C. elegans ZHP-4 is required at multiple distinct steps in the formation of crossovers and their transition to segregation competent chiasmata

Correct segregation of meiotic chromosomes depends on DNA crossovers (COs) between homologs that culminate into visible physical linkages called chiasmata. COs emerge from a larger population of joint molecules (JM), the remainder of which are repaired as noncrossovers (NCOs) to restore genomic integrity. We present evidence that the RNF212-like C. elegans protein ZHP-4 cooperates with its paralog ZHP-3 to enforce crossover formation at distinct steps during meiotic prophase: in the formation of early JMs and in transition of late CO intermediates into chiasmata. ZHP-3/4 localize to the synaptonemal complex (SC) co-dependently followed by their restriction to sites of designated COs. RING domain mutants revealed a critical function for ZHP-4 in localization of both proteins to the SC and for CO formation. While recombination initiates in zhp-4 mutants, they fail to appropriately acquire pro-crossover factors at abundant early JMs, indicating a function for ZHP-4 in an early step of the CO/NCO decision. At late pachytene stages, hypomorphic mutants exhibit significant levels of crossing over that are accompanied by defects in localization of pro-crossover RMH-1, MSH-5 and COSA-1 to designated crossover sites, and by the appearance of bivalents defective in chromosome remodelling required for segregation. These results reveal a ZHP-4 function at designated CO sites where it is required to stabilize pro-crossover factors at the late crossover intermediate, which in turn are required for the transition to a chiasma that is required for bivalent remodelling. Our study reveals an essential requirement for ZHP-4 in negotiating both the formation of COs and their ability to transition to structures capable of directing accurate chromosome segregation. We propose that ZHP-4 acts in concert with ZHP-3 to propel interhomolog JMs along the crossover pathway by stabilizing pro-CO factors that associate with early and late intermediates, thereby protecting designated crossovers as they transition into the chiasmata required for disjunction.