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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 NussleinVolhard • 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 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 Parasegments as defined by denticle pattern 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 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 Gap Genes • Example of mutation – Krüppel – Expressed in segments 4-6 – Mutant for Krüppel is missing these segments and those immediately adjacent to them 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 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 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 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 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! • Within these regions, different combinations of gene expression specify both the type of segment that will form and the proper order of segments. 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 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