Genome size and nucleotypic variation in Malus germplasm

The genus Malus has anywhere between 25 and 33 species along with several subspecies. Malus species as well as clones within the same species have varying ploidy levels, as these are more than likely collected from different trees and (or) from different locations. In recent years, large numbers of Malus germplasm accessions have been collected and maintained at the United States National Germplasm Clonal Repository; however, genome sizes of this material have not yet been determined. In this study, leaf tissues from young grafted trees of 100 Malus species and hybrids growing in a nursery at the University of Illinois were collected and immediately used for extracting nuclei. Leaf tissues from apple and maize line W-22, used as an internal standard, were co-chopped and prepared for flow cytometric analysis. Apple nuclei were stained with propidium iodide, an intercalating dye, and a minimum of 8000 nuclei per sample were analyzed. Mean fluorescence of apple nuclei was then determined. A total of four replications per sample was used. Among 100 Malus accessions analyzed, one tetraploid, three triploid, and 96 diploid genotypes were identified. Significant differences in genome size were identified among the three ploidy types observed and also within diploid genotypes. The 2C mean value for tetraploids was 3.13 pg and ranged from 2.27 to 2.41 pg for triploids, whereas 2C values for diploids ranged between 1.44 and 1.72 pg. In addition, leaf impressions of young, fully expanded leaves were collected from young trees of 10 selected genotypes based on their ploidy and flow cytometric analysis and used to measure the nucleotypic parameter stomatal length. Ten stomata were measured per slide, three slides were analyzed per leaf, and three leaves were analyzed per accession. Overall, mean length of stomata ranged between 19.47 μm (diploid) and 27.6 μm (tetraploid), indicating that stomatal length in a tetraploid Malus genotype was 1.4-fold higher than that of a diploid genotype. A positive correlation between genome size and the nucleotypic parameter stomatal length was observed.