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U of M BIOLOGY 4361 - The Genetics of Axis Specification in Drosophila

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Page 1Page 2Page 3Page 4Page 5Page 6Page 7Page 8Page 9Page 10Page 11Page 12Page 13Page 14Page 151Biology 4361Developmental BiologyThe Genetics of Axis Specification in DrosophilaNovember 2, 2006EARLY DROSOPHILA DEVELOPMENTFertilization 1) Drosophila egg activation occurs at ovulation - eggs are ovulated a few minutes before fertilization- oocyte nucleus has resumed meiotic division; cytoplasm begins translation from storedmRNAs 2) eggs begin to specify axes by the time of fertilization 3) sperm can enter only at the micropyle (a narrow tunnel in the chorion)- micropyle probably prevents polyspermy by allowing entry of only one sperm at a time- NOTE - no cortical granules in Drosophila eggs; although cortical changes seen 4) sperm competition; sperm can be many times as long as the adult fly- e.g. Drosophila melanogaster sperm tail is 1.8 mm; ~ as long as adult; 300X humansperm- entire sperm gets incorporated into eggCleavage Syncytial blastoderm Most insect eggs undergo superficial cleavage; large mass of centrally located yolk confinescleavage to the cytoplasmic rim of the egg- zygote nucleus undergoes eight divisions (256 nuclei) averaging 8 min each- nuclei then migrate to periphery of the egg- mitosis continues (but at a slower rate) - during the ninth cycle, ~5 nuclei reach the posterior pole- these nuclei become enclosed by membranes- generate the pole cells- pole cells give rise to the primordial germ cells, then gametes - most other nuclei arrive at the periphery of the embryo at cycle 10- undergo four more divisions - cytoplasm is non-uniform throughout the cell- each nucleus within syncytial blastoderm is contained within its own territory ofcytoskeletal proteins- each surrounded by microtubules and microfilaments- nuclei and surrounding cytoplasm = energids Cellular Blastoderm- following division 13, oocyte plasma membrane folds inward between the nuclei2- eventually partitions off each somatic nucleus into a single cell- actin-membrane complex begins to constrict at what will become the basal end of the cell- cellular blastoderm consists of ~6000 cells- formed within 4 hours of fertilization Mid-blastula transition- nuclear transcription starts at about cycle 11- speed of division decreases- eventually becomes asynchronousGastrulation - begins shortly after MBT - first movements segregate the presumptive mesoderm, endoderm, and ectoderm- prospective mesoderm: ~1000 cells constitute the ventral midline of the embryo foldinward to produce the ventral furrow- ventral furrow eventually pinches off from the surface to become a ventral tube- prospective endoderm invaginates to form two pockets at the anterior and posterior endof ventral furrow- pole cells are internalized along with the endoderm - embryo bends to form the cephalic furrow - ectodermal cells on the surface and the mesoderm undergo convergence and extension- migrate toward the ventral midline to form the germ band- germ band: collection of cells along the ventral midline- includes all cells that will eventually form the trunk of the embryo- germ band extends posteriorly- wraps around the dorsal surface of the embryo- at the end of germ band formation, the cells destined to from the most posteriorlarval structures are located immediately behind the future head region- body segments begin to appear: dividing ectoderm and mesoderm- germ band retracts; places presumptive posterior segments at the posterior tip of theembryo - key morphogenic events (occurring while germ band is in its extended position)- organogenesis- segmentation- segregation of imaginal discs- nervous system forms from two regions of ventral ectoderm- neuroblasts differentiate from neurogenic ectoderm within each segment- also form in non-segmented area of head ectoderm- NOTE - in insects (and other arthropods) nervous system located ventrally3 - Drosophila body plan: groups of repeated segments- head- thorax- formed from three segments- first thoracic segment (T1) - legs only- T2 - legs and wings- T3 - legs and halteres (balancing organs; modified wings)- abdomen (tail region)GENES THAT PATTERN THE DROSOPHILA BODY PLANOverview: - dorsal- ventral and anterior-posterior axes established by interactions between the developingoocyte and its surrounding follicle cells - dorsal-ventral patterning gradients formed within the embryo- gradients specify different tissue types - segment formation along anterior-posterior axis- segments become specialized - specification of tissues depends on position along primary axesPrimary Axis Formation during OogenesisAnterior-posterior polarity in the oocyteOocytes are derived from the oogonium - single cell surrounded by follicle cells - oogonium divides (with incomplete cytokinesis: cells connected by cytoplasmic bridges) into- 1 oocyte precursor and 15 nurse cells- oocyte positioned at posterior of the follicle - nurse cells synthesize gurken gene (homologue of vertebrate EGF)- gurken message transported to oocyte nucleus- localized between nucleus and cell membrane (at posterior end)- translated into Gurken protein - Gurkin signal received by follicle cells- Torpedo protein (homolog of vertebrate EGF receptor)- NOTE - Gurken diffuses a very short distance, so only follicle cells in close proximity tonucleus will be affected - Gurkin signal results in “posteriorization” of follicle cells- follicle cells differentiate into dorsal follicle cells- send signal (protein kinase A) back into the oocyte- PKA activity reorganizes the cytoskeleton (microtubules)- microtubules oriented specifically with their minus (cap) ends anterior and plus(growing) ends posterior4 - at the same time, nurse cells transport cytoplasm to oocyte; increase oocyte size at theexpense of nurse cells - nurse cell cytoplasm contains messages- messages transported via microtubules to distinct oocyte locations; e.g. - bicoid - anterior- bicoid message binds to dynein (actually binds to Exuperantia, anotherprotein that binds to dynein); associated with non-growing end ofmicrotubules- dynein moves bicoid message to anterior end of egg; stabilizes there- nanos - posterior- oskar message complexes with kinesin I; moves towards the growing endof microtubules- kinesin I moves oskar to posterior end of oocyteOskar protein binds nanos message; retains it in the posterior regionDorsal-ventral patterning in the oocyte - as oocyte volume increases,


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U of M BIOLOGY 4361 - The Genetics of Axis Specification in Drosophila

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