Neural precursor identity, cell division, and evolution in the insect central nervous system
Broadus, Julie Ann
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Permalink
https://hdl.handle.net/2142/20094
Description
Title
Neural precursor identity, cell division, and evolution in the insect central nervous system
Author(s)
Broadus, Julie Ann
Issue Date
1996
Doctoral Committee Chair(s)
Doe, Chris Q.
Department of Study
Neuroscience
Discipline
Neuroscience
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Molecular
Biology, Neuroscience
Biology, Entomology
Language
eng
Abstract
A central challenge for every organism is the generation of cell diversity during development. How does a single cell, the fertilized egg, give rise to the vast array of specialized cell types found in a mature organism? A related question is how has development evolved to generate diversity among organisms? I have pursued these questions by examining cell diversification during early central nervous system (CNS) development in Drosophila and the distantly related insect Schistocerca (grasshopper).
A key mechanism for specification of unique cell fates is regulated gene expression. In Drosophila, the spatio-temporal profile of seven-up (svp) gene expression in CNS precursors (neuroblasts, NBs) is suggestive of a role in the differential specification of progeny within NB lineages. Unexpectedly, we observe that svp is required for NB cell division rather than determination of neural cell fates. In embryos lacking svp function, NB cell division defects are characterized by sytokinesis failure, while the nuclear cell cycle continues. These abnormalities lead to NB polyploidy and the concomitant loss of identified neurons and glia. In svp embryos, NB polyploidization occurs nearly simultaneously among NBs with different svp expression profiles, suggesting that the generation of NB lineages may be extrinsically controlled.
As a first step toward understanding the evolution of nervous system development, we compared gene expression in NBs of two insects: Drosophila and Schistocerca, which are separated by over 300 million years during evolution. In Drosophila, the formation and fate of early-forming NBs are controlled by the pair-rule class of segmentation genes. The distantly related Schistocerca embryo has a similar arrangement of NBs, despite lack of known pair-rule gene function. We show that the pattern of gene expression in early NBs is also highly conserved. Our observation of homologous NB fates supports a principle raised by studies of insect segmentation: different gene hierarchies can create a conserved developmental consequence. In addition, we observe that later-forming NBs are often dissimilar. This result demonstrates that changes in upstream control of NB formation is an innovation that can also generate NB diversity during evolution.
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