Pomological and Molecular Characterization of Apple Cultivars in the German Fruit Genebank

Abstract

Traditional varieties are a valuable tool in modern apple breeding. However, the use of synonyms and missing source documentation hinder an effective identification and conservation of relevant cultivars. During several projects, the authenticity and diversity of the apple cultivar collection of the German Fruit Genebank (GFG) was evaluated extensively. The trueness-to-type of 7890 apple trees was assessed on a pomological and molecular level. Pomological evaluations were performed by at least two experienced experts to identify the original cultivar names. On the molecular level, a set of 17 SSR markers was used to determine a unique genetic profile for each apple cultivar. The pomological and molecular characterization was expressed in terms of a comprehensive trueness-to-type criterion and the results were previously published as a well-curated dataset. In this study, the published dataset was analyzed to evaluate the quality and diversity of the apple collection of the GFG and highlight new findings based on phylogenetic and parentage analysis. The dataset contains 1404 unique genetic profiles corresponding to unambiguous cultivar names. Of these 1404 cultivars, 74% were assessed as true-to-type. The collection of diploid apple cultivars showed a high degree of expected heterozygosity (Hexp = 0.84). Genetic diversity in terms of year and location of origin was investigated with a STRUCTURE analysis. It was hypothesized that genetic diversity might decline overtime due to restrictive breeding programs. The results showed a shift dynamic between older and newer cultivars in one specific cluster, but no significant decrease in genetic diversity was observed in this study. Lastly, a parentage analysis was performed to check parental relationships based on historical research. Out of 128 parent–child trios, 110 trios resulted in significant relationships and reconfirmed the information from the literature. In some cases, the information from the literature was disproven. This analysis also allowed for readjusting the trueness-to-type criteria for previously undetermined cultivars. Overall, the importance of authenticity evaluations for gene bank cultivars was highlighted. Furthermore, the direct use of the dataset was shown by relevant investigations on the genetic diversity and structure of the apple cultivar collections of the GFG.

Keywords

• <i>Malus</i> × <i>domestica</i>
• trueness-to-type
• genetic diversity
• genetic structure
• parentage analysis