The Role of AtMUS81 in Interference-Insensitive Crossovers in A. thaliana

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Abstract
MUS81 is conserved among plants, animals, and fungi and is known to be involved in mitotic DNA damage repair and meiotic recombination. Here we present a functional characterization of the Arabidopsis thaliana homolog AtMUS81, which has a role in both mitotic and meiotic cells. The AtMUS81 transcript is produced in all tissues, but is elevated greater than 9-fold in the anthers and its levels are increased in response to gamma radiation and methyl methanesulfonate treatment. An Atmus81 transfer-DNA insertion mutant shows increased sensitivity to a wide range of DNA-damaging agents, confirming its role in mitotically proliferating cells. To examine its role in meiosis, we employed a pollen tetrad–based visual assay. Data from genetic intervals on Chromosomes 1 and 3 show that Atmus81 mutants have a moderate decrease in meiotic recombination. Importantly, measurements of recombination in a pair of adjacent intervals on Chromosome 5 demonstrate that the remaining crossovers in Atmus81 are interference sensitive, and that interference levels in the Atmus81 mutant are significantly greater than those in wild type. These data are consistent with the hypothesis that AtMUS81 is involved in a secondary subset of meiotic crossovers that are interference insensitive. Meiosis is a specialized type of cell division in which one diploid progenitor cell divides into four haploid cells that are subsequently used for fertilization during sexual reproduction. During meiosis, chromosomes pair, synapse, and exchange genetic information, all of which are required for proper chromosome segregation during subsequent stages. Failure to properly segregate meiotic chromosomes often leads to genetic defects such as aneuploidy. Using the model plant A. thaliana, we have developed a powerful system for the visual analysis of meiotic recombination directly in the pollen, in which the four products of individual meioses are fused together in a tetrad. We have used this system to characterize the gene AtMUS81 and show that Atmus81 mutants have a moderate reduction in meiotic crossovers and are sensitive to a wide range of DNA-damaging agents. Importantly, the remaining crossovers in Atmus81still exhibit crossover interference, a phenomenon whereby one crossover inhibits the occurrence of other nearby crossovers. Our results suggest that AtMUS81 mediates a subset of meiotic recombination events in Arabidopsis that are insensitive to crossover interference.