S01 - Session O8 - Occurrence and mechanistic basis of 2n gamete formation in apple (Malus x domestica Borkh.)

Authors: Han Palmers *, Marijn Rymenants, Nico De Storme

In plants, aberrations in reproductive genome stability can lead to the formation of 2n gametes, and these are considered an important driver for polyploidization and speciation. The formation of 2n gametes is often controlled by genetic determinants, nevertheless can also be strongly affected by external factors such abiotic stress. 2n gametes are highly interesting from a breeding perspective due to their role in polyploidy induction, which often confers beneficial agronomical traits, such as increased yield and enhanced stress tolerance. In apple, the availability of several triploid genotypes, including many highly valued commercial cultivars, such as 'Jonagold' and 'Boskoop', indicates the occurrence of 2n gametes in the germplasm, however, little is known about their frequency, genetic origin and mechanistic basis. In order to gain more insights into the occurrence and genetic variation in 2n gamete formation in apple, we screened the available REFPOP population, a genetically diverse reference collection of apple cultivars, for alterations in reproductive genome stability via an integrated approach involving pollen size analysis and seed ploidy profiling. Accessions showing significant diplogamete production were further characterized using cytological analysis of meiosis and spore genotyping. In this screen we have identified multiple accessions that produce a significant proportion of larger pollen grains indicative of 2n gamete formation. Cytological characterization of meiosis in specific accessions has revealed enlarged and doubled tetrads as well as meiocytes with a duplicated chromosome set, indicating ectopic induction of pre-meiotic genome doubling. These results are the first report of pre-meiotic doubling as a mechanism for forming 2n gametes in apple. In addition, the 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.