HOMOLOGOUS CHROMOSOME-PAIRING IN DROSOPHILA-MELANOGASTER PROCEEDS THROUGH MULTIPLE INDEPENDENT INITIATIONS

The dynamics by which homologous chromosomes pair is currently unknown . Here, we use fluorescence in situ hybridization in combination with three-dimensional optical microscopy to show that homologous pairing o f the somatic chromosome arm 2L in Drosophila occurs by independent in itiation of pairing at discrete loci rather than by a processive zippe ring of sites along the length of chromosome. By evaluating the pairin g frequencies of 11 loci on chromosome arm 2L, over several timepoints during Drosophila embryonic development, we show that all 11 loci are paired very early in Drosophila development, within 13 h after egg de position. To elucidate whether such pairing occurs by directed or undi rected motion, we analyzed the pairing kinetics of histone loci during nuclear cycle 14. By measuring changes of nuclear length and correlat ing these changes with progression of time during cycle 14, we were ab le to express the pairing frequency and distance between homologous lo ci as a function of time. Comparing the experimentally determined dyna mics of pairing to simulations based on previously proposed models of pairing motion, we show that the observed pairing kinetics are most co nsistent with a constrained random walk model and not consistent with a directed motion model. Thus, we conclude that simple random contacts through diffusion could suffice to allow pairing of homologous sites.