Genetic dissection of MutL complexes in Arabidopsis meiosis

During meiosis, homologous chromosomes pair and exchange fragments, a process known as crossover. The main crossover pathway relies on ZMM proteins and is typically resolved by the MLH1/MLH3 heterodimer, known as MutLy. Through the analysis of plant fertility and bivalent formation, we demonstrated that another endonuclease can partially compensate for the loss of MutLy, though it is likely not MUS81. The loss of MutLy can be partially suppressed by eliminating FANCM helicase. Using F2 population sequencing, we examined the genome-wide distribution of crossovers in the mlh1 mutant and in a heterozygous mutant of the ZMM protein HEI10. Both mlh1 and hei10/+ mutants show reduced crossover numbers compared to the wild type. However, while crossover interference decreases in mlh1, it increases in hei10/+. Similarly, crossover assurance is reduced in mlh1, leading to a high level of aneuploidy in the offspring - an effect not observed in hei10/+. These findings highlight the distinct roles of ZMM and MutLy in crossover interference. Introducing additional copies of MLH3 significantly increases crossover frequency, with similar, though weaker, effects observed for MLH1. However, excessive expression of either gene results in a drastic reduction in crossover number and plant fertility, indicating the necessity for tight control of MLH1/3 levels.