Lorenzo L. Taping IIIBS Biology

IMPORTANCE OF FRUIT FLYIMPORTANCE OF FRUIT FLY

The fruit fly Drosophila melanogaster was first chosen as a model organism by geneticist T.H. Morgan and intensively studied by generations of geneticists after him.Small and easily grown in the laboratory.Generation time of only two weeks and produces

many offspring.Embryos develop outside the mother’s body.Vast amounts of information available on its genes

and other aspects of its biology.However, because first rounds of mitosis occurs

without cytokinesis, parts of its development are superficially quite different from what is seen in other organisms.

The fruit fly Drosophila melanogaster was first chosen as a model organism by geneticist T.H. Morgan and intensively studied by generations of geneticists after him.Small and easily grown in the laboratory.Generation time of only two weeks and produces

many offspring.Embryos develop outside the mother’s body.Vast amounts of information available on its genes

and other aspects of its biology.However, because first rounds of mitosis occurs

without cytokinesis, parts of its development are superficially quite different from what is seen in other organisms.

Embryonic development of Drosophila melanogaster

Embryonic development of Drosophila melanogaster

genetic analysis of embryogenesis in the fruit fly reveals conserved mechanisms that govern

development of most animals including humans

genetic analysis of embryogenesis in the fruit fly reveals conserved mechanisms that govern

development of most animals including humans

Our Frie

nd

Fruit Fly

Our Frie

nd

Fruit Fly

Development of the fruit fly from egg cell to adult fly

occurs in a series of discrete stages.

Development of the fruit fly from egg cell to adult fly

occurs in a series of discrete stages.

Fruit Fly development:

Fruit Fly development:

EggEgg

Larva

Larva

Drosophila Begins its Development as a Syncytium

Drosophila Begins its Development as a Syncytium

• Series of nuclear divisions without cell division• Early nuclear divisions are synchronus and

extremely rapid.• First nine divisions generate a cloud of nuclei that

will move toward the surface to create monolayer of syncytial blastoderm.

• Plasma membrane later grows inward to enclose each nucleus thus forming cellular blastoderm.

• 15 of the nuclei at the extreme posterior end segregate into pole cells (germ-line).

Superficial Cleavage in a Drosophila EmbryoSuperficial Cleavage in a Drosophila Embryo

• Early phase of development is controlled by maternal-effect genes.

• Gastrulation begins a little before cellularization is complete.

• As gastrulation nears completion, a series of indentations and bulges appear in the surface of the embryo, marking the subdivision of the body into segments along its anteroposterior axis.

• Soon a fully segmented larva emerges.• Imaginal discs- small groups of cell that are still

naïve. They will be destined to form most of the body structure of an adult fly.

The origins of the Drosophila body segments during embryonic development.

The origins of the Drosophila body segments during embryonic development.

The segments of the Drosophila larva and their correspondence with regions of the blastodermThe segments of the Drosophila larva and their correspondence with regions of the blastoderm

Comparison of Larval and Adult Segmentation in DrosophilaComparison of Larval and Adult Segmentation in Drosophila

Fate map of a Drosophila embryo at the cellular blastoderm stage

Fate map of a Drosophila embryo at the cellular blastoderm stage

Gastrulation in Drosophila

Gastrulation in Drosophila

Major Invagination and Furrows formed during Gastrulation

Major Invagination and Furrows formed during Gastrulation

Pole Cells Destined to Form Germ Line CellsPole Cells Destined to Form Germ Line Cells

Late Gastrulation Phase: Embryo’s Segmentation Becomes Apparent

Late Gastrulation Phase: Embryo’s Segmentation Becomes Apparent

Schematic Representation of Gastrulation in Drosophila

Schematic Representation of Gastrulation in Drosophila

Defining the Axes of Embryo:

Role of Egg-Polarity Genes

Defining the Axes of Embryo:

Role of Egg-Polarity Genes

Gradients of maternal molecules in the early embryo control axis formation

Gradients of maternal molecules in the early embryo control axis formation

Cytoplasmic determinants establish the axes of the Drosophila body.• These maternal effect genes, deposited in the

unfertilized egg, lead to an abnormal offspring phenotype if mutated.

In fruit fly development, maternal effect genes encode proteins or mRNA that are placed in the egg while in the ovary. • When the mother has a mutated gene, she makes

a defective gene product (or none at all), and her eggs will not develop properly when fertilized.

These maternal effect genes are also called egg-polarity genes, because they control the orientation of the egg and consequently the fly.• One group of genes sets up the anterior-posterior

axis, while a second group establishes the dorsal-ventral axis.

THE ORGANIZATION OF THE FOUR

EGG-POLARITY GRADIENT SYSTEMS.

THE ORGANIZATION OF THE FOUR

EGG-POLARITY GRADIENT SYSTEMS.

The receptors Toll and Torso are distributed all

over the membrane; the coloring in the

diagrams on the right indicates where they become activated by extracellular ligands.

A Drosophila oocyte in its follicle. The oocyte is derived from a germ cell that divides four times to give a family of 16 cells that remain in communication with one another via cytoplasmic bridges (gray). One member of the family group becomes the oocyte, while the others become nurse cells, which make many of the components required by the oocyte and pass them into it via the cytoplasmic bridges. The follicle cells that partially surround the oocyte have a separate ancestry. As indicated, they are the sources of terminal and ventral eggpolarizing signals.

Nurse Cell: Establishment of Oocyte’s Polarity

After a normal egg has been fertilized

and laid, its nucleus starts to divide rapidly without

division of cytoplasm.

After a normal egg has been fertilized

and laid, its nucleus starts to divide rapidly without

division of cytoplasm.

During the early nuclear divisions, the developing embryo starts to translate the Bicoid

and Nanos mRNAs into proteins.

During the early nuclear divisions, the developing embryo starts to translate the Bicoid

and Nanos mRNAs into proteins.

At this point, the embryo has not yet partitioned into separate cells, and proteins can diffuse

freely to form concentration gradients.

At this point, the embryo has not yet partitioned into separate cells, and proteins can diffuse

freely to form concentration gradients.

Segmentation Genes

The protein gradients are important because they regulate

the embryo’s own developmental genes. The

embryo contains a number of genes that control the fly’s

segmentation pattern. These genes, called segmentation

genes, operate in stages. Bicoid and nanos proteins regulate

genes in the first stage: the gap genes.

The protein gradients are important because they regulate

the embryo’s own developmental genes. The

embryo contains a number of genes that control the fly’s

segmentation pattern. These genes, called segmentation

genes, operate in stages. Bicoid and nanos proteins regulate

genes in the first stage: the gap genes.

Segmentation Genes are ZYGOTIC-EFFECT

GENES

Segmentation Genes are ZYGOTIC-EFFECT

GENES

The bicoid protein is a transcription factor that enters nuclei at the anterior pole and triggers the

transcription of a gap gene called hunchback.

The bicoid protein is a transcription factor that enters nuclei at the anterior pole and triggers the

transcription of a gap gene called hunchback.

The hunchback mRNA is then translated into hunchback protein. Nanos has an opposing function: at the posterior pole, it inhibits hunchback mRNA from being translated.

The hunchback mRNA is then translated into hunchback protein. Nanos has an opposing function: at the posterior pole, it inhibits hunchback mRNA from being translated.

Gap genes map out the basic subdivisions along the anterior-posterior axis.

Mutations cause “gaps” in segmentation.Pair-rule genes define the modular pattern in terms of pairs of segments.

Mutations result in embryos with half the normal segment number.

Segment polarity genes set the anterior-posterior axis of each segment.

Mutations produce embryos with the normal segment number, but with part of each segment replaced by a mirror-image repetition of some other part.

SUMMARY OF SEGMENTATION GENESSUMMARY OF SEGMENTATION GENES

Dorsoventral Axis EstablishmentDorsoventral Axis Establishment

Toll receptor controls the distribution of Dorsal (a gene regulatory protein).

Dorsal in cytoplasm is held inactive. In the newly laid egg, Dorsal mRNA is distributed

uniformly in the cytoplasm. After the nuclei have migrated to the surface of the embryo to form the blastoderm, however, a remarkable redistribution of the Dorsal protein occurs: dorsally the protein remains in the cytoplasm, but ventrally it is concentrated in the nuclei, with a smooth gradient of nuclear localization between these two extremes.

The concentration gradient of Dorsal protein in the nuclei of the blastoderm, as revealed by an antibody.

Dorsally, the protein is present in the cytoplasm and absent from the nuclei; ventrally, it is depleted in the cytoplasm and concentrated in thenuclei.

The concentration gradient of Dorsal protein in the nuclei of the blastoderm, as revealed by an antibody.

Dorsally, the protein is present in the cytoplasm and absent from the nuclei; ventrally, it is depleted in the cytoplasm and concentrated in thenuclei.

Once inside the nucleus, the Dorsal protein turns on or off the expression of different sets of genes depending on its concentration.

The regulatory DNA can be said to interpret the positional signal provided by the Dorsal protein gradient, so as to define a dorsoventral series of territories.

Most ventrally—where the concentration of Dorsal protein is highest—it switches on the expression of a gene Twist that is specific for mesoderm. Most dorsally, where the concentration of Dorsal protein is lowest, the cells switch on Decapentaplegic (Dpp).

And in an intermediate region, where the concentration of Dorsal protein is high enough to repress Dpp but too low to activate Twist, the cells switch on another set of genes, including one called Short gastrulation (Sog).

Morphogen gradientspatterning the dorsoventral axis of the embryo. (A) The gradient of Dorsal protein defines three broad territories of gene expression, marked here by the expression of three representative genes—Dpp, Sog, and Twist. (B) Slightly later, the cells expressing Dpp and Sog secrete, respectively, the signal proteins Dpp (a TGFb family member) and Sog (anantagonist of Dpp). These two proteins diffuse and interact with one another (and with certain other factors) to set upa gradient of Dpp activity that guides a more detailed patterning process.

Origin of the mesoderm from cells expressing TwistEmbryos were fixed at successive stages, crosssectioned, and

stained with an antibody against the Twist protein, a gene regulatory protein of the bHLH family. The cells that express Twist move into the interior of the embryo to form mesoderm.

Origin of the mesoderm from cells expressing TwistEmbryos were fixed at successive stages, crosssectioned, and

stained with an antibody against the Twist protein, a gene regulatory protein of the bHLH family. The cells that express Twist move into the interior of the embryo to form mesoderm.

Dorsal protein generate in turn more local signals that define finer subdivisions of the dorsoventral axis.

These signals act after cellularization. Dpp codes for the secreted Dpp protein, which forms a local morphogen gradient in the dorsal part of the embryo.

The gene Sog, meanwhile, codes for another secreted protein that is produced in the neurogenic ectoderm and acts as an antagonist of Dpp.

The opposing diffusion gradients of these two proteins create a steep gradient of Dpp activity.

The highest Dpp activity levels, in combination with certain other factors, cause development of the most dorsal tissue of all—extraembryonic membrane; intermediate levels cause development of dorsal ectoderm; and very low levels allow development of neurogenic ectoderm.

• In a normal fly, structures such as antennae, legs, and wings develop on the appropriate segments.

• The anatomical identity of the segments is controlled by master regulatory genes, the homeotic genes.

• Discovered by Edward Lewis, these genes specify the types of appendages and other structures that each segment will form.

Homeotic Genes and Segment IdentityHomeotic Genes and Segment Identity

• Mutations to homeotic genes produce flies with such strange traits as legs growing from the head in place of antennae.

Misplacement of Antennapedia Complex: a homeotic gene that determine proper location of antenna and legs.

Misplacement of Antennapedia Complex: a homeotic gene that determine proper location of antenna and legs.

Homeotic Gene Expression Domains in DrososphilaHomeotic Gene Expression Domains in Drososphila

Mutation in the Bithorax Complex (ultrabithorax) will give additional pair of wings from the original halteres in the 3rd thoracic segment.

• Like other developmental genes, the homeotic genes encode transcription factors that control the expression of genes responsible for specific anatomical structures.– For example, a homeotic protein made in a

thoracic segment may activate genes that bring about leg development, while a homeotic protein in a certain head segment activates genes for antennal development.

– A mutant version of this protein may label a segment as “thoracic” instead of “head”, causing legs to develop in place of antennae.

Amazingly, many of the molecules and mechanisms that regulate development in the Drosophila embryo, like the hierarchy below, have close counterparts throughout the animal kingdom.

Homeobox genes have been highly conserved in evolution

Homeobox genes have been highly conserved in evolution

• All homeotic genes of Drosophila include a 180-nucleotide sequence called the homeobox, which specifies a 60-amino-acid homeodomain.– An identical or very similar sequence of nucleotides

(often called Hox genes) are found in many other animals, including humans.

– Related sequences are present in yeast and prokaryotes.

– The homeobox DNA sequence must have evolved very early in the history of life and is sufficiently valuable that it has been conserved in animals for hundreds of millions of years.

In fact, the vertebrate genes

homologous to the homeotic genes of

fruit flies have even kept their

chromosomal arrangement.

In fact, the vertebrate genes

homologous to the homeotic genes of

fruit flies have even kept their

chromosomal arrangement.

• Most, but not all, homeobox-containing genes are homeotic genes that are associated with development.– For example, in Drosophila, homeoboxes are

present not only in the homeotic genes but also in the egg-polarity gene bicoid, in several segmentation genes, and in the master regulatory gene for eye development.

• The polypeptide segment produced by the homeodomain is part of a transcription factor.– Part of this segment, an alpha helix, fits neatly

into the major groove of the DNA helix.• Other more variable domains of the overall

protein determine which genes it will regulate.

• Proteins with homeodomains probably regulate development by coordinating the transcription of batteries of developmental genes.– In Drosophila, different

combinations of homeobox genes are active in different parts of the embryo and at different times, leading to pattern formation.