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DROSOPHILA DROSOPHILA DEVELOPMENDEVELOPMEN
TTArun Kumar PradhanArun Kumar Pradhan
ONAONA
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DEVELOPMENTDEVELOPMENT An organism arises from a fertilized egg as the An organism arises from a fertilized egg as the
result of three related processesresult of three related processes• Cell divisionCell division• Cell differentiationCell differentiation• MorphogenesisMorphogenesis
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DIFFERENTIATIONDIFFERENTIATION Cells may initially remain undifferentiatedCells may initially remain undifferentiated
• Embryonic stem cellsEmbryonic stem cells Cells ultimately Cells ultimately
differentiatedifferentiate• Become specialized in Become specialized in
structure and functionstructure and function
Odisha NET Academy ONA 09337727724Odisha NET Academy ONA 09337727724
DIFFERENTIATIONDIFFERENTIATION Virtually all cells within a multicellular Virtually all cells within a multicellular
organism are genetically identicalorganism are genetically identical Differences between cells are due to differences Differences between cells are due to differences
in gene expressionin gene expression• Different subsets of genes are “on” and “off”Different subsets of genes are “on” and “off”• Different cell types make different proteinsDifferent cell types make different proteins
Odisha NET Academy ONA 09337727724Odisha NET Academy ONA 09337727724
GENE EXPRESSIONGENE EXPRESSION Much of the regulation of gene expression Much of the regulation of gene expression
occurs at the level of transcriptionoccurs at the level of transcription Transcriptional regulation Transcriptional regulation
of gene expression is of gene expression is directed bydirected by• Maternal molecules in the Maternal molecules in the
cell’s cytoplasmcell’s cytoplasm• Signals from other cellsSignals from other cells
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PATTERN FORMATIONPATTERN FORMATION The development of a spatial organization in The development of a spatial organization in
which an organism’s tissues and organs are all in which an organism’s tissues and organs are all in their characteristic placestheir characteristic places
In animals, it begins in early embryoIn animals, it begins in early embryo• Basic body plan is establishedBasic body plan is established• Major axes are established earlyMajor axes are established early
Studied most extensively in Studied most extensively in Drosophila Drosophila melanogastermelanogaster
Odisha NET Academy ONA 09337727724Odisha NET Academy ONA 09337727724
Drosophila melanogasterDrosophila melanogaster Bilaterally symmetric segmented bodyBilaterally symmetric segmented body
• HeadHead• ThoraxThorax• AbdomenAbdomen
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Drosophila melanogasterDrosophila melanogaster Cytoplasmic determinants are present in the Cytoplasmic determinants are present in the
unfertilized eggunfertilized egg• Provide positional information for placement of axes Provide positional information for placement of axes
prior to fertilizationprior to fertilization• Establishes number and orientation of segmentsEstablishes number and orientation of segments• Ultimately trigger formation of specific structures Ultimately trigger formation of specific structures
within each segmentwithin each segment
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Drosophila melanogasterDrosophila melanogaster Egg develops in mother’s ovaryEgg develops in mother’s ovary
• Surrounding cells with nutrients, etc.Surrounding cells with nutrients, etc. Mitosis begins following fertilizationMitosis begins following fertilization
• First ten divisions include no growth, cytokinesisFirst ten divisions include no growth, cytokinesis• Single multinucleate cell resultsSingle multinucleate cell results
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Drosophila melanogasterDrosophila melanogaster Nuclei migrate to periphery of embryo at tenth Nuclei migrate to periphery of embryo at tenth
divisiondivision Plasma membranes finally partition ~6,000 Plasma membranes finally partition ~6,000
nuclei into separate cells at the thirteenth nuclei into separate cells at the thirteenth divisiondivision• Basic body plan already Basic body plan already
determined at this timedetermined at this time Body axes and segment Body axes and segment
boundariesboundaries
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Drosophila melanogasterDrosophila melanogaster Subsequent embryonic events create clearly Subsequent embryonic events create clearly
visible segmentsvisible segments• Initially look very similarInitially look very similar
Some cells move to new Some cells move to new positionspositions• Organs formOrgans form
Wormlike larva hatchesWormlike larva hatches• Eats, grows, & moltsEats, grows, & molts
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Drosophila melanogasterDrosophila melanogaster Larva eventually forms a Larva eventually forms a
pupapupa• Enclosed in a caseEnclosed in a case
Metamorphosis occursMetamorphosis occurs• Change from larva to adult flyChange from larva to adult fly
Adult fly emerges from caseAdult fly emerges from case• Each segment is anatomically Each segment is anatomically
distinctdistinct
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Drosophila melanogasterDrosophila melanogaster Each segment in the adult fly is anatomically Each segment in the adult fly is anatomically
distinctdistinct• Characteristic appendagesCharacteristic appendages
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Drosophila melanogasterDrosophila melanogaster Gradients of maternal molecules in the early Gradients of maternal molecules in the early
embryo control axis formationembryo control axis formation• Cytoplasmic determinants already present in Cytoplasmic determinants already present in
unfertilized eggunfertilized egg• Encoded by mother’s Encoded by mother’s maternal effect genesmaternal effect genes
a.k.a., “a.k.a., “Egg-polarity genesEgg-polarity genes”” Encode proteins or mRNAs that are placed into the egg Encode proteins or mRNAs that are placed into the egg
while still in the mother’s ovarywhile still in the mother’s ovary
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Drosophila melanogasterDrosophila melanogaster One group of maternal effect genes establishes One group of maternal effect genes establishes
the anterior-posterior axis of the embryothe anterior-posterior axis of the embryo Another set of maternal effect genes establishes Another set of maternal effect genes establishes
the dorsal-ventral axisthe dorsal-ventral axis
Female flies possessing mutations in Female flies possessing mutations in maternal maternal effect geneseffect genes appear phenotypically normal, but appear phenotypically normal, but produce offspring with mutant phenotypesproduce offspring with mutant phenotypes
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Drosophila melanogasterDrosophila melanogaster BicoidBicoid is an egg-polarity gene is an egg-polarity gene
• ““Two-tailed”Two-tailed” Mothers defective in Mothers defective in bicoidbicoid produce embryos produce embryos
lacking the front half of their bodylacking the front half of their body• Duplicate posterior structure at both endsDuplicate posterior structure at both ends
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Drosophila melanogasterDrosophila melanogaster BicoidBicoid gene product is concentrated at anterior gene product is concentrated at anterior
end of fly embryoend of fly embryo• Gradient of gene productGradient of gene product• Essential for setting Essential for setting
up anterior end of flyup anterior end of fly
Gradients of other proteins Gradients of other proteins determine the posterior end determine the posterior end and the dorsal-ventral axisand the dorsal-ventral axis
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The bicoid gradient regulates the expression of The bicoid gradient regulates the expression of segmentation genes in a concentration-segmentation genes in a concentration-
dependent fashion.dependent fashion.
There are peak levels of theThere are peak levels of the
Bicoid protein in anteriorBicoid protein in anterior
regions, intermediate levels inregions, intermediate levels in
certral regions, and low levelscertral regions, and low levels
in posterior regionsin posterior regions..
OrthodenticleOrthodenticle is activated only is activated only
by high levels of the Bicoidby high levels of the Bicoid
gradient in the head;gradient in the head;
hunchbackhunchback is activated by both is activated by both
high and intermediate levels inhigh and intermediate levels in
the head and thorax.the head and thorax.Odisha NET Academy ONA 09337727724Odisha NET Academy ONA 09337727724
In central In central regions of the regions of the embryo, the embryo, the
orthodenticle orthodenticle gene is off. In gene is off. In
contrast, contrast, hunchback hunchback is is on. These is on. These is
control by the control by the levels of Bicoid levels of Bicoid
protein.protein.Odisha NET Academy ONA 09337727724Odisha NET Academy ONA 09337727724
Hunchback expression is also Hunchback expression is also regulated at the level of regulated at the level of
translationtranslationThe translation is blocked in posterior regions by The translation is blocked in posterior regions by
RNA-binding protein called RNA-binding protein called NanosNanos. . After the After the NanosNanos mRNA is translated, the protein diffuses from mRNA is translated, the protein diffuses from posterior regions to form a gradient. The translation of posterior regions to form a gradient. The translation of the maternal the maternal hunchbackhunchback mRNA is arrested by the mRNA is arrested by the Nanos protein. The Nanos gradient thereby leads to the Nanos protein. The Nanos gradient thereby leads to the formation of a reciprocal Hunchback gradient in formation of a reciprocal Hunchback gradient in anterior regions.anterior regions.
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Hunchback and gapHunchback and gap proteins produce segmentation stripes of proteins produce segmentation stripes of
gene expression.gene expression.
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Drosophila melanogasterDrosophila melanogaster The The bicoidbicoid protein and the products of other protein and the products of other
egg-polarity genes are egg-polarity genes are transcription factorstranscription factors• Regulate the expression Regulate the expression
of some of the embryo’s of some of the embryo’s genesgenes
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Drosophila melanogasterDrosophila melanogaster Segmentation genesSegmentation genes
• Genes of embryoGenes of embryo• Expression regulated by products of egg-polarity Expression regulated by products of egg-polarity
genesgenes• Direct the actual formation of segments after the Direct the actual formation of segments after the
embryo’s major axes are definedembryo’s major axes are defined
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Drosophila melanogasterDrosophila melanogaster Three sets of segmentation genes are activated Three sets of segmentation genes are activated
sequentiallysequentially• Gap genesGap genes• Pair-rule genesPair-rule genes• Segment polarity genesSegment polarity genes
The activation of these sets of genes defines the The activation of these sets of genes defines the animal’s body plananimal’s body plan• Each sequential set regulates increasingly fine detailsEach sequential set regulates increasingly fine details
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Drosophila melanogasterDrosophila melanogaster Gap genesGap genes
• Map out basic subdivisions along the embryo’s Map out basic subdivisions along the embryo’s anterior-posterior axisanterior-posterior axis
• Mutations cause “gaps” in the animal’s segmentationMutations cause “gaps” in the animal’s segmentation
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Drosophila melanogasterDrosophila melanogaster Pair-rule genesPair-rule genes
• Define pattern in terms of pairs of segmentsDefine pattern in terms of pairs of segments• Mutations result in embryos having half the normal Mutations result in embryos having half the normal
number of segmentsnumber of segments
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Drosophila melanogasterDrosophila melanogaster Segment polarity genesSegment polarity genes
• Set the anterior-posterior Set the anterior-posterior axis of each segmentaxis of each segment
• Mutations produce Mutations produce segments where part of the segments where part of the segment mirrors another segment mirrors another part of the same segmentpart of the same segment
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Drosophila melanogasterDrosophila melanogaster The products of many of the segmentation genes The products of many of the segmentation genes
are transcription factorsare transcription factors• Directly activate the next set of genesDirectly activate the next set of genes
SummarySummary• Products of the egg-polarity genes regulate the Products of the egg-polarity genes regulate the
regional expression of the gap genesregional expression of the gap genes• Gap genes control the localized expression of the Gap genes control the localized expression of the
pair-rule genespair-rule genes• Pair rule genes activate specific segment polarity Pair rule genes activate specific segment polarity
genes in different parts of each segmentgenes in different parts of each segment• Segment polarity genes activate homeotic genesSegment polarity genes activate homeotic genes
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HOMEOTIC GENESHOMEOTIC GENES Master regulatory genesMaster regulatory genes Specify the types of appendages and other Specify the types of appendages and other
structures that each segment will formstructures that each segment will form Mutations produce flies with structures in Mutations produce flies with structures in
incorrect placesincorrect places
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HOMEOTIC GENESHOMEOTIC GENES Encode transcription factorsEncode transcription factors Control the expression of genes responsible for Control the expression of genes responsible for
specific anatomical structuresspecific anatomical structures• e.g., “Antennae go here”e.g., “Antennae go here”• e.g., “Legs go here”e.g., “Legs go here”
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HOMEOTIC GENESHOMEOTIC GENES Homeotic genes are Homeotic genes are
master genes that master genes that regulate the regulate the expression of expression of numerous other genesnumerous other genes• Some of the regulated Some of the regulated
genes are regulatory genes are regulatory themselvesthemselves
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DrosophilaDrosophila DEVELOPMENT DEVELOPMENTHierarchy of Gene ActivityHierarchy of Gene Activity Maternal genesMaternal genes Segmentation genes of embryoSegmentation genes of embryo
• Gap genesGap genes• Pair-rule genesPair-rule genes• Segment polarity genesSegment polarity genes
Homeotic genes of the embryoHomeotic genes of the embryo Other genes of the embryoOther genes of the embryo
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HOMEOTIC GENESHOMEOTIC GENES Homeotic genes of Homeotic genes of DrosophilaDrosophila all possess all possess
homologous segmentshomologous segments• 180-nucleotide sequence = 180-nucleotide sequence = homeoboxhomeobox• Encodes 60-amino-acid Encodes 60-amino-acid homeodomainhomeodomain
Homologous sequences have been found in Homologous sequences have been found in many other animalsmany other animals• e.g., Insects, nematodes, mollusks, fish, frogs, birds, e.g., Insects, nematodes, mollusks, fish, frogs, birds,
humans, etc.humans, etc.• Related genes are even found in yeast, etc.Related genes are even found in yeast, etc.• HoxHox genes genes
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HOMEOTIC GENESHOMEOTIC GENES Vertebrate genes Vertebrate genes
homologous to the homologous to the homeotic genes of homeotic genes of DrosophilaDrosophila have have maintained their maintained their chromosomal chromosomal arrangementarrangement
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Thank you
Odisha NET Academy ONA 09337727724Odisha NET Academy ONA 09337727724