TOPII and chromosome movement contribute to interlock resolution. (A) Immunolocalization of SC central element ZYP-1 (red) and lateral element ASY-1 (green) in late zygotene meiocytes of WT, topII-1, and topII-1^hom nup136-2^het, respectively. (a′ and a′′) Zoomed regions to illustrate chromosome paths in interlocks (ILs). (B) Mean number of interlocks per cell in WT, topII-1, topII-1^het nup136-2^hom, topII-1^hom nup136-2^het, and topII-1 nup136-2 double mutants. n > 100, 79, 49, 62, and 13, respectively. (C) Frequency of pairs of chromosomes involved in an interlock in topII-1^hom nup136-2^het metaphase I cells. n = 50 cells. (D) Ideogram of FISH of rDNA probes in A. thaliana chromosomes and representative metaphase I cells of topII-1^hom nup136-2^het. From left to right: two interlocks between bivalents 2 and 4 as well as 1 and 5; all bivalents involved in interlocks and probably other types of connections; and two interlocks between bivalents 2 and 4 as well as 1 and 3. (E) Model for interlock-resolution pathways depending on CO designation. Different types of interlocks (“open” vs. “closed”) were found in topII-1 and nup136-2, consistent with interlocks having two resolution pathways. A lack in TOPII would leave unresolved interlocks for which cleavage was essential for their resolution (closed interlocks involving ring bivalents), whereas impaired chromosome movement mutants would produce, mostly, open interlocks. See text for details. Bars, 5 µm. dsDNA, double-stranded DNA.