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2/15/2008 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 2.44: selector genes are arranged in two complexes 1 2/15/2008 Box4A Drosophila Vs. Mouse Figure 2.45 Mutations in selector genes result in homeotic transformations - one body part is transformed into another Vs. Caenorhabditis elegans Figure 5-12 Figure 2.45 Bithorax mutant - anterior compartment of haltere transformed into anterior wing compartment Postbithorax transformation of posterior compartment The dominant Antennapedia mutation causes legs to develop in place of antenna ((gain-of-function i ff i - gene is i expressed ectopically in the head as well as the thorax recessive mutation - (loss-offunction)) ggene is not expressed in thorax, antenna are formed in place of legs Double mutant = four wings 2 2/15/2008 Initial expression of the gap gene Krüppel predicts later expression of selector gene Antennapedia Box 4A: 3’--> 5’ organization reflects anterior---> posterior expression Figure 2.46 Expression of the 3 genes in the bithorax complex d fi defines parasegment identity Ubx gene has a number of regulatory regions that control expression in a particular region 3 2/15/2008 If delete all 3 genes of the bithorax compex, all segments g transformed to segment 4 If add Ubx back, get 4, 5 and 6… •Ubx specifies parasegments 5 and 6 Figure 2.46 If Ab-A is also restored, identity of parasegments 7 and 8 is restored (all remaining like parasegment 9) Figure 2.46 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 4 2/15/2008 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 Establishing A/P axis Defining regions more precisely Defining parasegments and their orientation Defining segment identity 5