The fly body plan: each segment has a unique identity and produces distinctive structures 3 head 3 thorax 8 abdomen
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
Thomas Hunt Morgan and the white eye mutant wildtype fly white mutant
Christiane Nüsslein-Volhard and Eric Wieschaus used genetics to identify proteins that set up the embryonic body plan
Wieschaus and Nüsslein-Volhard looked for mutants that affect the fly body plan wildtype
The fruit fly body plan is self-assembled in 24 hours: how is it specified?
Maternal effect genes establish the anterior/posterior axis of the embryo nurse cells
bicoid protein accumulates in a gradient 100 0 Level of bicoid AP headtail
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
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 A gradient of the bicoid transcription factor turns on different genes at different "thresholds" bicoid
wildtype larva bicoid mutant bicoid mutants have no head!! Figure 9.23
The “bicoid target genes” are known as the gap genes Hunchback Krüppel Knirps Expression pattern of proteins encoded by gap genes
Gap gene mutants are missing different regions of the body
Hunchback Krüppel The gap genes depend on each other to form boundaries and provide identity to unique regions where they overlap Fig. 9.17
The transcription factors encoded by gap genes cooperate to create even more complex patterns of gene expression Expression domain of Hunchback The expression domains of Hunchback and Krüppel overlap Some genes require both Hunchback and Krüppel present to be turned on Expression domain of Krüppel
Alberts Chapter 8 (p. 266) Pair-rule genes, such as Even-skipped, refine the segments
The segment-polarity gene Engrailed is activated by the Even-skipped and Fushi tarazu pair-rule transcription factors Figure 9.33
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
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 T3 specific gene A1 specific gene T1 specific gene A5 specific gene OFF ON Experiment #2 Does Ultrabithorax bind DNA and regulate genes specific for T3 and A1?
The Homeotic genes in Drosophila Fig 9.35 ANT-CBX-C
Antennapedia expression is negatively regulated by the Bithorax complex homeotic proteins Fig 9.35 ANT-CBX-C
Wildtype Ubx abdA AbdB triple mutant T2 T3 A1 A8 T2 All abdominal segments take on a T2 identity if the bithorax complex is deleted
Ultrabithorax, abdA, and AbdB normally repress expression of the thorax-specific “leg gene” Distalless in the abdominal segments wild-typeUbx abdA AbdB triple mutant T1 T2 T3 abdomen
Lewis hypothesized that the duplication and diversification of homeotic master regulators underlies the evolution of an increasingly complex body plan
The human body is also built up from reiterated units (segments) with different identities along the A/P axis
Mammals also have homeotic genes expressed at different places along the A/P axis
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 12th rib specific gene 13th rib specific gene 4th rib specific gene 15th rib specific gene OFF ON
The story of the epidermal vs. neural cell fate decision in Drosophila They started as one big happy ectodermal epithelium… I feel the need to be a neuroblast! you guys stay here and keep up the good work! 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
When the story takes a turn for the worse … the fly neurogenic mutants (mastermind, big brain, notch, delta) Nervous system Epidermis Extra nervous system No epidermis! Some cells become neuroblasts and signal their neighbors to remain epidermis If signal is missing... all cells eventually ingress and become neuroblasts
+ + + mutant + + +++ ++ +++ + ++++++++ + + Cells lacking signal behave differently than cells lacking receptor If mutant cells lack signal, they can be rescued by wild type neighbors which make signal. If mutant cells lack receptor, they cannot be rescued by wild type neighbors which make signal. + + + mutant +++ ++ ++ + ++++++ +++ ++++ Thanks, I needed that! What? I can't hear you!
+ + + mutant + + +++ ++ +++ + ++++++++ + + Cells lacking signal behave differently than cells lacking receptor + + + mutant +++ ++ ++ + ++++++ +++ ++++ Thanks, I needed that! What? I can't hear you! DELTA mutant cells can be rescued by wild type neighbors. Therefore, DELTA must be the SIGNAL. NOTCH mutant cells cannot be rescued by wild type neighbors. Therefore, NOTCH must be the Receptor.