Harnessing reproductive genome instability to facilitate ploidy breeding in apple (Malus x domestica Borkh.)

Genetic improvement in apple typically occurs via classical crosshybridization and subsequent selection of superior genotypes. As most parental lines are diploid, apple breeding mainly occurs at the 2x level and hence does not take advantage of karyotypic variation to exploit the full genetic potential of the available germplasm. Occasionally formed tri- and tetraploid varieties exhibit improved agronomic features, and thus confirm that ploidy manipulation constitutes a promising breeding approach in apple. However, the current strategy of interploidy crossing is highly inefficient and timeconsuming, and hence cannot be implemented in commercial pome fruit breeding. Here, we will assess and validate the use of reproductive genome instability as a means to induce novel karyotype variation in apple. At the one side we will screen an extensive population of apple cultivars to identify lines that generate di- or aneuploid spores. Via GWAS we will thereby identify causative QTLs and associated candidates genes. At the other side, we will use in vitro and in vivo assays to monitor the effect of temperature and hormone modulation on reproductive genome stability. For lines and treatments that confer dior aneuploid gamete formation, we will cytologically assess the underlying cellular mechanism and characterize the genetic make-up of resulting spores. Overall, our study will provide a toolbox to facilitate ploidy breeding in apple, with major relevance for other (fruit) crops.