Science Fair Project Encyclopedia
Drosophila has long been a favorite model system for geneticists and developmental biologists studying embroygenesis. The small size, short generation time, and large brood size makes it ideal for genetic studies. Transparent embryos facilitate developmental studies. Drosophila melanogaster was introduced into the field of genetic experiments by Thomas Hunt Morgan in 1909.
Drosophila are species of molting insects, meaning that they have two distinct stages of their life cycle with radically different body plans: larva and adults. During embryogenesis, the larva develops and then hatches from the egg. Cells that will produce adult structures are put aside in imaginal disks. During the pupal stage, the larval body breaks down as the imaginal disks grow and produce the adult body. This process is called complete metamorphosis.
The mother fly produces oocytes that already have anterior-posterior and dorsal-ventral axes defined by maternal activities.
Embroygenesis in Drosophila is unique among model organisms in that cleavage occurs in a syncytium. About 5,000 nuclei accumulate in the unseparated cytoplasm of the oocyte before they migrate to the surface and are encompassed by plasma membranes to form cells surrounding the yolk sac. Early on, the germ line segregates from the somatic cells through the formation of pole cells at the posterior end of the embryo.
Like other metazoa, gastrulation leads to the formation of three germ layers; the endoderm, mesoderm, and ectoderm. The mesoderm invaginates from the ventral furrow, as do the ectoderm that will give rise to the midgut. The pole cells are internalized by a different route.
Germ band elongation involves many rearrangements of cells, and the appearance of distinct differences in the cells of the three germ bands and various regions of the embryo. The posterior region (including the hindgut) expands and extends towards the anterior pole along the dorsal side of the embryo. The earliest signs of segmentation appear during this phase with the formation of parasegmental furrows. This is also when the tracheal pits form, the first signs of structures for breathing.
Germ band retraction returns the hindgut to the dorsal side of the posterior pole and coincides with overt segmentation. The remaining stages involve the internalization of the nervous system (ectoderm) and the formation of internal organs (mainly mesoderm).
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