TOPII and chromosome movement help remove interlocks between entangled chromosomes during meiosis

During the zygotene stage of meiosis, normal progression of chromosome synapsis and homologous recombination frequently lead to the formation of structural interlocks between entangled chromosomes. The persistence of interlocks through to the first meiotic division can jeopardize normal synapsis and occasionally chromosome segregation. However, they are generally removed by pachytene. It has been postulated that interlock removal requires one or more active processes, possibly involving topoisomerase II (TOPII) and/or chromosome movement. However, experimental evidence has been lacking. Analysis of a hypomorphic topII mutant and a meiosis-specific topII RNAi knockdown of Arabidopsis thaliana using immunocytochemistry and structured illumination microscopy (SIM) has now enabled us to demonstrate a role for TOPII in interlock resolution. Furthermore, analysis using a nucleoporin nup136 mutant, which affects chromosome movement, reveals that although TOPII activity is required for the removal of some interlock structures, for others, chromosome movement is also necessary. Thus, our study demonstrates that at least two mechanisms are required to ensure interlock removal.