Fate's grim intervention: Determining sibling relationships and mechanisms of cell fate specification in the NB7-3 lineage of the Drosophila embryonic CNS

The nervous system of Drosophila melanogaster has 30 neuroblasts (NBs) per hemisegment. These NBs delaminate from the neuroectoderm then divide to regenerate a NB and produce a ganglion mother cell (GMC). These divisions repeat to produce multiple GMCs, each of which typically divides symmetrically to form a pair of neurons. While many studies have focused on the differences between NBs and the differences between sibling neurons, few studies have looked at how the differences between GMCs arise. To identify the characteristics that make each GMC different in a single NB lineage, I selected a representative NB lineage to identify (1) the number of GMCs and (2) the birth order of their progeny. The lineage I chose was NB7-3, which gives rise to three interneurons and one motor neuron. In Chapter 2, I will describe how I identified the birth order of the progeny of NB7-3 as being: GMC-1 that produces an interneuron and a motor neuron; GMC-2 that produces an interneuron and a cell that undergoes programmed cell death (PCD); GMC-3 that appears to directly differentiate into an interneuron. I provide the first evidence of programmed cell death (PCD) acting in a lineage-specific manner. I show that Numb and Notch are playing an active role in directing sibling fate in the 7-3 lineage, including the PCD fate. I also show how prospero, a gene involved in NB to GMC specification in the 7-3 lineage. These studies are the first to fully describe a complete NB cell lineage including the number of GMCs, their birth order and their specific progeny. I also provide evidence of lineage specific programmed cell death acting in the CNS. In Chapter 3 I will describe how I use this lineage information to screen a collection of lethal P-element insertions on the X-chromosome for genes that affect the specification of the 7-3 lineage. In particular I will describe 3 mutant alleles, which appear to be in the same genomic region, that affect the 7-3 lineage. The last chapter will describe an expression screen in which a collection of GAL4 lines are studied to find lines that have an expression pattern in single NB lineages. While no single lineage expressing lines were found, I did find a GAL4 line with an insertion in the gene huckebein, which is known to be involved in NB specification. The information revealed in these experiments will help elucidate the mechanisms controlling GMC specification.