Evidence of Blastoderm Dpp Gradient Conservation in Drosophila. Juan S. Chahda, Priscilla Ambrosi, Claudia M. Mizutani. Biology, Case Western Reserve University, Cleveland, OH.

   The dorso-ventral axis of the Drosophila blastoderm is partly patterned by Decapentaplegic (Dpp)/BMP-4. High levels of Dpp in the dorsal ectoderm activate genes that specify the amnioserosa and dorsal epidermis, and lower levels of Dpp present in lateral regions of the embryo contribute to neuroectoderm specification via gene repression. The scaling properties of this morphogen have not been fully investigated across species. Here, we quantified the blastoderm expression of dpp and its target genes - rhomboid and race - in multiple Drosophila species and found that, between D. melanogaster and D. sechellia, the expression domain of rhomboid and race is relatively constant, compared to significant differences observed in the dpp expression domain. In D. simulans, we detected similar expression of rhomboid, but did not detect the expression of race within the embryo trunk, even though race is expressed in D. sechellia, a recently diverged sister species. We also show that the expression domain of dpp tends to scale with embryo width across evolutionary time. In order to test the dynamics of the Dpp gradient, we used mutants to expand zygotic dpp expression, and thus Dpp activity, and reduced peak Dpp activity by disrupting its extracellular regulation using multiple mutant vkg alleles. Vkg is a type IV collagen that extracellularly binds Dpp and its signaling complex. We have evidence that the Dpp gradient may not scale when augmented or reduced within the blastoderm, indicating the absence of a blastoderm Dpp expander that allows the gradient to scale in the growing wing disc. We are currently investigating if vkg embryos also exhibit increased Dpp activity in lateral regions of the embryo, which would result in altered gene expression profiles in the neuroectoderm. These results will help us understand the mechanisms of Dpp gradient conservation during evolution.