Chapter 9. Drosophila
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Drosophila Development Overview
Focus on genetics in Drosophila development.
Reading: Chapter 9, pages 273-290, 295-296.
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Importance of Drosophila in Development Studies
Drosophila provides unique insights into developmental genetics compared to other organisms like sea urchins.
While manipulating cells is challenging, genetic studies are straightforward.
Mutations serve as tools to understand affected genes' roles in development.
Many critical development-regulating genes in humans were first identified in Drosophila.
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Drosophila Life Cycle
Stages: embryo → larva → adult.
Body Structure:
Head
Thorax:
Prothorax (T1)
Mesothorax (T2)
Metathorax (T3)
Abdomen (A1-A8)
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Drosophila Anatomy
Adult consists of:
3 body regions:
Head (red)
Thorax (green)
Abdomen (orange)
Each region comprises structurally distinct segments.
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Head and Thorax Structure
Head:
3 segments (less visible)
Contains eyes, antennae, proboscis.
Thorax:
3 segments, each having a pair of legs (total 6).
2nd thorax segment has wings.
3rd thorax segment has halteres for balance.
Abdomen:
Composed of 9 segments (A1 to A9).
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Nuclear Organization in Early Embryos
Nuclei are arranged in early fly embryos (cell cycle numbers refer to divisions).
Syncytial embryos until nuclear division 13.
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Early Cell Division in Embryos
Development from fertilized egg to larva occurs within 24 hours.
Yolk is centered in the egg; cleavage is unusual—nuclei divide multiple times without membrane formation.
Syncytial stage: many nuclei without cell membranes.
256 nuclei result from 8 divisions, each taking about 8 minutes.
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Nuclear Migration and Membrane Formation
Nuclei migrate beneath the outer membrane by division 10, forming a syncytial blastoderm.
Midblastula transition occurs after the 10th division, leading to increased transcription of mRNA.
After division 13, membranes encase nuclei, forming a yolk core surrounded by a single cell layer.
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Nuclear Division in Fly Embryos
Diagrammatic representations (A, B, C, D) illustrate nuclear division processes in syncytial development.
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Fly Gastrulation: Initial Stages
Diagram illustrates internal ectoderm, amnioserosal covering, and pole cells.
Development of ventral furrow initiates gastrulation process.
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Gastrulation Process
Begins post-midblastula transition.
Ventral midline folds inward, shaping future endoderm and mesoderm.
Contrasts with processes in sea urchins; blastopore appears stretched along the embryo.
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Continued Gastrulation
Body segments arise during gastrulation.
Nervous system originates from the ventral ectoderm, positioned ventrally as opposed to dorsally in vertebrates.
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Nervous System Development
Body segmentation occurs; the nervous system forms from parts of the ventral ectoderm.
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Segment Formation and Gastrulation
Formation of various embryonic structures, highlighting anterior midgut invagination and cephalic and ventral furrows.
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Segment Identification in Classical Studies
Detailed diagrams showing segmentation and body structure (A8, T1, T2, T3).
Identification of medial and lateral segments of Drosophila.
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Key Genetic Materials in Development
Roles of maternal effect genes, gap genes, pair-rule genes, segment polarity genes, and homeotic genes in anterior-posterior axis development.
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Developmental Organization
Key tasks for establishing the anterior-posterior axis:
Specification of body ends.
Dividing segments appropriately.
Differentiating segments along the body.
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Anterior-Posterior Axis Specification
Stepwise formation of spatial organization with:
Maternal effect genes establishing the axis.
Gap genes dividing areas into broad segments (mutations cause gaps).
Differentiation through homeotic genes.
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Key Genes in Development
Discussion of maternal effect gene proteins, gap gene proteins, and pair-rule proteins' roles in segmentation processes.
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Maternal Effect Genes
Maternal effect genes are crucial for setting the anterior-posterior axis, with impacts observable on embryos based on maternal mRNA/proteins deposited.
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Protein Gradients and Axis Formation
The anterior-posterior axis is developed via gradients created by bicoid and nanos proteins.
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Bicoid Protein Characteristics
Description of bicoid as a homeodomain protein and its function in establishing body polarity via transcription factors within a 60 amino acid homeodomain.
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Experimental Validation of Bicoid's Role
Experimental results showing head development dependency on bicoid gene, including comparisons between normal and mutant phenotypes.
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Bicoid mRNA Manipulation in Experiments
Results of adding bicoid mRNA at varying embryo locations, illustrating altered head and tail development in mutants.
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Bicoid Protein Gradients
Concentration and spatial distribution of bicoid mRNA and its direct relationship with the anterior-posterior setup.
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Analysis of Bicoid Concentration
Graphical insights into bicoid concentrations and their influence across wild-type and mutant embryos.
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Mechanism of Bicoid Action
How bicoid inhibits the translation of posterior structures (caudal) and its implications for anterior-posterior development.
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Bicoid's Multifaceted Role
Explanation of bicoid's interactions with caudal mRNA for translation inhibition and transcription activation of hunchback.
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Distribution of Caudal Protein
Visual representation of caudal protein distribution in response to bicoid actions.
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Hunchback Transcription Activation
Role of bicoid in triggering hunchback transcription, crucial for thorax and head regions development.
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Concentration Variances Across Embryos
Comparison of oocyte mRNAs and embryo protein concentrations of significant developmental markers.
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Nanos Protein in Axis Formation
The significance of nanos in establishing the posterior axis via mRNA localization and translating inhibition of hunchback.
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Identification of Segments
Research insights on how specific segments in fly larvae are distinguishable in mutant analyses.
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Gap Genes in Segment Development
Gap gene Krüppel example illustrating mutations leading to segmentation pattern gaps.
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Function of Gap Genes
Morphological segmentation via gap genes characterized by how their mutations lead to segment absence.
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Regulatory Interactions Among Gap Genes
Explanation of gap genes' initial transcription being controlled by maternal effect genes and their mutual regulation.
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Interaction of Gap Gene Products
Visual representation of how gap gene products interact during segment development.
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Interactions of Gap Gene Products Continued
Detailed assessment of gap gene concentration variations along the embryo.
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Regulating Development with Gap Genes
Focusing on the distribution of gap gene products and their historical significance in Drosophila development.
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Overview of Genetic Regulation
Summary of how maternal effect, gap, pair-rule, segment polarity, and homeotic genes work in anterior-posterior organization.
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Homeotic Genes and Segment Identity
Examination of homeobox genes' effects on segment differentiation alongside mention of specific mutations.
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Homeotic Genes Example
Comparative images showing wild-type and altered head structures from the Antennapedia mutation.
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Homeotic Genes Localization
Identification of homeotic gene clusters along Drosophila chromosome 3 influencing segment types along the body axis.
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Regulation of Homeotic Gene Expression
Discussion on regulatory sequences affecting the expression of homeotic genes by gap genes at anterior/personal body regions.
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Interactions Among Homeotic Genes
Insight into how homeotic genes influence each other's expressions in different segments.
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Case Study: Ultrabithorax and Antennapedia
Effects of Ultrabithorax on Antennapedia expression revealing broader developmental implications when absent.
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Mutant Analysis
Example of a fly lacking expression of the ultrabithorax homeobox gene and its overall developmental impact.
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Role of Homeotic Genes in Development
Explanation of how homeotic genes govern the expression of specific developmental genes associated with structures or tissues, using antennapedia and eyeless as examples.
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Development of Wing Structures
Impact of homeotic gene ultrabithorax on segment development that normally leads to the formation of small balance structures called halteres.
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Key Developmental Regulatory Genes
Insight into gene classifications including selector, regulator, and realizator, alongside associated processes related to cell polarity and adhesion.