Meiotic chromosome structures constrain and respond to designation of crossover sites

Crossover (CO) recombination events between homologous chromosomes are required to form chiasmata, temporary connections between homologs that ensure their proper segregation at meiosis I(1). Despite this requirement for COs and an excess of the double-strand DNA breaks (DSBs) that are the initiating events for meiotic recombination, most organisms make very few COs per chromosome pair(2). Moreover, COs tend to inhibit the formation of other COs nearby on the same chromosome pair, a poorly understood phenomenon known as CO interference(3,4). Here we show that the synaptonemal complex (SC), a meiosis-specific structure that assembles between aligned homologous chromosomes, both constrains and is altered by CO recombination events. Utilizing a cytological marker of CO sites in Caenorhabditis elegans(5), we demonstrate that partial depletion of the SC central region proteins (SYPs) attenuates CO interference, elevating COs and reducing the effective distance over which interference operates, indicating that SYPs limit COs. Moreover, we show that COs are associated with a local 0.4-0.5 μm increase in chromosome axis length. We propose that meiotic CO regulation operates as a self-limiting system in which meiotic chromosome structures establish an environment that promotes CO formation, which in turn alters chromosome structure to inhibit other COs at additional sites.