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Download Segment polarity genes Defining A/P axis within a segment Figure
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Figure 5.2 Segment polarity genes Defining A/P axis within a segment Hierarchy of gene expression compartmentalizes embryo along A/P axis Figure 5.25 Pair-rule genes (fushi tarazu and even-skipped) define parasegments Figure 5.24: engrailed is expressed in the anterior region of each parasegment •Lineage restriction •Expressed through life of fly Engrailed expression is controlled by the action of these genes 1 In absence of ftz, engrailed is expressed only in odd stripes Expression of pair-rule genes defines expression of segment polarity genes Wingless repressed by eve and ftz Engrailed activated by eve and ftz Figure 5.29 •feedback between engrailed and wingless maintains and reinforces initial pattern •Induction of engrailed and hedgehog by wingless Figure 5.28: expression of segment polarity genes in parasegments (pre-gastrulation) and segments •Induction of wingless by hedgehog --> sharp boundaries 2 Figure 5.29 If wingless is mutated, no hedgehog or engrailed are expressed No parasegment boundaries (See Figure 5-28) Patched can suppress wg expression, but if Hh binds to smo, that suppression is inhibited Gradients of hedgehog and wingless define regions within the segments 3 The parasegments are examples of “compartments” regions of the organism that are lineage restricted Assessing compartments by somatic clones Box 5B •Mitosis generates clones from the different daughter cells •Heterozygous individual Box5B •X-rays induce mitotic recombination Figure 5.26: Clones mutant for minute divide more rapidly than normal cells engrailed defines A/P compartments throughout development Establishing A/P axis Defining regions more precisely Defining parasegments and their orientation Defining segment identity 4 Figure 5.2 Selector genes determine segment identity Combination of gap gene and pairrule gene expression patterns define the selector gene expression pattern Box 4A: 3’--> 5’ organization reflects anterior---> posterior expression Figure 5.35: selector genes are arranged in two complexes 5 Figure 5.36 Mutations in selector genes result in homeotic transformations - one body part is transformed into another The dominant Antennapedia mutation causes legs to develop in place of antenna (gain-of-function - gene is expressed ectopically in the head as well as the thorax recessive mutation - (loss-offunction) gene is not expressed in thorax, antenna are formed in place of legs Initial expression of the gap gene Krüppel predicts later expression of selector gene Antennapedia Box 4A: 3’--> 5’ organization reflects anterior---> posterior expression 6 Figure 5.37 Expression of the 3 genes in the bithorax complex defines parasegment identity If delete all 3 genes of the bithorax compex, all segments transformed to segment 4 Ubx gene has a number of regulatory regions that control expression in a particular region If add Ubx back, get 4, 5 and 6… •Ubx specifies parasegments 5 and 6 Figure 5.37 7 If Ab-A is also restored, identity of parasegments 7 and 8 is restored (all remaining like parasegment 9) Figure 5.37 Expression of selector genes defines parasegment identity If Ab-A and Ab-B are restored, identity of all parasegments except 5 and 6 are restored Expression of anterior homeotic genes is repressed by posterior homeotic genes Chromatin remodeling maintains selector gene expression in absence of gap genes “posterior dominance” Polycomb - maintains repression Trithorax - maintains activation 8 Establishing A/P axis Defining regions more precisely Defining parasegments and their orientation Defining segment identity 9