cosa1 reveals robust homeostasis and separable licensing and reinforcement steps governing meiotic crossovers crossovers cos between homologous chromosomes ensure their faithful segregation during meiosis we identify c elegans cosa1 a cyclinrelated protein conserved in metazoa as a key component required to convert meiotic doublestrand breaks dsbs into cos during late meiotic prophase cosa1 localizes to foci that correspond to the single co site on each homolog pair and indicate sites of eventual concentration of other conserved co proteins chromosomes gain and lose competence to load co proteins during meiotic progression with competence to load cosa1 requiring prior licensing our data further suggest a selfreinforcing mechanism maintaining co designation modeling of a nonlinear doseresponse relationship between irinduced dsbs and cosa1 foci reveals efficient conversion of dsbs into cos when dsbs are limiting and a robust capacity to limit cytologically differentiated co sites when dsbs are in excess cosa1 foci serve as a unique live cell readout for investigating co formation and co interference