Speaker: Prof. Piotr A. Ziolkowski (Adam Mickiewicz University in Poznan, Poland)

Title: "Anti-crossover functions of MMR and ZMM in plant meiosis", 

The seminar will start at 14:00 am in the conference room of the acting director (Genetic building, 0.93). 

Abstract
Our previous work demonstrated that MSH2 heterodimers, a critical component of the Mismatch Repair (MMR) system, stimulate Class I crossovers within polymorphic chromosome regions. However, in msh2 mutants, the reduction in crossover frequency is minimal due to robust mechanisms of crossover interference and assurance. In contrast, Class II crossovers are less efficient in polymorphic chromosomal regions. Given the typically low frequency of Class II crossovers, we enhanced their levels by utilizing mutants of the helicases FANCM and RECQ4 (the plant homolog of SGS1/BLM), which are well-known for their anti-crossover functions. In msh2 fancm and msh2 recq4 double mutants, recombination frequency increased significantly, highlighting the anti-recombination role of MSH2 heterodimers in Class II crossover pathways.
To investigate the exclusive role of MSH2 in ZMM-independent recombination, we combined the recq4 knockout with mutations in the ZIP4 gene, which encodes one of the ZMM proteins. Unexpectedly, the resulting recq4 zip4 mutant displayed a nearly twofold increase in crossovers compared to recq4 alone and a fivefold increase compared to the wild type. This finding suggests that ZMM proteins not only protect recombination intermediates from RECQ4 helicases but, in the absence of RECQ4, also shield them from Class II structure-specific endonucleases. As expected, heterochiasmy and crossover interference were entirely abolished in the recq4 zip4 mutant.
The knockout of MSH2 in the recq4 zip4 background further elevated crossover frequency, demonstrating that MSH2’s anti-recombination role is independent of ZMM proteins. Remarkably, combining fancm, recq4, and zip4 mutations with the msh2 mutation led to an unprecedented ninefold increase in crossover frequency compared to the wild type.
In summary, our findings reveal that ZMM proteins play a protective role in shielding recombination intermediates from helicase-mediated dissolution. However, in the absence of helicases, ZMM proteins also safeguard these intermediates from endonucleases that mediate Class II crossovers. This demonstrates that ZMM proteins have an anti-crossover role that operates independently of the MMR system’s anti-crossover function.