Basic HTML Version
Full Abstracts – PATTERN FORMATION
150
83
The Tbx-20 transcription factor Midline functions as a localized negative regulator of epidermal growth factor receptor signaling.
Mariana Fregoso
Lomas
1
, Fiona Hails
1
, Jean-Franςois Boisclair Lachance
1,2
, Laura Nilson
1
. 1) Biology, McGill University, Montreal, QC, Canada; 2) Biological Sciences,
University of Chicago, Chicago, IL, USA.
We use the ovarian follicular epithelium as a model to study signaling pathways and patterning during development. This epithelium is responsible for the
secretion of the eggshell, including the two dorsal appendages derived from two dorsal anterior primordia. These primordia are recognized through the high
expression of the Broad-Complex (Broad) transcription factor, and are separated by a region of cells at the dorsal midline that do not express Broad. The
remaining follicle cells express basal levels of Broad. Dorsally-localized EGF signaling coming from the underlying oocyte initiates this spatial pattern along
the dorsal-ventral axis, but the signals that pattern the anterior-posterior (AP) axis are less well understood. To better understand AP patterning, we
characterized a new locus called F27 required for this process. In F27 mutant epithelia, the posterior limit of the high Broad domain is shifted toward the
posterior, suggesting that dorsal anterior fates have been determined in posterior follicle cells. Consistent with this change in cell fate, the resulting eggshells
exhibit extra appendage material posterior to the normal appendage position. The altered fate in this region can be detected in earlier stages because the
mutant cells express the transcription factor
mirror
, which we have shown can induce dorsal-anterior fates. When we ectopically activate the EGFR
signalling cascade in the F27 background, the follicular epithelium can respond inducing dorsal fates accordingly. We mapped the F27 mutation to the
midline
gene, which encodes a Tbox transcription factor. Midline is expressed in the posterior follicular epithelium and, moreover, when we ectopically
express it in the anterior it represses dorsal fates. We show that the
midline
paralog
H15
participates in this process as well. Our data show that these factors
are regulating posterior fate in the follicular epithelium, and are acting as novel negative regulators of EGFR signaling output.
84
Evolutionary variation in the Dorsal gradient distribution.
J. Sebastian Chahda
1
, Claudia M. Mizutani
1,2
. 1) Case Western Reserve University,
Cleveland, OH. Deparment of Biology; 2) Case Western Reserve University, Cleveland, OH. Deparment of Genetics.
The specification of primary germ layers along the dorso-ventral (DV) axis by morphogenetic gradients is highly conserved among bilaterians. Currently,
little is known about how those gradients respond to modifications in embryonic size and how scaling affects DV cell fates. We recently found that related
Drosophila
species that vary in embryo size exhibit unequal scaling across their germ layers. While the neuroectodermal domain remains unchanged, there
are striking variations in the mesodermal domain of Drosophilids. Here we sought to investigate the underlying causes for this evolutionary variation in the
mesoderm size. Among possible factors affecting the mesodermal domain are alterations in nuclear size and densities, divergence in cis-regulatory sequence
of mesodermal genes, and variation in the range of Toll receptor activation, which modulates the nuclear intake of Dorsal, a key morphogen responsible for
mesoderm specification. Comparative quantifications of Dl reveal that Dl concentration levels have changed to an either broader or sharpened distribution
across the DV axis in different
Drosophila
species. To rule out the possibility that the nuclear intake of Dl was changed due to divergence in the Toll
signaling pathway, we generated
D. melanogaster
embryos with varying nuclear size and density packing and unmodified Toll signaling. Our data show that
alterations in the Dl gradient distribution similar to other species can be solely generated by physical changes in nuclear size and packing. Finally, we also
show that the activation of the mesodermal gene
snail
from sibling
melanogaster
species occurs at same threshold levels of Dl. Together, our results provide
an unexplored route to modifying primary germ layers without the alteration of cis-regulatory sequences of Dl target genes.
85
Computer-aided estimation of the motor neuron morphology patterns.
Xiao Chang
1
, Ashutosh Kale
1
, Lauren Dodge
1
, Jennifer Brazill
2
, Michael D.
Kim
2
, Gavriil Tsechpenakis
1
. 1) Computer and Information Science, Indiana University-Purdue University Indianapolis, Indianapolis, IN; 2) Department of
Molecular and Cellular Pharmacology, University of Miami School of Medicine, Miami, FL.
Our goal is to exploit the single-neuron resolution achieved by MARCM to create a comprehensive morphological categorization of motor neuron
subtypes in the Central Nervous System (CNS) of the Drosophila larva. Such morphology-based classification of motor neurons allows for their automated,
unbiased tracing and identification in complex single-cell resolution imagery, an essential step towards the bottom-up reconstruction of a fully annotated
brain. The morphological features that we use to classify motor neuron subtypes involve the topological relationship between compartments and the
reference ventral nerve cord (VNC) midline: relative positions between soma, axon, dendrites and reference; direction and extent of the axon; the extent and
approximate shape of dendrites. The major challenge in our task is the high variability between the morphology of neurons of the same subtype, and the
morphological similarities between motor neurons of different subtypes. The problem is translated computationally as clustering neuron morphologies into
distinct classes, given the relatively limited number of manually annotated neurons that can be used for computer training. Our solution consists of a
Machine Learning-based classification framework, namely a hidden Conditional Random Field, which incorporates the above features while avoiding the
accurate estimation of shapes and locations in three dimensions with image processing. Our study has revealed so far nine distinct morphological classes that
span across 30 motor neuron subtypes, independently from the target muscle and the terminal synaptic bouton morphology.