Transcript C elegans

Ch. 21: The Genetic Basis of Development
The application of genetic analysis and DNA technology has
changed our understanding of how a complex multicellular
organism develops from a single cell.
 In 1995, Swiss researchers demonstrated that a particular gene
functions as a master switch that triggers the development of the
eye in Drosophila.
 A similar gene triggers eye development in mammals.
 Developmental biologists are discovering remarkable similarities
in the mechanisms that shape diverse organisms.
Chapter 21: The Genetic Basis of Development
1. How do we study development in the genetics-based lab?
-Model organisms
-fruit fly, nematode worm, mouse, etc.
Figure 21.2 Model Organisms for Genetic Studies of Development
DROSOPHILA MELANOGASTER
(FRUIT FLY)
Drosophila
- small, easy & cheap to culture
- 2 week generation time
- 4 chromosomes
- LARGE literature of info
CAENORHABDITIS ELEGANS
(NEMATODE)
C elegans
- easy to culture
- transparent body with few cell types
- zygote to mature adult in 3 days
0.25 mm
ARABIDOPSIS THAMANA
(COMMON WALL CRESS)
MUS MUSCULUS
(MOUSE)
Zebra fish
- vertebrate
- external fertilization 
- external development 
Mouse
- mammalian vertebrate
- LARGE literature
- transgenics & knock-outs
- internal fertilization 
- internal development 
DANIO RERIO
(ZEBRAFISH)
Chapter 21: The Genetic Basis of Development
2. How does a zygote transform into an organism?
1) Cell division – early cell divisions are called cleavage – no cytokinesis
2) Morphogenesis—”creation of form/shape”
3) Cell differentiation – process by which cells become specialized
 differential gene expression!!!
Figure 21.3a, b
(a) Fertilized eggs
of a frog
(b) Tadpole hatching
from egg
(a) Animal development. Most
animals go through some
variation of the blastula and
gastrula stages. The blastula is
a sphere of cells surrounding a
fluid-filled cavity. The gastrula
forms when a region of the blastula
folds inward, creating a
tube—a rudimentary gut. Once
the animal is mature,
differentiation occurs in only a
limited way—for the replacement
of damaged or lost cells.
Cell
movement
Zygote
(fertilized egg)
Eight cells
Blastula
(cross section)
Gut
Gastrula
(cross section)
Adult animal
(sea star)
Cell division
Morphogenesis
(b) Plant development. In plants
with seeds, a complete embryo
develops within the seed.
Morphogenesis, which involves
cell division and cell wall
expansion rather than cell or
tissue movement, occurs
throughout the plant’s lifetime.
Apical meristems (purple)
continuously arise and develop
into the various plant organs as
the plant grows to an
indeterminate size.
Observable cell differentiation
Seed
leaves
Shoot
apical
meristem
Zygote
(fertilized egg)
Root
apical
meristem
Two cells
Figure 21.4a, b
Embryo
inside seed
Plant
Chapter 47: Animal Development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. What three things influence cell fate?
- Cytoplasmic determinants – mRNA & proteins in egg cytoplasm
- Cleavage pattern – divides cytoplasmic determinants
- Induction – cellular peer pressure—responses to signals from nearby cells
Figure 21.11 Sources of developmental information for the early embryo
Unfertilized egg cell
Molecules of another
cytoplasmic determinant
Sperm
Molecules of a
a cytoplasmic
determinant
Fertilization
Nucleus
Zygote
(fertilized egg)
Mitotic cell division
Two-celled
embryo
(a) Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its cytoplasm,
encoded by the mother’s genes, that influence development. Many of these cytoplasmic
determinants, like the two shown here, are unevenly distributed in the egg. After fertilization
and mitotic division, the cell nuclei of the embryo are exposed to different sets of cytoplasmic
determinants and, as a result, express different genes.
Figure 21.11b
Early embryo
(32 cells)
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
(b) Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing
chemicals that signal nearby cells to change their gene expression.
Cellular peer pressure
Figure 47.24 How does distribution of the gray crescent at the first
cleavage affect the potency of the two daughter cells?
EXPERIMENT
1
Left (control):
Fertilized
salamander eggs
were allowed to
divide normally,
resulting in the
gray crescent being
evenly divided
between the two
blastomeres.
Gray
crescent
Right (experimental):
Fertilized eggs were
constricted by a
thread so that the
first cleavage plane
restricted the gray
crescent to one
blastomere.
Gray
crescent
2 The two blastomeres were
then separated and
allowed to develop.
Normal
Belly
piece
Normal
RESULTS
Blastomeres that receive half or all of the gray crescent develop into normal embryos, but a blastomere
that receives none of the gray crescent gives rise to an abnormal embryo without dorsal structures. Spemann called it a
“belly piece.”
CONCLUSION
The totipotency of the two blastomeres normally formed during the first cleavage division depends on
cytoplasmic determinants localized in the gray crescent.
Now let’s look at induction….
Figure 47.25 Can the dorsal lip of the blastopore induce cells in another
part of the amphibian embryo to change their developmental fate?
EXPERIMENT Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastrula to the
ventral side of the early gastrula of a nonpigmented newt.
Pigmented gastrula
(donor embryo)
Dorsal lip of
blastopore
Nonpigmented gastrula
(recipient embryo)
RESULTS
During subsequent development, the recipient embryo formed a second notochord and neural tube in
the region of the transplant, and eventually most of a second embryo. Examination of the interior of the double embryo
revealed that the secondary structures were formed in part from host tissue.
Primary embryo
Secondary
structures:
Notochord (pigmented cells)
Neural tube (mostly nonpigmented cells)
Primary
structures: Secondary (induced) embryo
Neural tube
Notochord
CONCLUSION The transplanted dorsal lip was able to induce cells in a different region of the recipient to form
structures different from their normal fate. In effect, the dorsal lip “organized” the later development of an entire embryo.
Chapter 47: Animal Development
1.
2.
3.
4.
How do we study development in the genetics-based lab?
How does a zygote transform into an organism?
What three things influence cell fate?
Once cells have differentiated can they de-differentiate?
- Plant cuttings
- Animal cells????
Transverse
section of
carrot root
EXPERIMENT
2-mg
fragments
Fragments cultured in nutrient
medium; stirring causes
single cells to
shear off into
liquid.
Single cells
free in
suspension
begin to
divide.
Embryonic
plant develops
from a cultured
single cell.
Plantlet is cultured on agar
medium. Later
it is planted
in soil.
A single
RESULTS
Somatic (nonreproductive) carrot
cell developed into a mature carrot
plant. The new plant was a genetic
duplicate (clone) of the parent plant.
Adult plant
CONCLUSION At least some differentiated (somatic) cells in plants are toipotent, able
to reverse their differentiation and then give rise to all the cell types in a mature plant.
Figure 21.5
Figure 21.6 Can the nucleus from a differentiated animal cell
direct development of an organism?
EXPERIMENT Researchers enucleated frog egg cells by exposing them to ultraviolet light, which
destroyed the nucleus. Nuclei from cells of embryos up to the tadpole stage were transplanted into the
enucleated egg cells.
Frog embryo
Frog egg cell
Fully differentiated
(intestinal) cell
Less differentiated cell
Donor
nucleus
transplanted
Most develop
into tadpoles
Frog tadpole
Enucleated
egg cell
Donor
nucleus
transplanted
<2% develop
into tadpoles
Permanent (epigenetic) changes had occurred to the differentiated nucleus
-Methylation of DNA to turn off non-intestinal genes
-Only intestinal activators available
Chapter 21: Genetic basis of development
1.
2.
3.
4.
5.
How do we study development in the genetics-based lab?
How does a zygote transform into an organism?
What three things influence cell fate?
Once cells have differentiated can they de-differentiate?
How was Dolly cloned?
- nuclear transplantation
Fig. 21.7 Reproductive Cloning of a Mammal by Nuclear Transplantation
APPLICATION This method is used to produce cloned
animals whose nuclear genes are identical to the donor
animal supplying the nucleus.
1
RESULTS
The cloned animal is identical in appearance
and genetic makeup to the donor animal supplying the nucleus,
but differs from the egg cell donor and surrogate mother.
2
Egg cell
from ovary Nucleus
Nucleus
removed
3 Cells fused
removed
TECHNIQUE
Shown here is the procedure used to produce
Dolly, the first reported case of a mammal cloned using the nucleus
of a differentiated cell.
Egg cell
donor
Mammary
cell donor
Cultured
mammary cells
are semistarved,
arresting the cell
cycle and causing
dedifferentiation
Nucleus from
mammary cell
4 Grown in culture
Early embryo
Dolly only lived 6 yrs.
Premature signs of aging…no telomerase!
5 Implanted in uterus
of a third sheep
6 Embryonic
development
Surrogate
mother
Lamb (“Dolly”)
genetically identical to
mammary cell donor
Chapter 21: Genetic basis of development
1.
2.
3.
4.
5.
6.
How do we study development in the genetics-based lab?
How does a zygote transform into an organism?
What three things influence cell fate?
Once cells have differentiated can they de-differentiate?
How was Dolly cloned?
What is a stem cell?
- a relatively unspecialized cell
- can differentiate into different cell types under specific conditions
Figure 21.9 Working with stem cells
Embryonic stem cells
Adult stem cells
Early human embryo
at blastocyst stage
(mammalian equivalent of blastula)
From bone marrow
in this example
Totipotent
cells
Pluripotent
cells
Cultured
stem cells
Different
culture
conditions
Different
types of
differentiated
cells
Liver cells
Nerve cells
Blood cells
Chapter 21: Genetic basis of development
1.
2.
3.
4.
5.
6.
How do we study development in the genetics-based lab?
How does a zygote transform into an organism?
What three things influence cell fate?
Can cells de-differentiate?
How was Dolly cloned?
What is a stem cell?
- Embryonic – totipotent
- Adult – pluripotent
- iPS – induced pluripotent stem cell
- differentiated cells get re-programmed
- retroviruses used to introduce extra cloned copies of 4
stem cell master regulator genes in differentiated cells
- some “kinks” still exist
- WFIRM – WF Institute of Regenerative Medicine
7. When is a cell “determined” (fated)?
- Master regulator gene(s) turned “on”
- Muscle cells – MyoD transcription factor – activator turns on all muscle
genes
Figure 21.10 Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
Embryonic
precursor cell
OFF
OFF
Figure 21.10 Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
1
Myoblast
(determined)
Determination. Signals from other
cells lead to activation of a master
regulatory gene called myoD, and
the cell makes MyoD protein, a
transcription factor. The cell, now
called a myoblast, is irreversibly
committed to becoming a skeletal
muscle cell.
OFF
OFF
mRNA
MyoD protein
(transcription
factor)
Figure 21.10 Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
1
Myoblast
(determined)
2
Determination. Signals from other
cells lead to activation of a master
regulatory gene called myoD, and
the cell makes MyoD protein, a
transcription factor. The cell, now
called a myoblast, is irreversibly
committed to becoming a skeletal
muscle cell.
Differentiation. MyoD protein stimulates
the myoD gene further, and activates
genes encoding other muscle-specific
transcription factors, which in turn
activate genes for muscle proteins. MyoD
also turns on genes that block the cell
cycle, thus stopping cell division. The
nondividing myoblasts fuse to become
mature multinucleate muscle cells, also
called muscle fibers.
OFF
OFF
mRNA
MyoD protein
(transcription
factor)
mRNA
MyoD
Muscle cell
(fully differentiated)
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Chapter 21: Genetic basis of development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. What three things influence cell fate?
4. Once cells differentiate can they de-differentiate?
5. How was Dolly cloned?
6. What is a stem cell?
7. When is a cell “determined” (fated)?
8. How does morphogenesis (pattern formation) occur in animals?
- development of spatial organization of tissues & organs
- mostly occurs in embryonic & juvenile stages of animals
- occurs continually in plants
- consists of molecular cues that direct formation – differential gene
expression
Figure 21.12 Key developmental events in the life of Drosophila
Follicle cell
Nucleus
Egg cell
developing within
Egg cell
ovarian
Nurse
follicle
cell
Fertilization
Laying of egg
Fertilized egg
Egg
shell
Nucleus
1
Embryo
Multinucleate
single cell
2
Early blastoderm
Plasma
membrane
formation
3
Yolk
Late blastoderm
Cells of
embryo
4
Segmented
embryo
Body
segments
0.1 mm
5
Hatching
Larval stages (3)
6
Pupa
Metamorphosis
Head
Thorax
7
Abdomen
Adult fly
0.5 mm
Dorsal
BODY
AXES
Anterior
Posterior
Ventral
Chapter 21: Genetic basis of development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. What three things influence cell fate?
4. Once cells differentiate can they de-differentiate?
5. How was Dolly cloned?
6. What is a stem cell?
7. When is a cell “determined” (fated)?
8. How does morphogenesis (pattern formation) occur in animals?
9. How can gene function be determined (ch 20)?
- study mutants & knock-outs
- helped us understand animal development
Chapter 21: The Genetic Basis of Development
Eye
Antenna
Leg
Wild type
Mutant
Chapter 21: Genetic basis of development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. What three things influence cell fate?
4. Once cells differentiate can they de-differentiate?
5. How was Dolly cloned?
6. What is a stem cell?
7. When is a cell “determined” (fated)?
8. How does morphogenesis (pattern formation) occur in animals?
9. How can gene function be determined (ch 20)?
10. How do flies go from zygote to organism?
- cytoplasmic determinants in egg produced by maternal effect genes
- establish axes (anterior & posterior, left & right)
- bicoid
Chapter 21: The Genetic Basis of Development
Maternal effect genes are aka egg-polarity genes
bicoid – determines anterior end – transcription factor
Tail
Head
T1 T2
T3
A1 A2 A3 A4 A5
A6 A7
A8
Wild-type larva
Tail
Tail
A8
A7
Mutant larva (bicoid)
A8
A6
A7
(a) Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation
Figure 21.14a
in the mother’s bicoid gene leads to tail structures at both ends (bottom larva).
The numbers refer to the thoracic and abdominal segments that are present.
Egg cell
Nurse cells
1 Developing
egg cell
bicoid mRNA
2 Bicoid mRNA
in mature
unfertilized egg
Fertilization
Translation of bicoid mRNA
100 µm
3 Bicoid protein in
early embryo
Anterior end
(b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo.
Figure 21.14b
Chapter 21: The Genetic Basis of Development
In flies, after the body’s axes are determined….
- segmentation genes produce proteins that direct formation of body segments
- identity of body segments is directed by homeotic genes (many)
- all have 180 nucleotide (60 aa) homeobox domain common to many
invertebrates & vertebrates
- suggests that they developed early in history of life on Earth
- Hox genes
Hierarchy of Gene Activity in Early Drosophila Development
Maternal effect genes (egg-polarity genes)
Gap genes
Pair-rule genes
Segment polarity genes
Homeotic genes of the embryo
Other genes of the embryo
Segmentation genes
of the embryo
Chapter 21: Genetic basis of development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. What three things influence cell fate?
4. Once cells have differentiated can they de-differentiate?
5. How was Dolly cloned?
6. What is a stem cell?
7. When is a cell “determined” (fated)?
8. How does morphogenesis (pattern formation) occur in animals?
9. How can gene function be determined (ch 20)?
10. How do flies go from zygote to organism?
11. What have we learned from C elegans?
Chapter 21: The Genetic Basis of Development
-Cell lineage (cell history) of the C. elegans nematode is known…
Zygote
0
Time after fertilization (hours)
First cell division
Nervous
system,
outer
skin, musculature
Outer skin,
nervous system
Musculature,
gonads
Germ line
(future
gametes)
Musculature
10
Hatching
Intestine
Intestine
Eggs
Vulva
ANTERIOR
POSTERIOR
1.2 mm
Induction leads to many developmental changes in C elegans
Figure 21.16 Cell signaling & induction during development of C elegans
a. Induction of cell 3 by cell 4 causes…..
-posterior daughter cell to form intestines
-anterior daughter cell to form muscle & gonads
b. The anchor cell induces the 6 cells of the vulva
Chapter 21: Genetic basis of development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. What three things influence cell fate?
4. Once cells have differentiated can they de-differentiate?
5. How was Dolly cloned?
6. What is a stem cell?
7. When is a cell “determined” (fated)?
8. How does morphogenesis (pattern formation) occur in animals?
9. How can gene function be determined (ch 20)?
10. How do flies go from zygote to organism?
11. What have we learned from C elegans?
12. What is apoptosis?
-programmed cell death – cell suicide
-involves ced genes – cell death genes – stimulates production of
proteases and nucleases
Figure 21.18 Molecular basis of apoptosis in C. elegans
Ced-9
protein (active)
inhibits Ced-4
activity
Death
signal
receptor
Mitochondrion
Ced-4 Ced-3
Inactive proteins
Cell
forms
blebs
(a) No death signal
Ced-9
(inactive)
Death
signal
Active Active
Ced-4 Ced-3
Activation
cascade
(b) Death signal
Other
proteases
Nucleases
Figure 21.19 Effect of apoptosis during paw development in mice
Interdigital tissue
1 mm
Figure 21.19
Chapter 21: Genetic basis of development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. What three things influence cell fate?
4. Once cells have differentiated can they de-differentiate?
5. How was Dolly cloned?
6. What is a stem cell?
7. When is a cell “determined” (fated)?
8. How does morphogenesis (pattern formation) occur in animals?
9. How can gene function be determined (ch 20)?
10. How do flies go from zygote to organism?
11. What have we learned from C elegans?
12. What is apoptosis?
13. What is the relationship among the genetic basis of development across
organisms?
- Molecular analysis of the homeotic genes in Drosophila has shown
that they all include a sequence called a homeobox – Hox genes
- An identical (or very similar) DNA sequence has been discovered in
the homeotic genes of vertebrates and invertebrates
Figure 21.23 Conservation of fruit fly homeotic genes in fruit fly & mouse
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse