Development in drosophila ppt

DIFFERENT STAGES OF DROSOPHILA DEVELOPEMENT

1. Pt. R avishankar S hukla University S .o.s in Biotechnology Topic – Development in drosophila Guided by :- Dr. Jay S hankar P aul Submitted by :- P. S ujata Msc I sem

2. Content Drosophila introduction Drosophila life cycle Drosophila development Fertilization Cleavage G astrulation Egg polarity genes Dorsal - ventral axis Anterior – posterior axis Segmentation genes Homeotic genes Mutation in drosophila Drosophila in genetic analysis

3. Drosophila is a species of a fruit fly in the family drosophilidae . The species is known generally as the common fruit fly or vinegar fly. A wild drosophila or fruit fly has multi-faced brick red eyes, a thorax with arched black bristles, a striped abdomen and a pair of translucent wings. The thorax consists of three segments. The fly’s rapid life cycle lo chromosome number , small genome size, and giant salivary glands have many experimental advantages. Drosophila melanogoster

4. Drosophila Life cycle When a Drosophila egg has been fertilized, its diploid nucleus immediately divides nine times without division of the cytoplasm, creating a single, multinucleate cell These nuclei are scattered throughout the cytoplasm but later migrate toward the periphery of the embryo and divide several more times . Next, the cell membrane grows inward and around each nucleus, creating a layer of approximately 6000 cells at the outer surface of the embryo Nuclei at one end of the embryo develop into pole cells, which eventually give rise to germ cells. The early embryo then undergoes further development in three distinct stages: the anterior–posterior axis and the dorsal–ventral axis of the embryo are established the number and orientation of the body segments are determined the identity of each individual segment is established Different sets of genes control each of these three stages.

5. Drosophila development Drosophila develop a holometabolous method of development. They have three distinct stages of their life cycle with different body plans: larva , pupa and adult (imago). Drosophila development is orderly sequence of change which is controlled by different genes .

6. Drosophila development - overview Fertilization Cleavage Gastrulation Drosophila body plan Egg polarity genes Doral – ventral axis Anterior posterior axis Segmentation genes Homeotic gens

7. Fertilization

8. Cleavage

9. Drosophila body plan Egg polarity genes The egg-polarity genes play a crucial role in establishing the two main axes of development in fruit flies . You can think of these axes as the longitude and latitude of development: any location in the Drosophila embryo can be defined in relation to these two axes. The egg-polarity genes are transcribed into mRNAs in the course of egg formation in the maternal parent, and these mRNAs become incorporated into the cytoplasm of the egg . After fertilization, the mRNAs are translated into proteins that play an important role in determining the anterior–posterior and dorsal–ventral axes of the embryo. Because the mRNAs of the polarity genes are produced by the female parent and influence the phenotype of the offspring, the traits encoded by them are examples of genetic maternal effects

10. There are two sets of egg-polarity genes: one set determines the anterior–posterior axis, and the other determines the dorsal–ventral axis. These genes work by setting up concentration gradients of morphogens within the developing embryo . A morphogen is a protein that varies in concentration and elicits different developmental responses at different concentrations . Egg-polarity genes function by producing proteins that become asymmetrically distributed in the cytoplasm, giving the egg polarity, or direction. This asymmetrical distribution may take place in a couple of ways. An mRNA may be localized to particular regions of the egg cell , leading to an abundance of the protein in those regions when the mRNA is translated . Alternatively, the mRNA may be randomly distributed, but the protein that it encodes may become asymmetrically distributed by a transport system that delivers it to particular regions of the cell, by regulation of its translation, or by its removal from particular regions by selective degradation

11. Determination of dorsal-ventral axis The dorsal–ventral axis defines the back (dorsum) and belly ( ventrum ) of a fly (see At least 12 different genes determine this axis, one of the most important being a gene called dorsal. The dorsal gene is transcribed and translated in the maternal ovary, and the resulting mRNA and protein are transferred to the egg during oogenesis . In a newly laid egg, mRNA and protein encoded by the dorsal gene are uniformly distributed throughout the cytoplasm but, after the nuclei have migrated to the periphery of the embryo . Dorsal protein becomes redistributed. Along one side of the embryo, Dorsal protein remains in the cytoplasm; this side will become the dorsal surface. Along the other side, Dorsal protein is taken up into the nuclei; this side will become the ventral surface. At this point, there is a smooth gradient of increasing nuclear Dorsal concentration from the dorsal to the ventral side . The nuclear uptake of Dorsal protein is thought to be governed by a protein called Cactus, which binds to Dorsal protein and traps it in the cytoplasm. The presence of yet another protein, called Toll, leads to the phosphylation of Cactus, causing it to be degraded. When Cactus is degraded, Dorsal is released and can move into the nucleus.

12. Cactus and Toll regulate the nuclear distribution of Dorsal protein, which in turn determines the dorsal–ventral axis of the embryo. Inside the nucleus, Dorsal protein acts as a transcription factor, binding to regulatory sites on the DNA and activating or repressing the expression of other genes High nuclear concentration of Dorsal protein (as in cells on the ventral side of the embryo) activates a gene called twist, which causes ventral tissues to develop . Low nuclear concentrations of Dorsal protein (as in cells on the dorsal side of the embryo), activate a gene called decapentaplegic , which specifies dorsal structures . In this way, the ventral and dorsal sides of the embryo are determined

13. Determination of anterior- posterior axis One of the most important early developmental events is the determination of the anterior (head) and posterior (butt) ends of an animal . We will consider several key genes that establish this anterior–posterior axis of the Drosophila embryo An important gene in this regard is bicoid , which is first transcribed in the ovary of an adult female during oogenesis . The bicoid mRNA becomes incorporated into the cytoplasm of the egg; as it passes into the egg, bicoid mRNA becomes anchored to the anterior end of the egg by part of its 3′ end. This anchoring causes bicoid mRNA to become concentrated at the anterior end number of other genes that are active in the ovary are required for proper localization of bicoid mRNA in the egg.) When the egg has been laid, bicoid mRNA is translated into Bicoid protein . Because most of the mRNA is at the anterior end of the egg, Bicoid protein is synthesized there and forms a concentration gradient along the anterior–posterior axis of the embryo, with a high concentration at the anterior end and a low concen tration at the posterior end . This gradient is maintained by the continuous synthesis of Bicoid protein and its short half-life . The high concentration of Bicoid protein at the anterior end induces the development of anterior structures such as the head of the fruit fly. Bicoid —like Dorsal—is a morphogen . It stimulates the development of anterior structures by binding to regulatory sequences in the DNA and influencing the expression of other genes. One of the most important of the genes stimulated by Bicoid protein is hunchback, which is required for the development of the head and thoracic structures of the fruit fly.

14. The development of the anterior–posterior axis is also greatly influenced by a gene called nanos , an egg-polarity gene that acts at the posterior end of the axis. The nanos gene is transcribed in the adult female, and the resulting mRNA becomes localized at the posterior end of the egg . After fertilization, nanos mRNA is translated into Nanos protein, which diffuses slowly toward the anterior end. The Nanos protein gradient is opposite that of the Bicoid protein: Nanos is most concentrated at the posterior end of the embryo and is least concentrated at the anterior end. Nanos protein inhibits the formation of anterior structures by repressing the translation of hunchback mRNA. The synthesis of the Hunchback protein is therefore stimulated at the anterior end of the embryo by Bicoid protein and is repressed at the posterior end by Nanos protein. This combined stimulation and repression results in a Hunchback protein concentration gradient along the anterior–posterior axis that, in turn, affects the expression of other genes and helps determine the anterior and posterior structures.

15. Like all insects, the fruit fly has a segmented body plan. When the basic dorsal–ventral and anterior–posterior axes of the fruit-fly embryo have been established, segmentation genes control the differentiation of the embryo into individual segments. These genes affect the number and organization of the segments, and mutations in them usually disrupt whole sets of segments. The approximately 25 segmentation genes in Drosophila are transcribed after fertilization; so they don’t exhibit a genetic maternal effect, and their expression is regulated by the Bicoid and Nanos protein gradients. The segmentation genes fall into three groups. The three groups of genes act sequentially, affecting progressively smaller regions of the embryo . First, the products of the egg-polarity genes activate or repress gap genes, which divide the embryo into broad regions. The gap genes, in turn, regulate pair-rule genes, which affect the development of pairs of segments. Finally, the pair-rule genes influence segment-polarity genes, which guide the development of individual segments. Gap genes define large sections of the embryo; mutations in these genes eliminate whole groups of adjacent segments . Mutations in the Krüppel gene, for example, cause the absence of several adjacent segments. Pair-rule genes define regional sections of the embryo and affect alternate segments. Mutations in the even-skipped gene cause the deletion of even-numbered segments, whereas mutations in the fushi tarazu gene cause the absence of odd-numbered segments. Segment-polarity genes affect the organization of segments . Mutations in these genes cause part of each segment to be deleted and replaced by a mirror image of part or all of an adjacent segment. For example, mutations in the gooseberry gene cause the the posterior half of each segment to be replaced by the anterior half of an adjacent segment. Segmentation gene

16. Homeotic genes After the segmentation genes have established the number and orientation of the segments, homeotic genes become active and determine the identity of individual segments. Eyes normally arise only on the head segment, whereas legs develop only on the thoracic segments. The products of homeotic genes activate other genes that encode these segment-specific characteristics. Mutations in the homeotic genes cause body parts to appear in the wrong segments. In the late 1940s, Edward Lewis began to study homeotic mutations in Drosophila—mutations that cause bizarre rearrangements of body parts. Mutations in the Antennapedia gene, for example, cause legs to develop on the head of a fly in place of the antenna Homeotic genes create addresses for the cells of particular segments, telling the cells where they are within the regions defined by the segmentation genes. When a homeotic gene is mutated, the address is wrong and cells in the segment develop as though they were somewhere else in the embryo. Homeotic genes in Drosophila are expressed after fertilization and are activated by specific concentrations of the proteins produced by the gap, pair-rule, and segmentpolarity genes. The homeotic gene Ultrabithorax ( Ubx ), for example, is activated when the concentration of Hunchback protein (a product of a gap gene) is within certain values. These concentrations exist only in the middle region of the embryo; so Ubx is expressed only in these segments.

17. The homeotic genes in animals encode regulatory proteins that bind to DNA; each gene contains a subset of nucleotides, called a homeobox , that are similar in all homeotic genes. The homeobox encodes 60 amino acids that serve as a DNA-binding domain; this domain is related to the helix-turn-helix motif Homeoboxes are also present in segmentation genes and other genes that play a role in spatial development. There are two major clusters of homeotic genes in Drosophila. One cluster, the Antennapedia complex, affects the development of the adult fly’s head and anterior thoracic segments. The other cluster consists of the bithorax complex and includes genes that influence the adult fly’s posterior thoracic and abdominal segments . Together, the bithorax and Antennapedia genes are termed the homeotic complex (HOM-C). In Drosophila, the bithorax complex comprises three genes, and the Antennapedia complex has five; all are located on the same chromosome In addition to these eight genes, HOM-C contains many sequences that regulate the homeotic genes. Remarkably, the order of the genes in the HOM-C is the same as the order in which the genes are expressed along the anterior–posterior axis of the body . The genes that are expressed in the more-anterior segments are found at one end of the complex, whereas those expressed in the more-posterior end of the embryo are found at the other end of the complex.

18. Genetic mutations in drosophila Drosophila  genes are traditionally named after the  phenotype  they cause when mutated. For example, the absence of a particular gene in  Drosophila  will result in a mutant embryo that does not develop a heart. [ This system of nomenclature results in a wider range of gene names than in other organisms. bw : brown - The brown eye mutation results from pteridine (red) pigments inability to be produced or synthesized, due to a point mutation on chromosome II .  When the mutation is homozygous, the pteridine pigments are unable to be synthesized because in the beginning of the pteridine pathway, a defective enzyme is being coded by homozygous recessive genes . In all, mutations in the pteridine pathway produces a darker eye color, hence the resulting color of the biochemical defect in the pteridine pathway being brown.

19. vg : vestigial - A spontaneous mutation, discovered in 1919 by Thomas Morgan and Calvin Bridges. Vestigial wings are those not fully developed and that have lost function. Since the discovery of the vestigial gene in  Drosophila melanogaster The vestigial gene is considered to be one of the most important genes for wing formation, but when it becomes over expressed the issue of ectopic wings begin to form .   The vestigial gene acts to regulate the expression of the wing imaginal discs in the embryo and acts with other genes to regulate the development of the wings. A mutated vestigial allele removes an essential sequence of the DNA required for correct development of the wings .

20. Drosophila in genetic analysis Useful characters of drosophila in genetic analysis 61% human diseases have recognizable correspondence in genetic code of fruit fly. 50% of protein sequences have analogs with analogs. The life cycle is relatively short and each fly can produce number of progeny. Balancer chromosomes help preserve linkage.

21. Reference Genetics A Conceptual Approach FOURTH EDITION Benjamin A. Pierce w.w.w wikipedia.com www.slideshare.net

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