1. In Drosophila, maternal effect genes code for cytoplasmic determinants that establish the body axes and initial segmentation pattern through concentrations of proteins like Bicoid. 2. In C. elegans, cells send signaling molecules to neighboring cells, and the concentration of the signal received determines the cell's fate, such as forming the inner vulva in response to the highest signal concentration. 3. Both organisms use similar genetic and cellular mechanisms involving gradients of signaling molecules read out through cascades of gene regulation to develop their body plans in an orderly progression from fertilization to the fully formed animal.

1. CH 21.3: Pattern Formation in AnimalsCH 21.3: Pattern Formation in Animals
& Plants Results from Similar Genetic& Plants Results from Similar Genetic
and Cellular Mechanismsand Cellular Mechanisms

2. Cytoplasmic Determinants vs.
Inductive Signaling
• We will examine
Drosophila for the
effects of cytoplasmic
determinants
• We will examine C.
elegans for the effect
of cell signaling
Objective: Be able to describe how
cytoplasmic determinants affect development
of Drosophila and how cell signaling affects
development of C. elegans

3. Establishing A Body Plan
Body plan is determined by
positional information
molecules (P.I. molecules)
1.) molecules in the
cytoplasm
2.) signals from other cells
P.I. Molecules in cytoplasm
P.I. released from
other cells

4. Drosophila Development: A
Cascade of Gene Activations
Head
Thorax
Abdomen
Fruit flies are
bilaterally
symmetrical
Dorsal=back; Ventral=underbelly
Anterior=towards the head;
Posterior=towards the rear

5. Development
of Drosophila
– a case study
• Drosophila is the most well studied animal
• Observing and studying its development from
zygote to adult has lead to better
understanding of development in other
organisms, including humans

6. •Nurse & follicle cells supply egg with nutrients,
mRNA, etc. Egg already has cytoplasmic
determinants for orientation of the 2 axes
of fly
1. First 10 mitotic divisions: cells don’t grow before
mitosis. No cytokinesis=multinucleate cell
2. Nuclei move to cell’s edge. Cell=blastoderm
3. Plasma membranes surround nuclei. Axes &
segments are determined.
4. Clearly visible segments
5. Organs form. Fly goes through 3 larval stages
6. Case forms around larvae. Now called pupa.
7. Metamorphosisadult fly emerges

8. Genetic Analysis of Early Development
•Researchers studying
embryonic segmentation in
fruit flies focused on
comparing wild-type &
mutant flies
•By purposefully mutating specific genes, the researchers could
observe the phenotypic effect that the gene has on the body plan of
the fly

9. Axis Establishment
•Maternal effect genes code for the cytoplasmic determinants within
the mother’s egg
•If maternal effect genes are mutated in
the mom, then the offspring are
mutants, regardless of their genotype…
makes sense since these genes code for
determinants
•Maternal effect genes AKA egg-polarity genes
•Ex. of maternal effect gene = bicoid gene.
Bicoid gene seems to affect anterior
formation because when it’s mutated, the
fly develops with 2 posterior ends!

10. Axis Establishment
•It turns out that the
concentration of the bicoid
protein establishes the position
of the anterior end
High concentration of bicoid
protein = head formation
•In fact, you can stimulate any
part of the embryo to develop
into an anterior region if you
inject it with enough biocoid
•Bicoid=maternal effect gene
associated with axis formation

11. Segmentation Pattern
Segmentation genes direct the formation of body segments:
Head
Thorax
Abdomen
•Three sets of
segmentation
genes:
•Gap genes
•Pair-rule genes
•Segment polarity genes
•Some of the products of each set of
segmentation genes act as transcription
factors that activate the next group of
segmentation genes

12. Identity of Body Parts
•Once segments are
established, homeotic
genes determine the
body parts that will
grow from each
segment
•Proteins encoded by homeotic genes serve as activators or repressors forProteins encoded by homeotic genes serve as activators or repressors for
genes responsible for various appendagesgenes responsible for various appendages
•All of the fly’s cells have the same
genes. However, if a cell is in the head
segment, then its homeotic genes produce
proteins that only activate antenna growth
(for example). And in the cells of the
thoracic segment, the antenna genes are
not activated, but the leg genes are.

13. What we have learned from the fruit fly
• The molecules and
mechanisms present
in fruit fly formation
are found in many
other organisms

14. What we have learned from the
fruit fly
1. Maternal effect genes code for
cytoplasmic determinants that
direct the formation of the
body plan
2. First, the amount of bicoid
gene codes for head or butt
3. Second, segmentation genes
code for proteins that make
body segments form
4. Then homeotic genes code for
proteins that make body parts
grow from each segment

15. C. elegans: The Role of Cell Signaling
•In the development of multicellular organisms, communication
between neighboring cells is very important.
•Signaling between cells that are adjacent to
one another is called induction

16. Induction: Two Examples of Cell Signaling During
Development
A signal protein on the surface
of cell #4 induces the posterior
end of cell #3 to become a
daughter cell that will give rise
to intestinal tissue
So as early as the 4 cell
stage of development,
cells are communicating
with each other to direct
development of the
embryo

17. Induction: Two Examples of Cell Signaling
During Development
•Anchor cell releases signal proteins.
•The nearest cell receives the
highest concentration of the
signal and becomes the inner
vulva
•The two cells on either side
receive fewer signal proteins
(a weaker signal) and are thus
induced to form the outer
vulva
•Cells further out don’t receive
the signal, so they develop into
epithelial cells
Chemical signals from neighbor cells continue to direct development of
the organism

19. Key Points About DevelopmentKey Points About Development
The sequence of cell signals
drives the formation of
organs
The effect of an inducer
molecule can depend on its
concentration
The cell response is often
(in)activation of gene activity
characteristic of a particular
differentiated cell

20. Programmed Cell Death (AKA Apoptosis)
Normal
WBC
WBC undergoing
apoptosis
•Ced-3 & Ced-4Ced-3 & Ced-4
proteins areproteins are
essential foressential for
apoptosis,apoptosis, and areand are
always present inalways present in
the cell asthe cell as
inactive proteinsinactive proteins
•Proteases-enzymes that break down
protein
Nucleases-break down nucleic acids
•When a death signal binds to the receptor,
it triggers a cascade that activates ced-3
&ced-4 and leads to the production of
proteases & nucleases

21. Programmed Cell Death (AKA Apoptosis)
Why have cells that self-destruct???
•In vertebrates, it is essential for the proper development & functioning
of the nervous system, the immune system, and development of
separate digits
In humans, extra skin cells die to give us distinct and separated
fingers/toes.
The lack of apoptosis in these cells for ducks is what gives them
webbed feet

22. What we have learned from C. elegans
1. Cells send messages to
each other to direct
formation of the body
2. The effect of these
messages depends on the
dose the cell receives
3. The order of the signals
the cell receives leads to
organ formation
4. The death signal will lead
to the death of that cell

23. Quick Write
• Briefly describe how cytoplasmic
determinants affect development of
Drosophila
• Briefly describe how cell signaling
affects development of C. elegans