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Flies are quick!

Flies are quick!. 3 head. The fly body plan: each segment has a unique identity and produces distinctive structures. 3 thorax. 8 abdomen. Model Organisms: Drosophila. small (adult < 5 mm long). Can keep hundreds in a small vial. short generation time - 8 days

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Flies are quick!

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  1. Flies are quick!

  2. 3 head The fly body plan:each segment has a unique identity and produces distinctive structures 3 thorax 8 abdomen

  3. Model Organisms: Drosophila • small (adult < 5 mm long). Can keep hundreds in a small vial. • short generation time - 8 days • embryo develops outside the body in a short time - so can easily study development • history - scientists have been doing genetics and collecting mutations for many years (since 1910) • very cheap to keep • reproducible anatomy • segmentation visible • many anatomical, developmental, & behavioral similarities to vertebrates Small Genome = 120 Mb

  4. Thomas Hunt Morgan and the white eye mutant wildtype fly white mutant

  5. Christiane Nüsslein-Volhard and Eric Wieschaus used genetics to identify proteins that set up the embryonic body plan

  6. wildtype Wieschaus and Nüsslein-Volhard looked for mutants that affect the fly body plan

  7. The fruit fly body plan is self-assembled in 24 hours: how is it specified?

  8. Anterior-Posterior Pattern Formation in Flies Maternal effect genes Figure 6.17

  9. Oocyte Anterior Posterior Determinant bicoid mRNA Determinant nanos mRNA Maternal effect genes establish the anterior/posterior axis of the embryo nurse cells

  10. bicoid protein accumulates in a gradient head tail A P 100 Level of bicoid 0

  11. Remember that cleavage starts without cell division in Drosophila (superficial cleavage) Fig. 9.1 Syncytial specification: specification by interactions between cytoplasmic regions rather than cells

  12. A gradient of the bicoid transcriptionfactor turns on different genes at different "thresholds" bicoid Gene A- turned on only by high level of bicoid Gene B- turned on only by intermediate level of bicoid Gene C- turned off by bicoid and thus only on where bicoid is absent

  13. bicoid mutants have no head!! wildtype larva bicoid mutant Figure 6.24

  14. The “bicoid target genes” are known as the gap genes Hunchback Krüppel Knirps Expression pattern of proteins encoded by gap genes

  15. Gap gene mutants are missing different regions of the body

  16. The gap genes depend on each other to form boundaries and provide identity to unique regions where they overlap Krüppel Hunchback Fig. 6.17

  17. The transcription factors encoded by gap genes cooperate to create even more complex patterns of gene expression Expression domain of Hunchback Expression domain of Krüppel The expression domains of Hunchback and Krüppel overlap Some genes require both Hunchback and Krüppel present to be turned on

  18. Pair-rule genes, such as Even-skipped, refine the segments See Fig. 6.17D for beautiful localization of another pair-rule gene, Fushi tarazu

  19. The segment-polarity gene Engrailed is activated by the Even-skipped and Fushi tarazu pair-rule transcription factors Figure 6.33

  20. Anterior-Posterior Pattern Formation in Flies Maternal effect genes Figure 6.17

  21. wildtype Antennapedia mutant See Fig. 6.37 for a close-up of an Antennapedia mutant

  22. Ed Lewis was far ahead of his time …

  23. Fig. 6.35 Wildtype Ultrabithorax mutant Figure 6.36

  24. Is Ubx is expressed at the right time and place to make T3 different from T2? Yes! Ubx is expressed in T3 and A1 Experiment #1

  25. Does Ultrabithorax bind DNA and regulate genes specific for T3 and A1? Experiment #2 Ultrabithorax is expressed in the region of the embryo that will become the 3rd thoracic and 1st abdominal segments In these segments, the Ultrabithorax protein acts as a transcription factor, turning on genes specific for the 3rd thoracic and 1st abdominal segments ON OFF T3 specific gene ON OFF A5 specific gene T1 specific gene A1 specific gene

  26. The Homeotic genes in Drosophila ANT-C BX-C Fig 6.35

  27. Antennapedia expression is negatively regulated by the Bithorax complex homeotic proteins ANT-C BX-C Fig 6.35

  28. All abdominal segments take on a T2 identity if the bithorax complex is deleted UbxabdA AbdB triple mutant Wildtype T2 T3 A1 T2 T2 T2 A8 T2

  29. Bithorax complex homeotic proteins ANT-C BX-C Fig 6.35

  30. Ultrabithorax, abdA, and AbdB normally repress expression of the thorax-specific “leg gene” Distalless in the abdominal segments wild-type UbxabdA AbdB triple mutant abdomen T1 T2 T3

  31. Lewis hypothesized that the duplication and diversification of homeotic master regulators underlies the evolution of an increasingly complex body plan

  32. The human body is also built up from reiterated units (segments) with different identities along the A/P axis

  33. Mammals also have homeotic genes expressed at different places along the A/P axis

  34. Mouse homeotic genes also encode homeodomain transcription factors that act as master regulators of segment identity Hox 3.1 is expressed in the region of the embryo that will become the 12th and 13th ribs In these segments, Hox 3.1 protein acts as a transcription factor that turns on genes specific for the 12th and 13th ribs OFF 15th rib specific ON gene ON 12th rib specific OFF gene 13th rib specific 4th rib specific gene gene

  35. Notch and the competition to be a neuron

  36. I feel the need to be a neuroblast! you guys stay here and keep up the good work! The story of the epidermal vs. neural cell fate decision in Drosophila They started as one big happy ectodermal epithelium… then one of their number got some big ideas and started to ingress inside… as it left, it sent a message to its neighbors, telling them to stick with the epidermal fate

  37. When the story takes a turn for the worse … the fly neurogenic mutants (mastermind, big brain, notch, delta) If signal is missing... Some cells become neuroblasts and signal their neighbors to remain epidermis all cells eventually ingress and become neuroblasts Nervous system Extra nervous system Epidermis No epidermis!

  38. + + + + + + + + + mutant + mutant + + mutant + + + + + + + + + + + + + + + + + + mutant + mutant + + mutant + + + + + + + + + Cells lacking signal behave differently than cells lacking receptor Thanks, I needed that! + + If mutant cells lack signal, they can be rescued by wild type + neighbors which make signal. What? I can't hear you! + If mutant cells lack receptor, + they cannot be rescued by wild type + neighbors which make signal.

  39. + + + + + + + + + mutant + mutant + + mutant + + + + + + + + + + + + + + + + + + mutant + mutant + + mutant + + + + + + + + + Cells lacking signal behave differently than cells lacking receptor Thanks, I needed that! + + DELTA mutant cellscan be rescued by wild type neighbors. Therefore, DELTA must be theSIGNAL. + What? I can't hear you! + NOTCH mutant cellscannot be rescued by wild type neighbors. Therefore, NOTCH must be theReceptor. + +

  40. neuroblast epidermis

  41. neuroblast After binding Delta, the cytoplasmic domain of Notch undergoes proteolytic cleavage epidermal-specific genes Figure 3.33

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