Mesodermal cell migration underlies several morphogenetic and cell specification processes in the Drosophila embryo. Two examples are hematopoiesis and renal tubule formation. Prohemocytes are derived from the head (procephalic) mesoderm and are first identifiable during embryonic stage 5 by the expression of Serpent (Srp), a GATA transcription factor required for hematopoietic development (Rehorn et al. 1996; reviewed by Evans et al. 2003). Prohemocytes will eventually differentiate into either plasmatocytes, which migrate out of the head region to populate the embryo, or crystal cells that generally remain localized near their point of origin in the embryo. For many cell types, including blood cells, receptor tyrosine kinases are essential for proper navigation (reviewed by Starz-Gaiano and Montell 2004). The Drosophila homolog of the human platelet-derived growth factor and vascular endothelial growth factor receptors (PDGFR and VEGFR, respectively), PVR, is highly expressed in the hemocytes and required for their directed migration. Embryos carrying a mutation in pvr still specify hemocytes, but these cells are unable to migrate properly. The same phenotype results from inactivation of the PVR ligands, which are normally localized along the hemocyte-migration route (Cho et al. 2002). Hemocytes have several significant roles in development, seeking out and removing dead cells, secreting and remodelling extracellular matrix components, monitoring the environment for pathogens and finally signaling to the larval fat body (Evans et al. 2003).
Cells from the group of posterior mesoderm that gives rise to the caudal visceral mesoderm migrate out of their original location to contribute to the developing embryo as a physiologically distinctive subset of Malpighian tubule cells: the stellate cells (Denholm et al. 2003). These cells not only migrate but also undergo a complex mesenchymal-to-epithelial transition as they integrate into the ectodermal epithelial buds. The normal incorporation of stellate cells and the later physiological activity of the mature tubules depend on the activity of hibris (hbs), an ortholog of the mammalian protein NEPHRIN. Hibris is a transmembrane immunoglobulin-like protein that shows extensive homology to Drosophila Sticks and stones (Sns) (Bour et al. 2000; Artero et al. 2001; Dworak et al. 2001). Hibris is expressed in embryonic visceral, somatic and pharyngeal mesoderm among other tissues such as the mesectoderm and the hindgut. In the somatic mesoderm, Hibris is restricted to a subset of fusion competent myoblasts and embryos that lack or overexpress hibris show a partial block of myoblast fusion, followed by abnormal muscle morphogenesis. Thus, together with other transmembrane proteins such as Kirre (Ruiz-Gomez et al. 2000), Roughest (Strunkelnberg et al. 2001), and Sns (Bour et al. 2000), it has been proposed that Hibris acts in cell-cell recognition and attraction during embryonic myogenesis (reviewed by Chen and Olson 2004). Indeed, Hibris and Roughest are detected in distinct interommatidial precursor cell populations during Drosophila eye morphogenesis and were found to mediate preferential cell adhesion between specific interommatidial cell types through heterophilic interactions (Bao and Cagan 2005). By employing the hibris reporter line P[w+]36.1, which we confirm that reproduces hibris expression in several embryonic tissue types, here we show co-expression with the GATA factor Serpent in migrating prohemocytes and in posterior mesodermal cells before their integration into the Malpighian tubule primordia. We also show that hibris function is not essential for prohemocyte migration out of the procephalic mesoderm and maintenance of their Serpent expression.