The role of Xrp1 in the Minute syndrome and cell competition in Drosophila melanogaster
Lee, Chang Hyun
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Communication between cells to ensure tissue homeostasis and integrity may be important for normal development as well as for prevention of diseases in multicellular animals. A cell non-autonomous phenomenon, termed cell competition, can eliminate sub-optimal cells during development. Cell competition was first observed between normal cells and cells lacking a single copy of many ribosomal protein genes in Drosophila. It has generally been thought that reduced translational capability in these mutant cells accounts for their elimination by wild-type cells in mosaics, as well as their other phenotypes of developmental delay and small sensory organ structures, collectively termed the Minute Syndrome. Although many genes and signaling pathways involved in cell competition have been revealed, the mechanisms by which Rp-/+ cells are recognized in mosaics and cell competition initiates remains uncertain. This thesis describes a new unbiased genetic screen in search for novel genes required during cell competition between Minute and wild-type cells. Following EMS mutagenesis, mutant strains were sequenced at the whole-genome level to identify mutant lesions. In order to identify the mutation responsible for M2-73 mutant, which has a strong cell competition phenotype, we applied a method called iPLEX MassARRAY to efficiently genotype pooled recombinant strains. This approach identified the causative mutation as a premature stop codon in the Xrp1 gene, which was verified by analysis of existing deficiency strains. Mutation or deletion of a single copy of the Xrp1 gene is sufficient to eliminate competitive cell death, prevent the competitive expansion of wild type clones in a Minute background, and suppress the expression of genes that are induced in Minute cells. Furthermore, a surprising observation is the survival and limited proliferation of Rp-/- cells in the absence of Xrp1, which implicates this gene in apoptosis of these cells. Remarkably, mutation or deletion of Xrp1 suppressed the developmental delay of Minute animals, and cell-autonomously accelerated growth of Rp-/+ cells in wing imaginal discs. The only aspect of the Minute syndrome not affected by Xrp1 was bristle size. Xrp1 is elevated in Minute cells which suggest that Xrp1 is a stress response gene in cells perturbed of ribosome integrity.
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