Mitotic telomere clustering in Drosophila melanogaster. Natalia Wesolowska, Yikang Rong. Lab of Biochemistry and Molecular Biology, National Institutes of Health, Bethesda, MD.

   Telomeres are specialized structures that demarcate the ends of linear chromosomes. When their function is compromised, natural DNA ends can be improperly identified as broken ends and subjected to repair, resulting in chromosomal fusions and genomic instability. As obligatory chromosomal landmarks, telomeres can also serve to organize the genome. In yeast, telomeres cluster at the nuclear periphery, potentially to sequester the ends from the rest of the genome. To bring some insight into telomere organization in higher order organisms, we investigated the situation in interphase nuclei of Drosophila embryo. The syncytial blastoderm stage when nuclear divisions are still synchronized and take place at the surface of the embryo, presents a perfect experimental setting for imaging of a population of nuclei. To follow telomeres in vivo, we used a fluorescently labeled telomere protein HOAP. The 16 telomeres assemble into 4-6 fluorescent foci per nucleus. Furthermore, this organization appears to be present in other somatic tissues in the fly. In light of the findings from yeast, our results suggest that clustering may be a feature conserved through evolution. We made several testable predictions as to the rules governing telomere clustering and investigated them in embryos using fluorescence in situ hybridization to visualize telomeres. First, by inspecting anomalous embryos that develop without the paternal chromosome subset, we found that clustering is not mediated by associations between homologs. Second, using a fly with a novel telomere sequence at one of its ends, we showed that DNA sequence homology is irrelevant to clustering. Third, by marking the two ends of chromosome 3 with an exogenous sequence tag, we determined that clustering is not simply the association of telomeres of the same chromosome. Having ruled out these possibilities, we focus on a model where clustering is a protein-mediated process. So far, we found that telomere proteins do not play a major role in clustering. Through further mutant analysis we hope to bring insight to this mode of nuclear organization.