[Ishani7]

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.