Elstob, Philip Ronald; (2002) Hox gene function and cell identity in Drosophila. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Hox/Homeotic genes pattern the anteroposterior axis of animal embryos. However, the mechanisms by which these conserved transcription factors generate morphological diversity remain largely unknown. Here I describe both a molecular and a cellular study of Hox gene function. In the molecular approach, a model Hox target enhancer, the late neural enhancer (LNE) of the mouse Hoxb4 gene, was dissected in Drosophila. Individual analysis of two essential HOX binding sites (HS1 and HS2) revealed that each site has a different Hox specificity and modulates responses to more than one Hox input. Activation of the LNE requires the group 4-6 Hox genes and is largely dependent on the Hox cofactor Extradenticle. Furthermore, three conserved regions, remote to HS1/2, were found to influence LNE activity along the anteroposterior and dorsoventral axes. In summary, the LNE contains at least five regulatory modules required for correct enhancer expression. In a cellular approach, I have investigated how the Hox gene abdominal A micromanages segment identity in Drosophila by studying its role in specifying a single cell identity: the larval oenocyte. An initial study of this cell type revealed three stages of morphogenesis: 1) induction, 2) anterior movement out of the posterior compartment and 3) ventral migration. Induction occurs in response to EGFR signalling from primary chordotonal sensory organ precursors. Ectodermal cells are primed to become oenocytes by virtue of a genetic prepattern, one component of which is spalt. SPALT also suppresses EGFR-mediated induction of an alternative cell fate, the chordotonal organ. Given that both abdominal A and EGFR ligand can induce ectopic oenocytes, I propose that abdominal A might specify oenocytes non-cell autonomously, through regulating local EGFR ligand production and thus oenocyte induction.

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