setting up anterior end of fly  Gradients of other proteins determine the posterior end and the dorsal-ventral axis Odisha NET Academy ONA 09337727724
  18. 18. The bicoid gradient regulates the expression of segmentation genes in a concentration-dependent fashion. There are peak levels of the Bicoid protein in anterior regions, intermediate levels in certral regions, and low levels in posterior regions. Orthodenticle is activated only by high levels of the Bicoid gradient in the head; hunchback is activated by both high and intermediate levels in the head and thorax. Odisha NET Academy ONA 09337727724
  19. 19. In central regions of the embryo, the orthodenticle gene is off. In contrast, hunchback is on. These is control by the levels of Bicoid protein.Odisha NET Academy ONA 09337727724
  20. 20. Hunchback expression is also regulated at the level of translation The translation is blocked in posterior regions by RNA-binding protein called Nanos. After the Nanos mRNA is translated, the protein diffuses from posterior regions to form a gradient. The translation of the maternal hunchback mRNA is arrested by the Nanos protein. The Nanos gradient thereby leads to the formation of a reciprocal Hunchback gradient in anterior regions. Odisha NET Academy ONA 09337727724
  21. 21. Hunchback and gap proteins produce segmentation stripes of gene expression. Odisha NET Academy ONA 09337727724
  22. 22. Drosophila melanogaster  The bicoid protein and the products of other egg- polarity genes are transcription factors • Regulate the expression of some of the embryo’s genes Odisha NET Academy ONA 09337727724
  23. 23. Drosophila melanogaster  Segmentation genes • Genes of embryo • Expression regulated by products of egg-polarity genes • Direct the actual formation of segments after the embryo’s major axes are defined Odisha NET Academy ONA 09337727724
  24. 24. Drosophila melanogaster  Three sets of segmentation genes are activated sequentially • Gap genes • Pair-rule genes • Segment polarity genes  The activation of these sets of genes defines the animal’s body plan • Each sequential set regulates increasingly fine details Odisha NET Academy ONA 09337727724
  25. 25. Drosophila melanogaster  Gap genes • Map out basic subdivisions along the embryo’s anterior-posterior axis • Mutations cause “gaps” in the animal’s segmentation Odisha NET Academy ONA 09337727724
  26. 26. Drosophila melanogaster  Pair-rule genes • Define pattern in terms of pairs of segments • Mutations result in embryos having half the normal number of segments Odisha NET Academy ONA 09337727724
  27. 27. Drosophila melanogaster  Segment polarity genes • Set the anterior-posterior axis of each segment • Mutations produce segments where part of the segment mirrors another part of the same segment Odisha NET Academy ONA 09337727724
  28. 28. Drosophila melanogaster  The products of many of the segmentation genes are transcription factors • Directly activate the next set of genes  Summary • Products of the egg-polarity genes regulate the regional expression of the gap genes • Gap genes control the localized expression of the pair-rule genes • Pair rule genes activate specific segment polarity genes in different parts of each segment • Segment polarity genes activate homeotic genes Odisha NET Academy ONA 09337727724
  29. 29. HOMEOTIC GENES  Master regulatory genes  Specify the types of appendages and other structures that each segment will form  Mutations produce flies with structures in incorrect places Odisha NET Academy ONA 09337727724
  30. 30. HOMEOTIC GENES  Encode transcription factors  Control the expression of genes responsible for specific anatomical structures • e.g., “Antennae go here” • e.g., “Legs go here” Odisha NET Academy ONA 09337727724
  31. 31. HOMEOTIC GENES  Homeotic genes are master genes that regulate the expression of numerous other genes • Some of the regulated genes are regulatory themselves Odisha NET Academy ONA 09337727724
  32. 32. Drosophila DEVELOPMENT Hierarchy of Gene Activity  Maternal genes  Segmentation genes of embryo • Gap genes • Pair-rule genes • Segment polarity genes  Homeotic genes of the embryo  Other genes of the embryo Odisha NET Academy ONA 09337727724
  33. 33. HOMEOTIC GENES  Homeotic genes of Drosophila all possess homologous segments • 180-nucleotide sequence = homeobox • Encodes 60-amino-acid homeodomain  Homologous sequences have been found in many other animals • e.g., Insects, nematodes, mollusks, fish, frogs, birds, humans, etc. • Related genes are even found in yeast, etc. • Hox genes Odisha NET Academy ONA 09337727724
  34. 34. HOMEOTIC GENES  Vertebrate genes homologous to the homeotic genes of Drosophila have maintained their chromosomal arrangement Odisha NET Academy ONA 09337727724
  35. 35. Thank you Odisha NET Academy ONA 09337727724

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