Bio 120 2005 Lecture 9 - PowerPoint PPT Presentation

1 / 37
About This Presentation
Title:

Bio 120 2005 Lecture 9

Description:

basic biology is likely to be similar --we are mammals. but low success rates ... The red-banded leaf-hopper (not Euscelis) Experimental embryology of insects ... – PowerPoint PPT presentation

Number of Views:60
Avg rating:3.0/5.0
Slides: 38
Provided by: andrewc80
Category:

less

Transcript and Presenter's Notes

Title: Bio 120 2005 Lecture 9


1
Bio 120 2005 Lecture 9
  • Cloning implications
  • Insect development

2
human reproductive cloning
  • basic biology is likely to be similar --we are
    mammals
  • but low success rates pose a problem
  • USA no federally funded research
  • UK, Canada restricted, but allowed
  • Germany total ban

3
Proposed outcomes (if it ever worked)
  • Vanity cloning
  • eternal life? but your clone would be even less
    similar than an identical twin
  • Eugenics, Dysgenics
  • clone Einstein, Hitler, armies
  • Sentimental cloning
  • clone dead child, parent, pet
  • all the above rest on wrongheaded genetic
    determinism, i.e., that DNA determines identity.
  • cloning is not photocopying
  • Infertility treatment?
  • there are easier ways...

4
therapeutic cloning
  • Goal is to make genetically matched ES cells
  • Proof-of-concept in mice
  • allows somatic (not germline) gene/cell therapy

Rideout WM et al. 2002. Correction of a genetic
defect by nuclear transplantation and combined
cell and gene therapy. Cell 109 17-27.
5
(No Transcript)
6
therapeutic cloning
  • goal is to make ES cells
  • ethical worries are with use of blastocysts
  • isnt there any other way to convert regular
    cells into ES cells?
  • not yet
  • adult stem cells very controversial, and not as
    pluripotent as ES

7
therapeutic cloning questions
  • what makes blastocysts special
  • biologically?
  • ethically? why are other cells not so
    privileged?
  • they are potential individuals
  • but any cell is

8
The ethical debate
  • Kant humans are an end in themselves, not a
    means
  • Bentham the greatest good for the greatest
    number
  • is a zygote an individual? totipotency means any
    cell is a potential individual!

Immanuel Kant
Jeremy Bentham
9
(No Transcript)
10
The questions
  • How are body axes set up?
  • How are germ layers specified
  • How are germ layers subdivided and patterned?

11
Arthropods
  • Hardened exoskeleton, articulated body segments,
    Jointed appendages
  • probably monophyletic
  • gt 1 million described species, estimated 10-30
    million
  • Outnumber humans by ratio of 108 1
  • Subphylum Uniramia
  • Class Myriapoda centipedes, millipedes
  • Class Insecta insects
  • Other subphyla Crustacea, Chelicerates

12
The insect body plan
  • Three body regions
  • head (5-6 segments)
  • thorax (3)
  • abdomen (8-11)
  • body is segmented unsegmented terminal regions
  • 3 pairs of legs (on the three thoracic
    segments)-Hexapoda

m ? f
13
Insects
Wingless insects (e.g. silverfish)
Hemimetabolous insects Incomplete
metamorphosis (dragonflies, bugs, roaches,
earwigs, lice)
Holometabolous insects Complete
metamorphosis (beetles, butterflies, wasps, flies)
14
Drosophila life cycle
Fig 2.29
15
Drosophila development
http//flymove.uni-muenster.de/ Movies of
development and anatomy Interactive animations
of genetic mutants e.g. Processes/segmentation/
Highly recommended
16
The Drosophila egg
  • About 0.5 mm long
  • Yolk in center
  • Visible AP and DV asymmetry before fertilization
  • eggshell (chorion) is also polarized--made by
    follicle cells
  • Sperm entry via micropyle (little gate) in
    eggshell

dorsal
A
P
ventral
17
Cleavage and cellularization
Fig 2.30
Tubulin actin
Tubulin myosin
  • nuclei divide every 9 min without cytokinesis
  • Cellularization all at once
  • Movies from Bill Sullivans lab
    http//bio.research.ucsc.edu/people/sullivan/

18
Gastrulation
  • Ventral blastoderm invaginates to form mesoderm
    (muscles etc)
  • Anterior, posterior invaginations form gut

Cross sections of embryos immunostained for
Twist, a bHLH protein expressed in mesoderm
Fig 2.31
19
The germ band
  • Ventral blastoderm, after gastrulation, will give
    rise to most of embryo
  • Undergoes extension then retraction
  • segmentation first visible during extended germ
    band stage (top)
  • embryonic units are parasegments, different from
    larval segments

20
long-germ versus short-germ insects
  • Drosophila (and other advanced insects)
    Long-germ development
  • germ band develops from most of blastoderm
  • Segments form simultaneously
  • Beetles ( other primitive insects) Short or
    intermediate germ development
  • Part of blastoderm first forms anterior segments
  • Posterior segments develop progressively from
    growth zone (cf. somitogenesis)
  • different routes to similar extended germ band
    stages

Fig 5.34
21
The Drosophila larva
  • Feeding machine
  • Segmented, obvious AP and DV pattern in cuticle
  • Pupates, larval tissues self-destruct (autolysis)
  • Adult (imago) rises from the ashes via imaginal
    discs

Figs 2.33, 2.34
22
Today axis formation
  • 1. Experimental embryology suggests that simple
    mosaic models are not enough
  • 2. Morphogen gradient models of pattern
    formation
  • 3. Using genetics in Drosophila to identify the
    morphogens

The red-banded leaf-hopper (not Euscelis)
23
Experimental embryology of insects
  • Leaf hoppers short-horned bugs (Hemimetabola)
  • Euscelis incisus (formerly E. plebejus)
  • Intermediate-germ development
  • Large eggs, soft egg shell, amenable to
    manipulations
  • Endosymbiotic bacteria in posterior
  • See section 5.18

24
1. evidence for a posterior organizer
  • Suck out tiny bit of cytoplasm from posterior
  • Lose thorax and abdominal segments
  • Long-range effect the activation center

123456789
12
Friedrich Seidel (1897-1992)
25
2. Ligature experiments
123456789
1 2 7 8 9
  • Klaus Sander (1950s) ligate egg with thread
  • lose segments in middle of pattern
  • remaining segments in correct order, spread out
  • Earlier ligature gives bigger gap

26
Cytoplasmic transfer experiments
(1)
(2)
2 hours
123456789
123456789
9 8 7 7 8 9
9 8 7 7 8 9
(same experiment as Fig 5.36)
  • Move posterior cytoplasm by poking with needle
  • ligate immediately--posterior bicaudal,
    anterior makes nothing
  • same experiment except wait 2 hours before
    ligating
  • now anterior forms complete pattern

27
Conclusions
  • Posterior cytoplasm is special
  • Probably nothing to do with the bacteria, these
    just a convenient marker
  • Rest of egg highly regulative
  • Long-range effects, not explained by simple
    mosaic model
  • Sanders model diffusible morphogen made in
    posterior
  • Also independence of A-P and D-V axes

28
Two questions of pattern formation
  • How can cell fate be determined by relative
    position? (what is the positional information)
  • How can a cells response vary depending on its
    history?

29
Morphogenetic fields
shoulder
limb
X
  • Newt limb development (Spemann)
  • Remove limb disc, limb flank regulates
  • Disk flank constitute a developmental field
    region in which cell fate determined by relative
    position

30
Response to signals depends on history (I.e.
genome)
  • Spemann Schotte 1932
  • Transplant newt ventral cells into frog gastrula,
    newt teeth where frog mouth (no teeth) should be
  • cells fates were appropriate for their position
    and for their ancestry

31
Fields
  • Embryonic territories that communicate to form a
    structure
  • E.g. the limb field etc
  • Cells in a field are equivalent in developmental
    potential (at first)
  • Cells become different in response to signals
  • Signals produced from signaling centers, and have
    concentration-dependent effects

32
The French Flag analogy
  • Flag area field
  • Cells read out position relative to boundary
    (flagpole)
  • Response depends on
  • Local concentration of morphogen relative to
    threshold values
  • Cells own history

Fig 1.22
33
Response depends on history (genotype)
  • Cells are newt (UK) or frog (French) in genotype
  • Both respond to same signals
  • Response (UK or French) depends on history

(newt mouth)
(frog gastrula)
Fig 10.36
34
How do you get gradients?
  • Localized source of morphogen that can diffuse
    over gt1 cell diameter
  • Morphogen must be unstable if degraded
    everywhere, a dispersed sink
  • Exponential decay gradient from source
  • Localized source/Dispersed sink (LSDS) model
  • Gradient could form over small (gt 1 mm)
    territories in 1 hour given known diffusion
    constants

morphogen
Distance from source
35
Simple LSDS models
  • Can explain
  • Why organizers can pattern large groups of
    cells--because they are morphogen sources
  • How ordered patterns form--because morphogen
    gradient has polarity
  • defect regulation--why pattern reforms if small
    bits of field removed or added (because source
    still intact)
  • Have difficulty explaining
  • Size invariance (e.g. Dictyostelium)
  • Ability of sources to re-form (e.g. limb field)
  • And do not address
  • how cells can read out local morphogen
    concentrations
  • Box 10A reaction-diffusion models that
    self-organize

36
Gradient explanation of gaps
123456789
1 2 7 8 9
37
Gradient explanation of cytoplasmic transfers
Wait a couple of hours
123456789
123456789
9 8 7 7 8 9
Write a Comment
User Comments (0)
About PowerShow.com