Occurrence and mechanistic basis of 2n gamete formation in apple (Malus × domestica)

In plants, aberrations in reproductive genome stability can lead to the formation of 2n gametes, which are considered an important driver for polyploidization and speciation. The formation of 2n gametes is often controlled by genetic determinants, nevertheless can also be affected by external factors such as abiotic stress. 2n gametes are highly interesting from a breeding perspective as they form a route for polyploidy induction, which often confers beneficial agronomical traits, such as increased yield and enhanced stress tolerance. In apple, the existence of several triploid genotypes, including many highly valued commercial cultivars, such as ‘Jonagold’ and ‘Boskoop’, indicates for the occurrence of 2n gametes in the germplasm; however, little is known about their frequency, specific origin and mechanistic basis. In order to gain more insights into the occurrence and natural genetic variation in 2n gamete formation in apple, we screened the REFPOP population, a genetically diverse reference collection of apple cultivars, for alterations in reproductive genome stability via an integrated approach involving pollen size analysis. Accessions showing significant production of 2n gametes were further characterized using cytological analysis of microsporogenesis and -gametogenesis as well as via spore genotyping. In the screen performed in 2021 we identified multiple apple accessions that produced a significant proportion of larger pollen grains, which is indicative for 2n gamete formation. Preliminary cytological characterization of male meiosis of one of these lines revealed enlarged and doubled tetrads as well as meiocytes with a duplicated chromosome set, indicating ectopic occurrence of pre-meiotic genome doubling. These results are the first report of the characterization of 2n gamete formation in apple germplasm, and provide evidence for the occurrence of pre-meiotic doubling as a mechanistic basis underlying 2n pollen production in apple. In addition, natural variation in 2n gamete formation in the apple germplasm forms a fundamental basis for elucidating underlying cellular and genetic mechanisms, and thus may provide novel insights into the regulation of reproductive genome stability, with implications for apple genome evolution and breeding.