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Full Abstracts – PLENARY SESSION II
117
Oogenesis - where physiology and development meet.
Denise J. Montell, Xiaobo Wang, Li He, Ho Lam Tang, Aeri Cho, Jessica Sawyer, Meg Waghray,
Jennifer Jensen, Danfeng Cai, Jingchuan Luo, Fang Bu. Dept Biological Chemistry, Center for Cell Dynamics, Johns Hopkins Sch Med, Baltimore, MD.
The development of the Drosophila ovary serves as a useful model for general aspects of organogenesis. In contrast to embryonic development, which is
highly stereotyped and insulated from environmental perturbations, oogenesis is responsive to changing physiological conditions. Starvation for example
slows the rate of stem cell division and induces some egg chambers to undergo apoptosis while others arrest until conditions improve. The development of
robust live imaging of most stages of oogenesis has opened the door to the use of sophisticated new tools, including photoactivatable proteins and FRET
biosensors, to measure and manipulate molecules and physical forces in real time. I will describe recent genetic and live imaging studies, touching on the
topics of somatic stem cell maintenance, mechanisms of cell fate diversification, cell motility, epithelial morphogenesis and the influence of physiological
conditions on ovarian development.
Evolution and Phenotypic Effects of New Genes in Drosophila.
Manyuan Long. Dept Ecology & Evolution, Univ Chicago, Chicago, IL.
New gene evolution is a generally important process. The young genes which originated recently, e.g. as short as 1~10 million years (mys) of divergence
time between D. melanogaster and its eight close relatives, provided an excellent system to understand the origin of genes, because the signature of evolution
in their early stage is detectable in their sequences before its disappearance in subsequent evolution. Several genetic mechanisms have been found
responsible for the initial structures of new genes. New genes were fixed into a species with an appreciable rate, in which the lineage toward D. melanogaster
generated ~1000 new genes since the Drosophila genus diverged 40-60 mys ago. These frequent origination events revealed a stable process of gene
evolution in which the excess male genes, whose expression is male-biased or male-specific, frequently copied into autosomes; and the excess female genes
into the X in a slower pace. This “gene traffic”, manifesting the interaction between the sex chromosome and autosomes that impacted genome evolution, is
shaped by natural selection through various evolutionary genetic and mechanistic processes. Its consequence was evolutionarily and functionally to
reorganize the genomes in the contents and types of genes between sex chromosomes and genomes. Furthermore, the functional analyses revealed an
unexpected evolutionary role of new genes in phenotypic evolution: ~1/3 of young genes quickly evolved essential functions, terminating developmental
process and blocking the formation of organs/tissues when silenced with knockdown or disruptive mutagenesis. These findings revealed that in Drosophila
there are species-specific or lineage-specific components of the genetic system that controls development, defining a paradox to understand the divergence
between closely related species with a potential solution that may lie in the evolution of gene interaction. The materials for presentation are based on the
published (http://longlab.uchicago.edu/publication) and unpublished data in the Long lab at Chicago.
PIPs control cell morphogenesis in Drosophila.
Julie A. Brill. Cell Biology Program, The Hospital for Sick Children, Toronto, ON.
Phosphatidylinositol (PI) phosphates (PIPs) regulate cell signaling, cytoskeletal organization and membrane trafficking in yeast and mammalian cells, yet
surprisingly little is known about their effects on the development of multicellular organisms. We are investigating the roles of PI 4-phosphate (PI4P) and PI
4,5-bisphosphate (PIP
2
) in cytokinesis, gametogenesis and organelle biogenesis using Drosophila as a model system. We discovered that the different PI 4-
kinases, which synthesize PI4P, play distinct roles in these fundamental biological processes: Fwd (PI4KIIIβ) regulates Rab11 during spermatocyte
cytokinesis; PI4KIIIα is essential for moesin activation and membrane trafficking during oogenesis; and PI4KII regulates glue granule biogenesis and acts
with the PI 4-phosphatase Sac1 to promote pigment granule formation. We also identified roles for the PIP 5-kinase Sktl, which synthesizes PIP
2
, in
cytokinesis, cell polarity and nuclear shaping during sperm development. Our current understanding of the molecular mechanisms and targets of PIP
regulation in these different processes confirms that PIPs serve as critical signaling molecules during Drosophila development.
Spindle orientation in neural stem cells.
Chris Q. Doe. Inst Neuroscience, Univ Oregon, Eugene, OR.
I will discuss our work on spindle orientation in Drosphila neuroblasts and sense organ precursors. We have been using both in vivo genetics as well as an
"induced cell polarity" culture system to dissect the mechanisms of spindle orientation. Neuroblasts and SOPs are polarized cell types, and the mitotic
spindle aligns with cortical polarity cues to induce a molecularly asymmetric cell division in both cases. We show that loss of spindle orientation leads to
failure of neuroblast homeostasis -- there is always an increase in neuroblast number and never a loss of neuroblast number. We use the induced cell polarity
system to dissect the protein domains and amino acids in each domain that are required for proper spindle orientation, including structural studies of the
relevant domains. We uncover a mechanism by which the mitotic kinase Aurora-A positively regulates the Pins-Mud-dynein spindle orientation pathway,
and evidence for a second Pins-Dlg-Khc73 pathway. More recently we have characterized the role of the fly Afadiin ortholog Canoe in recruiting Mud to the
cortex to promote spindle orientation. Lastly, I will discuss our progress in understanding the mechanisim and function of Dishevelled-mediated spindle
orientation.
Deciphering the
cis
-regulatory code.
Eileen E. Furlong. Genome Biology, EMBL, Heidelberg, Germany.
A central challenge in biology is to understand how the genome is utilized to generate diverse cell types. Embryonic development occurs through
progressive restriction of cell fates, from pluripotent fields of cells to complex organs and tissues. This requires a directed progression through interlinked
regulatory states, each defined by the total set of active transcription factors. At each development stage, the inputs of signaling and transcriptional networks
regulate the expression of specific sets of genes that drive the transition to the next state. Hence, understanding how the underlying
cis
-regulatory networks
produce spatial and temporal gene expression is a vital step towards deciphering metazoan development and many diseases. Recent approaches assaying
transcription factor (TF) binding enable the location and combinatorial occupancy of enhancers to be experimentally measured at specific development
stages genome-wide. A current challenge is to interpret these TF binding data in terms of their resulting spatio-temporal
cis
-regulatory activity. Our work
uses mesoderm specification in
Drosophila
as a well-defined model system. Using machine learning, we demonstrated that transcription factor occupancy
alone is sufficient to predict spatio-temporal enhancer activity.
In vivo
transgenic reporter assays demonstrated a high accuracy, with 80 percent of
enhancers’ activity matching their predicted expression domains. We have now complemented this study by generating cell-type specific information on
chromatin state within the context of a developing embryo, using a new method that we have developed. The data reveals heterogeneous combinations of
chromatin marks linked to enhancers in an active state. Using a Bayesian network, we show that chromatin state is sufficient to predict not just the location,
but activity state, of regulatory elements
de novo
. The model thereby enables the visualization of dynamic enhancer usage during development and
uncovered a temporal link between RNA polymerase II enhancer occupancy and the precise timing of enhancer activity.