Functional and molecular charecterization of targets of ultrabithorax in drosophila

Abstract

The Homeotic/Hox genes are highly conserved throughout metazoan evolution and code for homeodomain containing transcription factors that control specific developmental pathways. The Hox gene, Ultrabithorax (Ubx) is expressed in third thoracic segment of Drosophila and regulates many wing patterning genes to form haltere. Loss of Ubx from developing haltere and ectopic expression of Ubx in wing causes haltere-to-wing, wing-to-haltere transformations, respectively. Genome wide and transcriptome studies identified many genes/pathways involved in wing development as targets of Ubx in haltere. Ubx regulates cellular processes such cell division, cell proliferation, cell differentiation, cell size, and cell affinity in developmental stage specific manner to provide haltere a distinct morphology. As compared to wing, haltere is highly reduced in size, they lack wing type marginal bristles, vein/inter-vein, have reduced cell size, different cell shape and densely arranged trichomes. However the functionality of genes and pathways required by Ubx to provide identity to haltere are not completely known. Functional relevance of only few Pathways like Wingless (Wg), Decapentaplegic (Dpp), and epidermal growth factor receptor (EGFR) has been studied previously. Here, we have addressed how differences in cell size and shape influence organ size and shape. We have explained the role of two major organ size determining pathways in haltere i.e. Insulin/Insulin like signaling (IIS) pathway and Hippo pathway. Akt a central component of IIS pathway is down-regulated in haltere. Hippo pathway component Yorkie (Yki), a transcriptional co-activator protein is differentially regulated in wing and haltere. Also, regulation of upstream component like Expanded (Ex) and down-stream component like bantam microRNA is critical in haltere. Interestingly, while both overexpression of Yki and knock-down of ex caused significantly increased haltere size, degree of increase were much higher when ex expression is down regulated. This, suggests that de-regulation of both Yki-dependent and Yki 112 independent components of Hippo pathway is critical for liberating Ubx mediated regulation of haltere identity. While over-expression of Akt of IIS pathway or Vein of EGFR pathway did not affect haltere growth, when overexpressed in the background of down regulation of ex caused dramatic increase in haltere growth and differentiation of cell shape and organization to wing type. We conclude that regulation of Hippo pathway by Ubx is central to the modification of wing identity to that of haltere.