The genetic background of the phenotypic variability observed in apple autotetraploids

The aim of the research is to obtain tetraploids with new functional traits, including increased resistance to infection by dangerous apple pathogens. In the previous studies, numerous tetraploid versions were obtained for five apple (Malus × domestica Borkh.) cultivars: ‘Free Redstar’, ‘Gala Must’, ‘Pinova’, ‘Redchief’ and ‘Sander’, using the in vitro technique. Polyploidization caused a marked change of the phenotype. In comparison with diploids, the autotetraploids had shorter shoots and smaller leaves of altered shape; the chlorophyll content of the tetraploids was higher. Transpiration rate and stomatal conductivity were generally higher in tetraploids. But, the photosynthetic activity and maximum PSII quantum yield (Fv/Fm) in tetraploids were comparable to those observed in diploids. Several tetraploid genotypes were much less susceptible to Venturia inaequalis infection than their diploid counterparts. Molecular analysis confirmed that the differences within tetraploids of a given cultivar are associated with genetic and epigenetic changes as indicated by the variation in DNA structure and methylation degree, detected using AFLP and MSAP analysis, respectively. The mean degree of genetic differentiation of the tetraploids tested in three cultivars analyzed with the AFLP markers was 2.4% compared to the diploid counterparts. Using MSAP analysis, higher DNA methylation level was detected in tetraploid clones compared to their diploid counterpart. Moreover, the tetraploid clone with tendency to premature dormancy (most of the plants of such clone manifested growth inhibition) was characterized by the highest DNA methylation rate (22.95%), which was twice as high as in diploid.