Over 90% of neurons in the adult CNS of Drosophila are born from neuronal stem cells (neuroblasts) during the post-embryonic phase of neurogenesis. Most of the post-embryonic neurons derive from type I neuroblasts, which undergo repeated asymmetric divisions to produce a series of ganglion mother cells (GMCs). Each GMC then divides once resulting in two neurons, the “A” (Notch-on) and “B” (Notch-off) daughters. The respective daughter neurons of each type then constitute the A and B hemilineages for that neuroblast. 33 postembryonic hemilineages contribute neurons to each thoracic hemisegment, and these immature neurons arrest their development at a similar stage until metamorphosis. These arrested neuroblast lineages are uniquely identifiable by morphology. Access to a large pool of clonally-related and morphologically similar neurons makes this system tractable to RNA-seq analysis, since one can genetically label and isolate many cells per animal, which are predicted to share similar gene expression profiles. Our primary focus is to examine hemilineages with similar targets (e.g. leg neuropil) to identify genes that are required to establish and maintain hemilineage identity early in development. Given that activating these hemilineage neurons as a group drives distinct behaviors and that they form morphologically coherent structural units during development, we hypothesized that these hemilineages should express patterns of genes that are: 1) distinct from other hemilineages and 2) characteristic of individual hemilineages. We have used hemilineage-specific GAL4 lines to isolate hemilineages for RNA-seq analysis, ultimately gathering data for 11 of the 33 hemilineages as well as for some larger populations of neurons. We found that, in addition to combinatorial patterns of genes specifying the hemilineage neurons, there are some genes that are expressed by only a single hemilineage within the ventral nervous system (VNS). Most hemilineages display unique expression of certain transcription factors (TFs) and axon guidance genes. We collected data for two pairs of sibling hemilineages (lineage 1 and lineage 12) in order to identify differences between the A and B hemilineages derived from a common neuroblast. While A neurons display greater overall transcriptional diversity than B neurons, sibling hemilineages share very similar expression profiles. Comparing the gene expression between immature and mature larval neurons revealed that mature neurons express many genes not expressed in immature neurons, such as neuropeptide signaling genes and many neurotransmitter and ion channel genes associated with mature neuron function. Birth order also appears to dictate many differences in expression profile. Late-born immature neurons are typified by a period of transient Notch-related gene expression that is absent from early-born neurons. We are characterizing the function of many differentially expressed genes in particular hemilineages.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:744421 |
| Date | January 2017 |
| Creators | Etheredge, Jack |
| Contributors | Brand, Andrea ; Truman, James |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.repository.cam.ac.uk/handle/1810/270548 |
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