Live imaging of somatic homolog pairing in Drosophila supports a button-based mechanism for pairing facilitated by the genetic insulator gypsy
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In the somatic cells of the model organism Drosophila melanogaster, homologous chromosomes will pair from end to end during embryogenesis and once paired, remain paired throughout development. Somatic homolog pairing begins following the maternal-to-zygotic transition of transcription and pairing rates increase with embryonic time. Recent literature suggests that somatic homolog pairing initiates through a set of random independent contacts that occur along the length of the homologous chromosomes between regions of the homologs which have a highly specific affinity for one another, called “pairing buttons”. Some evidence supports that the molecular basis for these pairing buttons are genetic insulators and/or the architectural proteins that bind them. Here, I show that gypsy, a genetic insulator, increases the proportion of homologous loci pairing in the earliest stages of the maternal-to-zygotic transition of transcription. Using a transgenic live imaging system to fluoresce and therefore tag homologous loci, I imaged the pairing dynamics of homologous loci with adjacent gypsy insulators on both homologs, neither homolog, and on only one homolog. I observed that gypsy’s presence at both loci increases the number of loci that are transitioning from a stably unpaired to paired state in nuclear cycle 13 and early cycle 14. Furthermore, I found that gypsy does not increase the proportion of homologous loci that experience a chance to pair, supporting a random walk model for pairing initiation. My results demonstrate that the gypsy insulator likely functions as a pairing button, stabilizing chance contacts that occur between homologs and facilitating end-to-end somatic homolog pairing.
Available for download on Monday, May 18, 2026