CH 21.3: Pattern Formation in CH 21.3: Pattern Formation in Animals & Plants Results from Animals & Plants Results from Similar Genetic and Cellular Similar Genetic and Cellular

MechanismsMechanisms

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

Establishing A Body PlanBody 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

Drosophila Development: A Cascade of Gene Activations

HeadThorax

Abdomen

Fruit flies are bilaterally symmetrical

Dorsal=back; Ventral=underbelly

Anterior=towards the head; Posterior=towards the rear

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

•Nurse & follicle cells supply egg with nutrients,

mRNA, etc. Egg already has cytoplasmic determinants for orientation of the 2 axes of fly1. 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. Metamorphosisadult fly emerges

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

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!

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

Segmentation PatternSegmentation genes direct the formation of body segments:

HeadThorax

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

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 for Proteins 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.

What we have learned from the fruit fly

• The molecules and mechanisms present in fruit fly formation are found in many other organisms

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

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

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

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 cellsChemical signals from neighbor cells continue to direct development of the organism

Key Points About DevelopmentKey Points About DevelopmentThe 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

Programmed Cell Death (AKA Apoptosis)

Normal WBC

WBC undergoing apoptosis

•Ced-3 & Ced-4 Ced-3 & Ced-4 proteins are proteins are essential for essential for apoptosis, apoptosis, and are and are always present in always present in the cell as the cell as inactive proteinsinactive proteins

•Proteases-enzymes that break down proteinNucleases-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

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

What we have learned from C. elegans1. 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

Quick Write

• Briefly describe how cytoplasmic determinants affect development of Drosophila

• Briefly describe how cell signaling affects development of C. elegans