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Transcript
9.35 Homeotic gene expression in Drosophila After the parasegmental boundaries are set, the pair-rule and gap genes interact to regulate the homeotic selector genes, which determine the identity of each segment. Antennapedia complex: labial (lab), Deformed (dfd)- head Antennapedia (Antp), sex comb reduced (scr)- thoracic segments proboscipedia (pb)- appears to act only in adults, but in its absence, the labial palps of the mouth are transformed into legs. 9.35 Homeotic gene expression in Drosophila Bithorax complex: Ultrabithorax (Ubx)required for the identity of the 3rd thoracic segment. abdominal A (abdA) and abdominal B (abdB)responsible for the segmental identities of the abdominal segments. Homeotic complex (Hom-C) 9.36 A four-winged fruit fly constructed by putting together three mutations in cis regulators of the Ultrabithorax gene Wild type: 3 thoracic segments. 2nd segment produces a pair of wings and the 3rd segment produces a pair of wings and a pair of balancers known as halteres. When the Ultrabithorax gene is deleted, the 3rd thoracic segment is transformed into another 2nd thoracic segment. 9.37 (A) Head of a wild-type fruit fly. (B) Head of a fly containing the Antennapedia mutation that converts antennae into legs When Antennapedia gene is expressed in the head, legs rather than antennae grow out of the head sockets. Initiating the patterns of homeotic gene expression The initial domains of homeotic gene expression are influenced by the gap genes and pair rule genes. - Expression of abdA and abdB genes is repressed by the gap gene proteins Hunchback and Kruppel. - The Antennapedia gene is activated by particular levels of Hunchback. Gap genes and pair-rule genes are transient. Polycomb proteins- repression of homeotic genes Trithorax proteins- maintain the chromatin active conformation 9.38 Antibody staining of the Ultrabithorax protein in (A) the wing disc and (B) the haltere disc of third instar Drosophila larvae The homeotic genes work by activating or repressing a group of “realisator genes”, which are the targets of homeotic gene proteins and which function to form the specified tissue or organ primordia. Ultrabithorax protein is able to repress the expression of wingless in those cells that will become the halteres of the fly.
