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LIFE CYCLE: Drosophila is a fruit fly seen on bananas, and its life cycle happens really quickly. This helps scientist to easily study them, it is highly preferred, and it helps answer the question of the link between genetics and developmental biology. Drosophila get to their adult stage by nine days. An organism that has larval and pupal stage without a nymph stage before being an adult is called holometabolous, and drosophila is one of them. Drosophila mates in the ten weeks of their life span, and the sperm is stored in the female's body in a seminal receptacle for internal fertilization. Even before an egg is fertilized in a female's body, females still lay eggs from 50 to 70 in a day. The ovum of drosophila had layers in the outer and inner. The outer layer is tough, and it is called the chorion, while the inner layer is a thin envelope. There are two small filaments at the anterior end of the egg called respiratory filaments that helps in gas exchange. The egg needs these filaments because the females usually lay the eggs almost half-way into a rotten fruit. Within 22-24 hours the eggs hatch at room temperature.

The larva then comes out as a little worm, and this first larva appearance is called the first instar larva. The food the instar larva feeds on is the substrate that the eggs were laid in. After about an additional 25 hours the larva, molts into a big worm which is the second instar larva. After enough feeding, it molts into the third instar larva which happens after 24 hours. The third instar larva is the largest of the three. As it feeds it also climbs out of the substrate to be clean and dry awaiting pupation. After about 30 hours the third instar larva molts into the pupa. At the early stages, the pupa is very immobile and yellowish-white in color. It darkens as it continues to develop. The pupal stage is where the development into an adult fly happens. The adult fly is also known as the imago. As it is metamorphosing, some of the larval organs are lysed but some are also kept. An example of an organ that is not lysed is the nervous system. Some and maybe most of the adult organs form fresh from two cells. These cells are what is referred to as the imaginal disc and the histoblasts. These cells lay dormant through the instar stages. Imaginal discs, which are formed from epithelial cells, are what will later form wings, legs, eyes, etc. The histoblasts form the abdominal epidermis and internal organs. It takes about 3-4 days for the pupal stage and then the imago breaks out of the pupal case known as the eclosion. When they emerge, the female eggs are not ripe for reproduction until two days after eclosion for the cycle to begin again even though the male is very sexually active.

GASTRULATION: Gastrulation is after cleavage and its main contribution to embryonic development is by infolding of a band of cells in the midventral region. Before gastrulation happens, the drosophila embryo is divided into seven parts. There is the anterior midgut, posterior midgut, neural and epidermal ectoderm, hindgut ectoderm, stomodeal ectoderm, extraembryonic amnioserosa, mesoderm. The initial step in the process of gastrulation is the formation of a ventral furrow, where two cell groups invaginate to form the endoderm and mesoderm. This happens in conjunction with the infolding of the mesoderm. Mesoderm invaginates and the germ band and pole cells move forward. The furrow that is formed as the cells move forward has an anterior and posterior end, and at these two positions there is an invagination of the endoderm which results in the formation of the anterior and posterior midgut respectively, the ventral endoderm invaginates and creates the anterior midgut, while the dorsal endoderm create the posterior midgut. The anterior stomodeum, also known as the foregut, and posterior proctodeum, also known as hindgut, are later formed by the invagination of the ectoderm and as the endoderm thickens. The stomodeum and the hindgut invaginate and get deeper. The anterior and posterior midgut start to extend and get longer. The cells around the ventral midline then change shape to form the ventral furrow. After everything is formed, the germ band and cells start to move back down, and this creates the posterior gut or airduct opening. Anterior and posterior midget joins together, and segmentation becomes obvious. Germ band moves to the head region and make head disappear to the interior. The process is very quick, and it turns epithelium cells that are identical to invaginated tube cells. Another form of infolding process called the cephalic furrow also happens in addition to that of the ventral infolding discussed above. This lateral infolding indicates the exact position of the future head.

The consequence of gastrulation is the creation of three germ layers that extends around the ventral circumference of the egg. This multilayered germ band formed stretches along the dorsal side of the egg and with this the embryo performs, for the lack of a better term, acrobatic maneuvers to touch its head to the posterior end. Following this the germ band bounces back to its position. As this happens, major parts of the embryo appear. These include the mandibular, maxillary, labial, thoracic and abdominal regions. One important thing to note during gastrulation is the fact that at some point the head that forms disappears causing the thoracic regions to overgrow the head region. Only a little region of the head will appear.

CITATIONS
 * Leptin, M. (1999). Gastrulation in Drosophila: the logic and the cellular mechanisms. The EMBO Journal, 18(12), 3187-3192. doi:10.1093/emboj/18.12.3187


 * Kam, Z., Minden, J. S., Agard, D. A., Sedat, J. W., & Leptin, M. (1991). Drosophila gastrulation: analysis of cell shape changes in living embryos by three-dimensional fluorescence microscopy. Retrieved from, http://dev.biologists.org/content/develop/112/2/365.full.pdf


 * Development of the Fruit Fly Drosophila melanogaster. Bainbridge and Bownes, (1981). Retrieved from, http://web.as.uky.edu/Biology/faculty/cooper/Population%20dynamics%20examples%20with%20fruit%20flies/08Drosophila.pdf


 * Ratnaparkhi, A., & Zinn, K. (2007). The secreted cell signal Folded Gastrulation regulates glial morphogenesis and axon guidance in Drosophila. Developmental Biology, 308(1), 158-168. doi:10.1016/j.ydbio.2007.05.016