Lectures 17 & 18 Notes - Vandan Desai BIOL...

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Vandan Desai BIOL 442—Developmental Biology 1 Lecture #17 & #18—Anterior-Posterior Pattern Formation in the Drosophila Embryo I & II I. Reminder About Drosophila Development :: A. Anterior-posterior patterning of the Drosophila embryo and adult ( LEFT ) = i. Anterior region = very different ; gives rise to head ii. Posterior region segmented : 3 thoracic & 8 abdominal ; 3 thoracic => legs , legs + wings , and legs + vestigial wing B. The Drosophila life cycle ( MIDDLE ) = i. Male fertilizes female egg => embryo => three stages of instar larva => pupae ii. Most larval structures undergo apoptosis in the pupae iii. Tissues within pupae that were already present in the instar larva (imaginal discs) grow and give rise to structures in adult => wings, antenna, etc. C. Cleavage in Insects ( RIGHT ) = i. Insects undergo superficial cleavage—the large mass of centrally located yolk confines cleavage to the rim of the egg Embryo at this stage of development (from stage 1 to stage 13) is therefore called syncytial blastoderm Only have one cell ; nuclei divide within that one cell Then they move to periphery where they’ll undergo cellularization (at cycle 13) ii. Although the nuclei divide within a common cytoplasm , this doesn’t mean that the cytoplasm is itself uniform (there are gradients of proteins ) In these early embryos , specification along the A-P and D-V axes is accomplished by the interactions of cytoplasmic materials within the single, multinucleated cell (via microtubules) After cellularization , specification occurs via paracrine signals II. Overview of A-P Axis Specification in Drosophila :: A. Maternal genes = expressed in the mother’s ovary ; produce mRNAs that are placed in specific regions of the egg (divide the embryo into broad regions) i. Bicoid and Hunchback —regulate the production of anterior structures ii. Nanos and Caudal —regulate the formation of posterior parts B. Gap genes = regulated by maternal genes and become expressed in broad , partially overlapping regions ( mutations cause gaps in the segmentation pattern) C. Differing concentrations of Gap proteins regulate the transcription of pair-rule genes, which divide the embryo into periodic units = seven vertical bands perpendicular to the A-P axis D. Pair-rule genes activate the transcription of segment polarity genes, whose RNA and protein divide the embryo into 14 segments
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Vandan Desai BIOL 442—Developmental Biology 2 E. Combined protein products of gap , pair-rule , and segment polarity genes regulate homeotic selector genes belonging to the Hox family III. Maternal Genes (Morphogens) :: A. Discovery of Morphogens— i. Classical expts. demonstrating bipolar gradient (of maternal mRNAs ) = Classical experiments done in the late 70s showed that there are at least two “organizing centers” in the insect egg: (1) one in the anterior of the egg, and (2) one in the posterior Ligation of the egg (separation of the anterior and posterior half) = Half developing into an anterior embryo and the other half into a posterior embryo Neither half contained middle segments
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