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Drosophila, known as fruit fly is widely used as a model organism in genetic studies for studying mutations, inheritance patterns etc. Drosophila melanogaster is a holometabolous insect. An adult fly has three basic body parts; head, thorax and abdomen. Thorax consists of three segments with legs, winds and halters. Abdomen has 11 segments. Large number of mutations in the fruit fly influences all aspects of their development and there mutations have been subjected to molecular analysis to find how the genes control early development in Drosophila.

When a Drosophila egg has been fertilized, its diploid nucleus immediately divides nine times without division of the cytoplasm creating a single multinucleating cell. After 8th division, nuclei get scattered in the cytoplasm followed by formation of 4 polar nuclei in 10th division. In the 13th division, nuclei divides and numerous nuclei are found in the periphery. This is called syncytial blastoderm. Each of these nuclei proved to have their own cytoplasmic environment rich in microtubules. Plasma membrane invaginates and each nuclei are surrounded by a membrane, which is called as cellular blastoderms. This has about 6000 cells. Polar cells give rise to germ cells and embryo undergoes further development. Three important genes are involved in the development of Drosophila; maternal effector genes/ egg polarity genes, segmentation genes, homeotic genes.

Egg polarity genes function in axis specification. These genes act by setting up a concentration gradient of morphogens in the developing embryo. A morphogen is a protein whose concentration gradient affects the developmental fate of the surrounding region. The egg polarity genes are transcribed into mRNA in the course of egg formation in the maternal parent and this maternal mRNA are incorporated in the cytoplasm of the egg. Proteins encoded by these mRNA play an important role in axis determination. These proteins are examples of maternal inheritance as the offspring will have similar phenotype.

Like in all other insects, fruit fly has a segmented body. When axis specification has taken place, segmentation genes control the differentiation of the embryo into individual segments. About 25 genes are included in segmentation genes. They are zygotic genes whose expression is controlled by bicoid and nanos protein gradients. Gap genes in segmentation genes are involved in defining large segments of embryo. Pair rule genes define regional sections of the embryo. Segment polarity genes are involved in organization of segments. Mutations in these genes lead to absence of certain segments.

Gap genes, which are regulated by maternal genes, are involved in dividing the embryo into broad regions each containing parasegment primodia. Hunchback proteins are expressed at the anterior end. Transcription of anterior gap genes are initiated by the different concentrations of hunchback and bicoid proteins. Higher concentration of hunchback proteins results in expression of giant proteins and prevents transcription of posterior gap genes in the anterior part whereas lower hunchback concentrations result in expression of kruppel proteins. Giant gene has two methods of activation; one for anterior expression band and one for posterior expression band. After this, gap genes become stabilized and maintained by interactions between different gap gene products. Protein products of gap genes interact with neighbouring gap gene proteins to activate transcription of pair rule genes. These proteins divide the embryo into areas that are precursors of segmented body plan. Expression of these genes results in zebra stripe pattern along the anterior posterior axis, dividing the embryo into 15 subunits. Eight pair rule genes are known which include hairy, even skipped, runt etc. Mutations in these genes results in deletion of particular stripe. Pair rule proteins activate the segment polarity genes.

Segment polarity genes are responsible for organization of the segments. Mutations in segment polarity genes lead to deletion of part of the segment and replaced by a mirror image of the adjacent segment. Gene products of segment polarity genes play an important role in cell to cell signaling. They encode proteins that are involved in signal transduction pathways.

Homeotic genes are involved in determining the identity of individual segments. Homeotic gene products activate genes that encode segment specific characters. Mutations lead to specific body parts to appear in wrong segments. Homeotic genes create addresses for the cells of particular segments indicating the cells where they are within the region defined by segmentation genes.
Drosophila is one of the widely used experimental model due to its similarities with humans. There is a significant similarity in the genome of fruit fly Drosophila with respect to various diseases in human. This makes it ideal for use as model for study of various diseases and in other research. Almost 52% of sequence of protein of fly has mammalian homologs. The well known diseases in which Drosophila is used as genetic model are neurodegenerative disorders like Huntington’s, Parkinson’s, Spinocerebellar ataxia and even Alzheimer’s disease. The effect of many antioxidants with respect to aging and oxidative stress is also studied in this fruit fly. Other diseases in which it is used as genetic model are diabetes, cancer and other immunological diseases.
The scientific reasons for which it is selected as genetic model are:-
1) It has a short reproductive cycle that is generation time. It is only 10 days so result of experiments are concluded soon in research work.
2) It requires little equipments and space thus save cost.
3) It has high Fecundity. Fecundity means females lay up to 100 eggs in one day and almost 2000 in her lifetime. This increase scope in understanding wide range of controls at a given time.
4) The facilitation of genetic studies in this fruit fly is due to absence of meiotic recombination in males.
These are the unique characteristics of drosophila which make it genetic model!
Few more details on Drosophila development and sex determination

Drosophila melanogaster is well studied and often used model organism in various experiments. Drosophila life cycle and development is temperature dependent - it’s ectothermic species. At temperature of ~29° C, drosophila will live for 30 days. Transition from egg to adult can last from 7 to 50 days, depending on the outer temperature. At 25° C (ideal temperature), adult stage will be reached in 8.5 days. At 30° C, heat stress will prolong development to 11 days. At 19° C development will last 18, or even 50 days if temperature drop to 12° C.
Drosophila is holometabolous insect (undergoes complete metamorphosis). Female is lying 400 eggs, that will hatch after 12-15 hours. During 4 days of larval stage, two molting will happen. In the following stage, encapsulated in the puparium, drosophila will finish metamorphosis and emerge as fully developed adult after 4 days.

8-12 h after emerging, females are ready for courtship and sexual intercourse. Mating ritual is divided in couple of stages. First, male will position himself in front of the female and serenade (sound is produced by wing vibration) to attract female attention. In the next phase, he will turn to the abdominal part of the female and excite her sexually by taping and licking her genitalia. At the end, he will attempt copulation by curling his abdomen. Copulation will last 15-20 minutes and seminal fluid (containing very long sperm cells) will be transported to the female’s body. Females are mating with more males, and sperm cells are competing for fertilization. Tubular receptacles and 2 spermathecae are used for sperm storing. Incapacitation (one sperm is incapacitated by other) and displacement of the sperm (done by female) will finally “decide” which sample will be the most successful in fertilization. It’s believed that ~80% of offspring is result of the last mating.

Drosophila gender determination is interesting as well. Genome consists of four chromosomes: three autosomal and X/Y pair. Y chromosome is carrying genes that will encode sperm production but it doesn’t determine the male sex like in humans. Ratio of X to autosomal chromosomes will determine the gender. More importantly, each cell in the body will “decide” whether to be male or female depending on the mentioned ratio. Normal female will develop each time X to autosomal chromosome ratio is 1, despite the number of chromosome pairs present (XXXX:AAAA, XXX:AAA or XX:AA). Normal male will develop when X to autosomal ratio is 0.5 (X:AA). When 3 X chromosome pairs are present (XXX:AA), 1.5 ratio will result in metafemale will development. This genetic combination is associated with impaired development and inability of fly to emerge from the pupae. If autosomal chromosomes are dominating (X:AAA), 0.33 ratio will lead to metamale development. Drosophila with this genetic combination is weak and sterile. Finally, when ratio of X to autosomes is 0.66 (XX:AAA), intersex will be formed. Organism with both male and female characteristics is known as gynandromorph.

All this combinations are associated with three genes present on autosomal and X chromosome that will determine the gender. Deadpan is autosomal gene and it will inhibit expression of the Sex-lethal gene. Sisterless is located on the X and it will inhibit activity of the Deadpan. When autosomal chromosomes are present in higher ratio (over X chromosome), Deadpan will inhibit Sex-lethal, and male will develop. When same amount of X and autosomal chromosomes are present, Sisterless will inhibit the action of Deadpan and female will develop. Different forms of Sex-lethal gene are present in males and females. This gene is controlling further stages of the gender development by splicing its own mRNA and producing truncated (males) or fully developed (females) protein that are affecting expression of Doublesex gene, which is responsible for yolk production in females.

Drosophila genome is sequenced back in 2000. Out of 15,016 genes, 60% are associated with gene expression control. Mating is not the only thing where Drosophila is showing high similarity with humans. Genes for 75% of known human diseases are matching Drosophila genes. Besides genetic experiments, Drosophila is excellent model organism for the study of cancer, diabetes, oxidative stress or even drug abuse. Very interesting and more than useful creature, for sure.
Cancer causes many issues to health, with that man can lost their life early. In this diseases many types are exist, i think all are to be dangerous. But many clinics and doctors are having solution for that.