HEIP1 is required for efficient meiotic crossover implementation and is conserved from plants to humans.

Crossovers (CO) shuffle genetic information and physically connect homologous chromosomal pairs, ensuring their balanced segregation during meiosis. COs arising from the major class I pathway require the activity of the well-conserved group of ZMM proteins, which, in conjunction with MLH1, facilitate the maturation of DNA recombination intermediates specifically into COs. The HEI10 Interacting Protein 1 (HEIP1) was identified in rice and proposed to be a new, plant-specific member of the ZMM group. Here, we establish and decipher the function of the HEIP1 homolog in meiotic crossover formation and report its wide conservation in eukaryotes. We show that the loss of Arabidopsis HEIP1 elicits a marked reduction in meiotic COs and their redistribution toward chromosome ends. Epistasis analysis showed that acts specifically in the class I CO pathway. Further, we show that HEIP1 acts both prior to crossover designation, as the number of MLH1 foci is reduced in , and at the maturation step of MLH1-marked sites into COs. Despite the HEIP1 protein being predicted to be primarily unstructured and very divergent at the sequence level, we identified homologs of HEIP1 in an extensive range of eukaryotes, including mammals.