Feb 28, 2005; Biology 2005 Albert Harris
Drosophila (fly) embryologyTwo Nobel prizes were given for embrological discoveries:1) To Spemann for the discovery of induction (in salamader gastrulation). Awarded in 1935. 2) To Nusslein-Volhard, Wieschaus, and E. B. Lewis, for discovering genes that control early development in Drosophila. Awarded in 1995. Fly embryos have two special properties, very different from vertebrates: A) Their axes of asymmetry are already decided, before fertilization, by molecules from the surrounding cells.
B) For the first twelve mitotic cells, the nuclei do not become separated into different cells. The embryos remain syncytial! Until then, even proteins are free to diffuse from one end of the embryo to the other; or from the top to the bottom.
Regarding #A) There are some species of wasps whose eggs inside caterpillars, which develop into multicellular masses that bud off variable numbers of wasp embryos. This phenomenon is called polyembryony.
Regarding #B) From books I have read, some species of insects do not remain syncytial like this during the early mitoses; but many other arthropods do have syncytial embryos (crayfish). Nusslein-Volhard and Wieschaus did a "genetic screen", seeking mutations that caused embryos to become structurally abnormal and/or stop development at early stages. Later, they did "in situ" labeling of normal embryos with "probe" nucleic acids whose base sequences were complementary to the normal versions of the genes they had discovered in their screen for early extreme birth defects. (Nature vol 287: 795-801) Experiments were done introducing extra copies of genes, injecting proteins, etc.
Among other genes, they discovered one "Bicoid" Another gene's proteins vary in behavior along the dorso-ventral axis "Dorsal"
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