ASY3 has dosage-dependent diverse effects on meiotic crossover formation

Crossovers create genetic diversity and are required for equal chromosome segregation during meiosis. Their number and distribution are highly regulated by different, yet not fully understood mechanisms including crossover interference. Crucial for crossover formation is the chromosome axis. Here, we explore the function of the axis protein ASY3. To this end, we use the allotetraploid species Brassica napus and due to its polyploid nature, this system allows a fine-grained dissection of the dosage of meiotic regulators. The simultaneous mutation of all four ASY3 alleles results in defective synapsis and drastic reduction of crossovers, which is largely rescued by the presence of only one functional ASY3 allele. Crucially, while the number of class I crossovers in mutants with two functional ASY3 alleles is comparable to that in wildtype, this number is significantly increased in mutants with only one functional ASY3 allele, indicating that reducing the dosage of ASY3 increases crossover formation. Moreover, the class I crossovers on each bivalent in mutants with one functional ASY3 allele follow a random distribution, indicating compromised crossover interference. These results reveal the dosage-dependent distinct effects of ASY3 on crossover formation, and provide insights into the role of chromosome axis in patterning recombination.