The anterior -posterior specification and positioning of the cardiac progenitor cells in <italic>Drosophila</italic>.

The Drosophila heart is a highly ordered structure with only a limited number of cell types, which are arranged in a stereotyped and precise fashion. How these cell types are specified and positioned at discrete segmental locations along the anterior/posterior axis within the cardiac mesoderm has not been resolved. Here I show that the striped pattern of the secreted factor Hedgehog (Hh), in combination with the RAS pathway, specifies and positions neighboring groups of cardiac progenitors within each segment: the anterior ladybird- (lbe ) and the posterior even skipped-expressing ( eve) cardiac progenitors. Ectodermal Hh controls dorsal mesodermal Ras signaling by transcriptional regulation of the EGF receptor ligand protease, encoded by rhomboid (rho). In the absence of hh (but in the maintained presence of wg), the lack of mesodermal Eve progenitors is partially rescued by mesodermal overexpression of ras. hh, via inhibition of the Ci repressor, also functions independently of ras in repressing Lbe expression in cardiac progenitors located just anterior to those expressing Eve. Thus, the Eve precursors next to the Hh stripe are distinguished from more distant Lbe precursors by locally augmenting Ras signaling via elevating rho transcripts. The eve and lbe pattern in the cardiac mesoderm is further elaborated by the mutual repressive interaction, which results in their mutually exclusive expression domains. While Lbe acts directly on the eve mesodermal enhancer (eme) to restrict its expression anteriorly, additional repressive activities are required to converge on the enhancer to generate precisely targeted gene (eve) expression. Here, I study the combinatorial transcriptional repressive mechanisms leading to precisely localized (eve) gene expression in the cardiac mesoderm. I identified two additional repressor motifs: the AT rich M1 and GC rich M2 motifs in eme. I have also identified three potential negative regulators of eve: the hemeodomain proteins Lim3, Msh and C15, which are sufficient for repression of mesodermal eve expression or wildtype but not the mutated eme. Lim3 seems to require only an AT-rich M1 site, but not the GC-rich M2a site, to confer efficient repression. In contrast, mutated M1b or M2a site renders eme less sensitive to repression by lbe overexpression similar to the lbe (Lb2) site, suggesting that multiple sites can confer repression requiring Lbe. Thus, I propose that these repressor sites and associated proteins form a complex or are in some other way dependent on each other to delineate the spatial pattern of eve expression within the cardiac mesoderm.PhDBiological SciencesGeneticsMolecular biologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126496/2/3253343.pd