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Full Abstracts – PATTERN FORMATION
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86
Physical mechanisms shaping the
Drosophila
dorsoventral compartment boundary.
Christian Dahmann
1,2
, Maryam Aliee
3
, Jens-Christian Röper
1
,
Katharina Landsberg
1
, Constanze Pentzold
1
, Thomas Widmann
1
, Frank Jülicher
3
. 1) Max Planck Institute of Molecular Cell Biology and Genetics, Dresden,
Germany; 2) Institute of Genetics, Dresden University of Technology, Dresden, Germany; 3) Max Planck Institute for the Physics of Complex Systems,
Dresden, Germany.
Separating cells with distinct identities and fates by straight and sharp compartment boundaries is important for growth and pattern formation during
animal development. The physical mechanisms shaping compartment boundaries, however, are not fully understood. We combine theory and quantitative
experiments to investigate the roles of different mechanisms to shape compartment boundaries. Our theoretical work shows that cell elongation created by
anisotropic stress, cell area pressure, cell proliferation rate, orientation of cell division, and cell bond tension all have distinct effects on the morphology of
compartment boundaries during tissue growth. Our experiments using the developing
Drosophila
wing reveal that the roughness of the dorsoventral
compartment boundary is dynamic and that it decreases during development. By measuring tissue relaxation in response to laser ablation of cell bonds at
different developmental times, we demonstrate that decreased boundary roughness correlates with increased cell bond tension along the compartment
boundary. Finally, by using experimentally determined values for cell bond tension, cell elongation and bias in orientation of cell division in simulations of
tissue growth, we can reproduce the main features of the time-evolution of the dorsoventral boundary shape. We conclude that a local increase of cell bond
tension along the interface as well as global anisotropies in the tissue contribute to shaping interfaces in cell networks. We propose a simple scenario that
combines time-dependent cell bond tension at the boundary, oriented cell division, and cell elongation in the tissue that can account for the main features of
the dynamics of the shape of the dorsoventral compartment boundary.
87
Inverse Regulation of Target Genes Distant from the Dpp Morphogen Source.
Offer Gerlitz
1
, Oren Ziv
1
, Rutie Finkelstein
1
, Yaron Suissa
1
, Tama
Dinur
1
, Girish Deshpande
2
. 1) Developmental Biology and Cancer Research, IMRIC, The Hebrew University-Hadassah Medical School, Jerusalem; 2)
Department of Molecular Biology, Princeton University.
Morphogenetic capacity of Dpp is contingent upon how an extracellular gradient is established and read across the A-P axis in the wing disc. It can thus
pattern the cellular field by modulating gene expression in a concentration-dependent manner. Furthermore, the profile of the transcriptional response to the
graded activity of Dpp relies upon two counter active gradients of pMad and Brk. However the conclusions, thus far, have relied heavily on a region
proximal to the source in the wing pouch,. Earlier studies have assumed that the target gene expression in the regions distant from the source is independent
of the signaling influence. We demonstrate that this supposition is clearly incorrect i.e. Dpp signaling is active all across the patterning field including at the
periphery of the wing disc. We show that the current patterning model is inadequate to explain the expression pattern of Dpp targets, such as vestigial and
spalt (sal), in lateral regions of the wing disc far from the Dpp source where Brk levels peak. Most importantly, in a classic ‘role reversal’ mode, the
expression of the same targets is positively regulated by Brk and negatively by Dpp signaling. How is this opposite regulation achieved? By studying sal
expression in the different regions of the wing disc, we identify a new mechanism where a classic Dpp target is regulated through an enhancer that contains
neither Brk nor Mad-Med-Shn complex binding sites. We provide evidence that Brk induces expression of sal at the periphery of the wing disc indirectly
through repression of a negative regulator (NRS). On one hand, NRS represses sal expression by binding to a cis-regulatory element that does not contain
Brk binding sites. On the other, NRS is itself negatively regulated by Brk. Our findings constitute an important first step towards unraveling the workings of
a morphogen gradient at the edges.
88
Association Mapping to Identify New Leg Development Genes.
Megan Leach, Nathaniel Grubbs, Xin Su, Tiffany Petrisko, Catherine Longo, James
Mahaffey. Genetics Department, North Carolina State University, Raleigh, NC.
In order to further understand how genes, pathways, and networks establish and differentiate appendage systems, we used a genome wide association study
to identify previously unknown genes that are involved in establishment and differentiation of Drosophila legs. Femur, tibia, and tarsal segments of the first
(T1) and second (T2) thoracic leg were measured from a total of 12 males and 12 females of 117 inbred lines from the Drosophila Genetic Reference Panel
(DGRP). Natural variation in proportionality of leg segments was used to conduct association mapping of the variance in segment proportionality between
DGRP lines. We examined proportionality of segments instead of total leg length to 1) avoid only looking at larger vs. smaller flies, and 2) the
proportionality variation studied can help to better explain the effect of altering Wingless or Decapentaplegic signal strength in the current bullseye model of
Drosophila leg development. A change in the signal strength can alter the proportion of Drosophila imaginal discs expressing the leg determinant genes
homothorax, dachshund, or Distal-less. Single Nucleotide Polymorphisms (SNPs) with the lowest p-values across male and female T1 and T2 were chosen
for further study. Genes near SNPs of interest were identified independently for each leg segment analyzed. After candidate genes were identified, we used
in situ hybridization to determine gene expression in Drosophila imaginal discs. RNAi lines, obtained from Bloomington and Vienna Stock Centers, were
crossed with several GAL4 drivers for loss-of-function analysis. We identified genes that, when reduced, resulted in phenocopies with deleted leg segments,
reduction of segment leg lengths and altered leg segment morphology, as well as, some that altered antenna segments and caused deformation of wing. Our
findings include several newly discovered genes, implicating new genetic pathways with specific effects on leg development.