Identification of ‘Sorrento’ walnut using simple sequence repeats (SSRs)

Abstract

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.

Long-term human impacts on genetic structure of Italian walnut inferred by SSR markers

“…The analysis was based on ten nuclear microsatellite markers originally developed in Juglans nigra (Woeste et al 2002), but successfully used in related species for a wide range of genetic applications (Pollegioni et al 2009a). In this study, mean number of alleles per locus (6.2) and effective number of alleles (2.99) computed over all 456 walnut samples were relatively high compared to the levels of variability detected in 48 J. regia cultivars (Dangl et al 2005), SORRENTO (Foroni et al 2007) and five J. regia populations from central and southwestern China (Wang and Pei 2008). Conversely, the ranges of allelic richness and observed heterozygosity (0.597) observed across the same subset of SSR loci were lower than the corresponding values observed in autochthonous populations of Juglans mandshurica (Bai et al 2007), in 39 open-pollinated J. nigra families (Robichaud et al 2006), and 43 indigenous populations of J. nigra collected in the Central Hardwood Region of the United States (Victory et al 2006).…”

“…It originated from the Sorrento peninsula (Campania region) but is cultivated in the entire Italian peninsula. According to Foroni et al (2007), in the last century, "some farmers appear to have performed a strong selection in favor of a small group of walnut trees to improve yield and nut quality on Sorrento peninsula".…”

“…For comparison, 20 genotypes of the non-clonal variety (landrace) Sorrento, previously sampled and conserved at the CNR-IBAF repository (Porano, TR) were also included in this study. Foroni et al (2007) pointed out that Italian walnut varieties are usually composed of genotypes that are similar but not identical. For this reason, they are defined as "landraces" and labeled using the name of their region of origin (e.g., Sorrento).…”

“…These data were confirmed by analyzing chloroplast PCR-RFLP markers in 29 Euro-Asiatic populations (Fornari et al 2001), and a marked erosion of genetic resources was supposed. Recently Woeste et al (2002) developed a panel of 30 nuclear microsatellites (SSR) for a wide range of genetic investigations in Juglans, including clonal identification (Robichaud et al 2006;Dangl et al 2005;Foroni et al 2007), a broad-scale study of the genetic structure of Juglans nigra populations in the Central Hardwood Region of the United States of America (Victory et al 2006), and the identification of hybridogenic walnut plants (Pollegioni et al 2009a, b). In a recent study, Gunn et al (2010) evaluated 220 walnut trees from six Tibetan villages in China using 14 SSR markers.…”

DNA markers identify hybrids between butternut (Juglans cinerea L.) and Japanese walnut (Juglans ailantifolia Carr.)

“…About 100 mg of fresh leaf tissue and two 1/4 in. cylindrical ceramic beads (Bio 101-Savant, Carlsbad, CA, (Victory et al 2006;Foroni et al 2007). After grinding, samples were incubated at 65°C overnight.…”