Identification of 'Sorrento' walnut using simple sequence repeats (SSRs)

Among the Italian walnut (Juglans regia L.) landraces, 'Sorrento' is the most famous. It originated in the Sorrento peninsula, but today it is grown in the entire Campania region. 'Sorrento' is a mixture of genetic entities and is often heterogeneous for important commercial traits such as fruit size and yield. Consequently, the morphological description of 'Sorrento' is extremely difficult. To evaluate the genetic diversity in 'Sorrento' walnut, we analyzed 16 'Sorrento' plants grown in Caserta (10 originated from seeds and six from grafts), and 26 grafted 'Sorrento' clones grown in the Sorrento peninsula. We compared their genotypes along with six other walnut cultivars using 12 microsatellite (SSR) markers. A total of 66 putative alleles were detected, 16 of which were unique to one individual. Two loci, WGA9 and WGA71, were particularly useful for distinguishing Caserta samples from Sorrento peninsula clones. The phylogenetic and STRUCTURE analysis highlighted the genetic distance between the Sorrento peninsula and Caserta groups, assigning the samples to two different clusters (or populations) corresponding closely, but not perfectly, to each sample's geographic origin. The name 'Sorrento' walnut should not be assigned to samples falling outside the two genotypic clusters, even if they have phenotypes traditionally associated with the 'Sorrento' walnut.