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Genetics of Axis Specification in Drosophila: Segmentation Genes

Genetics of Axis Specification in Drosophila: Segmentation Genes. Gilbert - Chapter 9. Today’s Goals. Describe the use of a Genetic screen to identify functionality of individual gene products Identify Eric Weischaus and Christianne Nusslein-Volhard

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Genetics of Axis Specification in Drosophila: Segmentation Genes

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  1. Genetics of Axis Specification in Drosophila: Segmentation Genes Gilbert - Chapter 9

  2. Today’s Goals • Describe the use of a Genetic screen to identify functionality of individual gene products • Identify Eric Weischaus and Christianne Nusslein-Volhard • Identify key molecular players in establishing polarity and segments in various types of embryos • Define the term “positional information” • Describe how evolutionarily conserved genes play similar roles in development of a wide variety of species • Define transcription factor

  3. The Segmentation Genes • Now we have a mechanism for setting up the Anterior and Posterior ends of the embryo • BUT- there are many specific segments along the axis of the embryo • How can segmentation be achieved? • We must also examine the formation of parasegments • Parasegments make up the segments of the embryo - embryonic building blocks

  4. Parasegments as defined by denticle pattern

  5. Segmentation Genes • Three classes of Segmentation genes • Hierarchical arrangement based on what they control • Now - all are zygotic genes (not maternal-effect) • Gap genes • Divide embryo into broad regions (several parasegments) • Pair-rule genes • Divide regions defined by gap genes into smaller regions (fewer parasegment) • Segment Polarity genes • Control patterns within each parasegment

  6. Gap Genes • Ex. Hunchback, Krüppel, knirps • Mutations affect broad areas of the body plan • These genes are activated or repressed by the maternal effect genes (ex. Bicoid) • Expressed in broad domains along A-P axis • Expression patterns correspond very well with segments that are lost when mutations occur

  7. Gap Genes • Example of mutation • Krüppel • Expressed in segments 4-6 • Mutant for Krüppel is missing these segments and those immediately adjacent to them

  8. Gap Genes • Expression in the embryo changes over the course of development • Begins as low levels across whole embryo • Becomes consolidated into discrete regions within the embryo • Proteins become localized to specific areas along axis • Initially controlled by concentration gradients of maternal effect genes • Expression patterns eventually defined by interactions with one another

  9. Maternal Effect Genes Influence Gap Genes • High levels of Bicoid and Hunchback • Induce expression of Giant near the anterior end • Inhibit expression of knirps • Slightly lower levels of Hunchback • Krüppel expression induced • More complex - several pathways that lead to activation of genes in multiple places

  10. SO… • The net result of all of these interactions • Precise expression patterns of overlapping mRNAs in specific sub-regions of the embryo • Overlap is only 8 cells or so • GAP genes set up sub-regions of the embryo - not individual segments, but smaller sections than just Anterior-Posterior

  11. Pair-rule Genes • Begin to define segments • Ex. Fushi tarazu (ftz),hairy, even-skipped (eve) • Mutation in pair-rule gene deletes every other parasegment • Ex. Ftz • At least eight pair-rule genes act to divide the embryo into a series of stripes that overlap

  12. Pair-rule genes • Expressed in stripes along the embryo that represent the future segmentation of the body • Gene is transcribed in a vertical stripe of cells, not transcribed in next vertical stripe - continues to alternate along axis • Divide the embryo into 15 subunits • PHENOMENALLY specific control of expression of these genes!

  13. Within these regions, different combinations of gene expression specify both the type of segment that will form and the proper order of segments.

  14. Pair-rule Gene Regulation • How is the transcription of these genes regulated to appear in alternating stripes of cells? • Appears to be under control of Gap genes • In places where the edges of bands of gap gene expression overlap (ex. Hunchback with Kruppel) expression of a pair rule gene is activated • In places where other bands of gap genes overlap, a pair rule gene gets turned off

  15. Pair-rule genes: Summary • Tight control of expression by enhancer/repressor interactions in the promoter regions results in 7 stripes of each gene • This divides the embryo into the 14 parasegments along the axis • Each gene is expressed in different parasegments • Each parasegment has a different combo of pair-rule genes

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