Drosophila Embryogenesis

8/12/2019 Drosophila Embryogenesis

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Drosophila embryogenesis 1

Drosophila embryogenesis

Drosophila embryogenesis, the process by which Drosophila (fruit fly) embryos form, is a favorite model system

for geneticists and developmental biologists studying embryogenesis. The small size, short generation time, and

large brood size make it ideal for genetic studies. Transparent embryos facilitate developmental studies. Drosophila

melanogaster was introduced into the field of genetic experiments by Thomas Hunt Morgan in 1909.

Life cycle

Drosophila display a holometabolous method of development, meaning that they have three distinct stages of their

post-embryonic life cycle, each with a radically different body plan: larva, pupa and finally, adult. The machinery

necessary for the function and smooth transition between these three phases develops during embryogenesis. During

embryogenesis, the larval stage fly will develop and hatch at a stage of its life known as the first larval instar. Cells

that will produce adult structures are put aside in imaginal discs. During the pupal stage, the larval body breaks down

as the imaginal disks grow and produce the adult body. This process is called complete metamorphosis. About 24

hours after fertilization, an egg hatches into a larva, which undergoes three molts taking about 5.5 to 6 days, afterwhich it is called a pupa. The pupa metamorphoses into an adult fly, which takes about 3.5 to 4.5 days. The entire

growth process from egg to adult fly takes an estimated 10 to 12 days to complete at 25C.[1]

The mother fly produces oocytes that already have anterior-posterior and dorsal-ventral axes defined by maternal

activities.

Embryogenesis in Drosophila is unique among model organisms in that cleavage occurs in a multinucleate

syncytium (strictly a coenocyte). Early on, 256 nuclei migrate to the perimeter of the egg, creating the syncytial

blastoderm. The germ line segregates from the somatic cells through the formation of pole cells at the posterior end

of the embryo. After thirteen mitotic divisions and about 4 hours after fertilization, an estimated 6,000 nuclei

accumulate in the unseparated cytoplasm of the oocyte before they migrate to the surface and are encompassed by

plasma membranes to form cells surrounding the yolk sac producing a cellular blastoderm.

Like other triploblastic metazoa, gastrulation leads to the formation of three germ layers: the endoderm, mesoderm,

and ectoderm. The mesoderm invaginates from the ventral furrow (VF), as does the ectoderm that will give rise to

the midgut. The pole cells are internalized by a different route.

Germ band elongation involves many rearrangements of cells, and the appearance of distinct differences in the cells

of the three germ bands and various regions of the embryo. The posterior region (including the hindgut) expands and

extends towards the anterior pole along the dorsal side of the embryo. At this time, segments of the embryo become

visible, creating a striped arrangement along the anterior-posterior axis. The earliest signs of segmentation appear

during this phase with the formation of parasegmental furrows. This is also when the tracheal pits form, the first

signs of structures for breathing.Germ band retraction returns the hindgut to the dorsal side of the posterior pole and coincides with overt

segmentation. The remaining stages involve the internalization of the nervous system (ectoderm) and the formation

of internal organs (mainly mesoderm).

Anterior-posterior axis patterning in Drosophila

One of the best understood examples of pattern formation is the patterning along the future head to tail

(antero-posterior) axis of the fruit flyDrosophila melanogaster. There are three fundamental genes that give way to

the developmental structure of the fly. The three genes that are involved are: maternal effect genes, segmentation

genes, and homeotic genes. The development ofDrosophila is particularly well studied, and it is representative of a

major class of animals, the insects or insecta. Other multicellular organisms sometimes use similar mechanisms for

axis formation, although the relative importance of signal transfer between the earliest cells of many developing
http://en.wikipedia.org/w/index.php?title=Insecthttp://en.wikipedia.org/w/index.php?title=Insecthttp://en.wikipedia.org/w/index.php?title=Drosophila_melanogasterhttp://en.wikipedia.org/w/index.php?title=Segmentation_%28biology%29http://en.wikipedia.org/w/index.php?title=Germ_layerhttp://en.wikipedia.org/w/index.php?title=Gastrulationhttp://en.wikipedia.org/w/index.php?title=Animalhttp://en.wikipedia.org/w/index.php?title=Triploblastyhttp://en.wikipedia.org/w/index.php?title=Pole_cellhttp://en.wikipedia.org/w/index.php?title=Germlinehttp://en.wikipedia.org/w/index.php?title=Coenocytehttp://en.wikipedia.org/w/index.php?title=Syncytiumhttp://en.wikipedia.org/w/index.php?title=Oocytehttp://en.wikipedia.org/w/index.php?title=Metamorphosis_%28biology%29http://en.wikipedia.org/w/index.php?title=Imaginal_dischttp://en.wikipedia.org/w/index.php?title=Embryogenesishttp://en.wikipedia.org/w/index.php?title=Thomas_Hunt_Morganhttp://en.wikipedia.org/w/index.php?title=Drosophila_melanogasterhttp://en.wikipedia.org/w/index.php?title=Drosophila_melanogasterhttp://en.wikipedia.org/w/index.php?title=Embryogenesishttp://en.wikipedia.org/w/index.php?title=Developmental_biologyhttp://en.wikipedia.org/w/index.php?title=Geneticshttp://en.wikipedia.org/w/index.php?title=Model_organismhttp://en.wikipedia.org/w/index.php?title=Drosophila


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organisms is greater than in the example described here.

Maternal effect genes

Figure 1. mRNA distributions.

Figure 2. Protein distributions.

The building-blocks of anterior-posterior axis patterning inDrosophila

are laid out during egg formation (oogenesis), well before the egg is

fertilized and deposited. The maternal effect genes are responsible for

the polarity of the egg and of the embryo. The developing egg (oocyte)

is polarized by differentially localized mRNA molecules.

The genes that code for these mRNAs, called maternal effect genes,

encode for proteins that get translated upon fertilization to establish

concentration gradients that span the egg. Bicoid and hunchback are

the maternal effect genes that are most important for patterning of

anterior parts (head and thorax) of the Drosophila embryo.Nanos and

Caudal are maternal effect genes that are important in the formation of

more posterior abdominal segments of theDrosophila embryo.[][2]

In embryos from bicoid mutant mothers, the head and thoracic

structures are converted to the abdomen making the embryo with

posterior structures on both ends, a lethal phenotype.[]

Cytoskeletal elements such as microtubules are polarized within the

oocyte and can be used to allow the localization of mRNA molecules

to specific parts of the cell. Maternally synthesized bicoid mRNAs

attach to microtubules and are concentrated at the anterior ends of

formingDrosophila eggs. In unfertilized eggs, transcripts are still strictly localized at the tip, but immediately after

fertilization, a small mRNA gradient is formed in the anterior 20% of the eggs. nanos mRNA also attaches to a

Drosophila egg's cytoskeleton but is concentrated at the posterior end of the egg. hunchback and caudal mRNAs

lack special location control systems and are fairly evenly spread throughout the entire interior of the egg cells.

When the mRNAs from the maternal effect genes are translated into proteins, a Bicoid protein gradient forms at the

anterior end of the egg. Nanos protein forms a gradient at the posterior end. The Bicoid protein blocks translation of

caudal mRNA so Caudal protein is of lower concentration at the anterior part of the cell and at higher concentration

at the posterior part of the cell. This is of opposite direction of the Bicoid protein. The caudal protein then activates

later to turn genes on to form the posterior structures during the segmentation phase. Nanos protein creates a

posterior-to-anterior slope and is a morphogen that helps in abdomen formation. Nanos protein binds to the

hunchback mRNA and blocks its translation in the posterior end ofDrosophila embryos.

The Bicoid, Hunchback, and Caudal proteins are transcription factors. The Bicoid protein is a morphogen as well.The Nanos protein is a translational repressor protein. Bicoid has a DNA-binding homeodomain that binds both

DNA and the nanos mRNA. Bicoid binds a specific RNA sequence in the 3' untranslated region, called the Bicoid

3'-UTR regulatory element, of caudal mRNA and blocks translation.

Hunchback protein levels in the early embryo are significantly augmented by new hunchback gene transcription and

translation of the resulting zygotically produced mRNA. During early Drosophila embryogenesis there are nuclear

divisions without cell division. The many nuclei that are produced distribute themselves around the periphery of the

cell cytoplasm. Gene expression in these nuclei is regulated by the Bicoid, Hunchback, and Caudal proteins. For

example, Bicoid acts as a transcriptional activator of hunchback gene transcription. In order for development to

continue, Hunchback gene is needed in an area that is declining in amount from anterior to posterior. This is created

by the Nano protein whose existence is at a declining slope from posterior to anterior ends.
http://en.wikipedia.org/w/index.php?title=Zygotehttp://en.wikipedia.org/w/index.php?title=Bicoid_3%27-UTR_regulatory_elementhttp://en.wikipedia.org/w/index.php?title=Bicoid_3%27-UTR_regulatory_elementhttp://en.wikipedia.org/w/index.php?title=3%27_UTRhttp://en.wikipedia.org/w/index.php?title=Homeoboxhttp://en.wikipedia.org/w/index.php?title=Morphogenhttp://en.wikipedia.org/w/index.php?title=Transcription_factorhttp://en.wikipedia.org/w/index.php?title=Translationhttp://en.wikipedia.org/w/index.php?title=Microtubulehttp://en.wikipedia.org/w/index.php?title=Cytoskeletonhttp://en.wikipedia.org/w/index.php?title=Messenger_RNAhttp://en.wikipedia.org/w/index.php?title=Oocytehttp://en.wikipedia.org/w/index.php?title=Oogenesishttp://en.wikipedia.org/w/index.php?title=File%3ADrosophila_early_embryo_protein_gradients.pnghttp://en.wikipedia.org/w/index.php?title=File%3AMaternal_effect_mRNAs.png


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Figure 3. bicoid mRNA + protein gradient Figure 4. Nanos protein gradient

Figure 5. Gap genes.

The other important function of the gradients of Bicoid, Hunchback,

and Caudal proteins is in the transcriptional regulation of other

zygotically expressed proteins. Many of these are the protein products

derived from members of the "gap" family of developmental control

genes. giant, huckebein, hunchback, knirps,Krppel and tailless are all

gap genes. Their expression patterns in the early embryo are

determined by the maternal effect gene products and shown in the

diagrams on the right side of this page. The gap genes are part of a

larger family called the segmentation genes. These genes establish the

segmented body plan of the embryo along the anterior-posterior axis.

The segmentation genes specify 14 parasegments that are closely

related to the final anatomical segments. The gap genes are the first

layer of a hierarchical cascade of the segmentation control genes.

Additional segmentation genes

Figure 6. Pair rule.

Two additional classes of segmentation genes are expressed after the

gap gene products. The pair-rule genes are expressed in striped

patterns of seven bands perpendicular to the anterior-posterior axis (see

Figure 6, even-skipped). These patterns of expression are established

within the syncytial blastoderm. After these initial patterning events,

cell membranes form around the nuclei of the syncytial blastoderm

converting it to a cellular blastoderm.
http://en.wikipedia.org/w/index.php?title=File%3APair_rule.jpghttp://en.wikipedia.org/w/index.php?title=Gap_genehttp://en.wikipedia.org/w/index.php?title=File%3AGap_ene_expression.pnghttp://en.wikipedia.org/w/index.php?title=File%3ANanos_gradient.pnghttp://en.wikipedia.org/w/index.php?title=File%3ABicoid_gradient.png


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Figure 7. Reciprocal signaling between Wingless

and Hedgehog producing cells.

The expression patterns of the final class of segmentation genes, the

segment polarity genes, are then fine-tuned by interactions between

the cells of adjacent parasegments (see the example, engrailed, Figure

7). The Engrailed protein is a transcription factor (yellow in Figure 7)

that is expressed in one row of cells at the edge of each parasegment.

This expression pattern is initiated by the pair-rule genes (likeeven-skipped) that code for transcription factors that regulate the

engrailed gene's transcription in the syncytial blastoderm.

Cells that make Engrailed can make the cell-to-cell signaling protein

Hedgehog (green in Figure 7). The motion of Hedgehog is limited by

its lipid modification, and so Hedgehog activates a thin stripe of cells

anterior to the Engrailed-expressing cells. Only cells to one side of the

Engrailed-expressing cells are competent to respond to Hedgehog

because they express the receptor protein Patched (blue in Figure 7).

Cells with activated Patched receptor make the Wingless protein (red

in Figure 7). Wingless is a secreted protein that acts on the adjacent

rows of cells by activating its cell surface receptor, Frizzled.

Wingless acts on Engrailed-expressing cells to stabilize Engrailed

expression after the cellular blastoderm forms. The Naked cuticle protein is induced by Wingless to limit the

number of rows of cells that express Engrailed. The short-range, reciprocal signaling by Hedgehog and Wingless,

held in check by the Patched and Naked proteins, stabilizes the boundary between each segment. The Wingless

protein is called "wingless" because of the phenotype of some wingless mutants. Wingless and Hedgehog also

function in multiple tissues later in embryogenesis and also during metamorphosis.

The transcription factors that are coded for by segmentation genes regulate yet another family of developmental

control genes, the homeotic selector genes. These genes exist in two ordered groups onDrosophila chromosome 3.

The order of the genes on the chromosome reflects the order that they are expressed along the anterior-posterior axis

of the developing embryo. The Antennapedia group of homeotic selector genes includes labial, antennapedia, sex

combs reduced, deformed, andproboscipedia. Labial and Deformed proteins are expressed in head segments where

they activate the genes that define head features. Sex-combs-reduced and Antennapedia specify the properties of

thoracic segments. The bithorax group of homeotic selector genes control the specializations of the third thoracic

segment and the abdominal segments. Mutations in some homeotic genes can often be lethal and the cycle of life

will end at embryogenesis.

In 1995, the Nobel Prize for Physiology or Medicine was awarded for studies concerning the genetic control of early

embryonic development to Christiane Nsslein-Volhard, Edward B. Lewis and Eric Wieschaus. Their research on

genetic screening for embryo patterning mutants revealed the role played in early embryologic development by

Homeobox genes like bicoid. An example of a homeotic mutation is the so-called antennapedia mutation. In

Drosophila, antennae and legs are created by the same basic "program", they only differ in a single transcription

factor. If this transcription factor is damaged, the fly grows legs on its head instead of antennae. See images of this

"antennapedia" mutant and others, at FlyBase[3]

. Another example is in the bithorax complex. If nonlethal mutations

occur in this complex, it can cause the fly to have 2 sets of wings, instead of 1 pair of wings and 1 pair of halteres,

which aid in balance in flight.
http://flybase.bio.indiana.edu/http://en.wikipedia.org/w/index.php?title=Homeoboxhttp://en.wikipedia.org/w/index.php?title=Eric_F._Wieschaushttp://en.wikipedia.org/w/index.php?title=Edward_B._Lewishttp://en.wikipedia.org/w/index.php?title=Christiane_N%C3%BCsslein-Volhardhttp://en.wikipedia.org/w/index.php?title=Nobel_Prize_in_Physiology_or_Medicinehttp://en.wikipedia.org/w/index.php?title=Homeotic_selector_genehttp://en.wikipedia.org/w/index.php?title=Metamorphosis_%28biology%29http://en.wikipedia.org/w/index.php?title=Phenotypehttp://en.wikipedia.org/w/index.php?title=Hedgehog_%28cell_signaling%29http://en.wikipedia.org/w/index.php?title=Engrailed_%28gene%29http://en.wikipedia.org/w/index.php?title=File%3AWingless.png


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Dorsal-ventral Axis

Formation of the Dorsal-Ventral Axis is dependent on a maternally synthesized transcription factor known as dorsal

protein. The production of dorsal protein is stimulated by the localization of the embryonic nuclei. The nuclei

secretes a protein called Gurken. Gurken inhibits the production of PIPE protein by interacting with Torpedo

receptor on flanking oocyte follicle cells. PIPE positive cells are able to secrete dorsal protein and form the ventral

side of the egg, while PIPE negative cells do not secrete dorsal protein and form the dorsal side of the egg.

Dorsal induces the transcription of two genes twist& snail while repressingzerknllt& decapentaplegic.

Intra-membranous dorsal receptor proteins, known as Toll receptors are responsible for transporting dorsal protein

into the embryonic nuclei. These Toll receptors are the product of Toll gene, and are uniformly spaced across the

embryoinic plasma-membrane.

Since dorsal protein is secreted by PIPE positive-ventral follicular cells of an egg, dorsal protein enters the embryo to

the ventral side. Once transported into the nuclei, dorsal protein is most concentrated at the ventral side of the

embryo.

This process sets up a gradient differential between the ventral and dorsal side of an immature embryo, the

repression or induction of these four genes are differentially regulated. For example;

At the ventral end of the embryo, blastoderm nuclei exposed to high concentrations of dorsal protein induce the

transcription of twist and snail while repressingzerknllt& decapentaplegic.

In the middle of the embryo, blastoderm nuclei exposed to mild concentrations of dorsal protein don't express any

genes.

At the dorsal end of the embryo, blastoderm nuclei exposed to little or no dorsal protein express onlyzerknllt&

decapentaplegic.

References

[2][2] Rivera-Pomar,R., ad Jackle, H. 1996. From gradients to stripes in Drosophilia embryogenesis: Filling in the gaps. Trends Genet. 12: 478-483.

[3] http:/ /flybase.bio. indiana.edu

External links

Fly Move: http://flymove.uni-muenster.de

The Interactive Fly: http://www.sdbonline.org/fly/segment/bicoid1.htm

April, 2012 Cell commentary on recent advances that challenge scientific understanding of

concentration-dependent morphogenesis (http://www.sciencedirect.com/science/article/pii/

S0092867412004667)
http://www.sciencedirect.com/science/article/pii/S0092867412004667http://www.sciencedirect.com/science/article/pii/S0092867412004667http://www.sdbonline.org/fly/segment/bicoid1.htmhttp://flymove.uni-muenster.de/http://flybase.bio.indiana.edu/http://en.wikipedia.org/w/index.php?title=Toll_%28gene%29http://en.wikipedia.org/w/index.php?title=Decapentaplegichttp://en.wikipedia.org/w/index.php?title=Zerkn%C3%BCllthttp://en.wikipedia.org/w/index.php?title=Snail_%28gene%29http://en.wikipedia.org/w/index.php?title=Twist_%28gene%29http://en.wikipedia.org/w/index.php?title=Regional_specificationhttp://en.wikipedia.org/w/index.php?title=Regional_specification


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Article Sources and Contributors 6

Article Sources and ContributorsDrosophila embryogenesis Source: http://en.wikipedia.org/w/index.php?oldid=539085953 Contributors: AdamRetchless, AnnaFrance, BenKovitz, Brodyt66, Celefin, Dr d12, Flyguy649,

Geneticsjgarcia, Gongoozler123, Havardnh, IceCreamAntisocial, Ilyas1978, JWSchmidt, Jacopo Werther, Jebus989, Josh Parris, Kazkaskazkasako, Kupirijo, Kwhartonjr, Lexor, Magioladitis,

Mild Bill Hiccup, MitraE, Naraht, PDH, PhilKnight, Ppgardne, Rentaferret, Retama, Samsara, Skytrooper21, Smartse, SpectraValor, Stan Shebs, Stemonitis, Sundustsj, Szquirrel, TheLimbicOne,

TheObtuseAngleOfDoom, TimVickers, TwoOneTwo, Vanished user 19794758563875, Xatnoc, 45 anonymous edits

Image Sources, Licenses and ContributorsFile:Maternal effect mRNAs.png Source: http://en.wikipedia.org/w/index.php?title=File:Maternal_effect_mRNAs.pngLicense: Public Domain Contributors: Bestiasonica, Lauranrg

File:Drosophila early embryo protein gradients.png Source: http://en.wikipedia.org/w/index.php?title=File:Drosophila_early_embryo_protein_gradients.pngLicense: Public Domain

Contributors: Bestiasonica, Lauranrg

File:Bicoid gradient.png Source: http://en.wikipedia.org/w/index.php?title=File:Bicoid_gradient.png License: Creative Commons Sharealike 1.0 Contributors: Cayte, Deerstop, Jacopo

Werther

File:Nanos gradient.png Source: http://en.wikipedia.org/w/index.php?title=File:Nanos_gradient.png License: Public Domain Contributors: Cayte, Deerstop, EugeneZelenko, Nard the Bard

File:Gap ene expression.png Source: http://en.wikipedia.org/w/index.php?title=File:Gap_ene_expression.pngLicense: Public Domain Contributors: Jacopo Werther, Lauranrg

File:Pair rule.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Pair_rule.jpg License: GNU Free Documentation License Contributors: Original uploader was JWSchmidt at

en.wikipedia

File:Wingless.png Source: http://en.wikipedia.org/w/index.php?title=File:Wingless.png License: GNU Free Documentation License Contributors: Original uploader was JWSchmidt at

en.wikipedia. Later version(s) were uploaded by DragonflySixtyseven at en.wikipedia.

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Drosophila Embryogenesis