Abstract
Walnut (Juglans regia L.) is the most important species in the genus Juglans due to the high commercial value of its nuts and timber. Simple sequence repeats (SSRs) are considered as the markers of choice owing to their codominant nature. Since very few SSRs have been developed in walnut, this study aimed to develop numerous polymorphic SSRs from Class I repeats by using DNA sequences of the Chandler cultivar. In all, 800 SSRs were designed and tested in 8 Turkish, 3 French, and 5 US walnut cultivars, of which 88 (11%) did not produce bands, 161 (20.1%) were monomorphic, and 551 (68.9%) were polymorphic. In all, 2696 alleles were produced by the 551 polymorphic SSR loci in the 16 walnut cultivars, ranging from 2 to 14 alleles per locus with an average number of 4.9. Polymorphism information content ranged from 0.21 to 0.89 with an average of 0.62. Cluster analysis produced a very robust dendrogram. The walnut cultivars were separated into two main groups: all Turkish cultivars were included in one group, whereas the US and French cultivars were included in the other group. A set of 20 SSRs was selected for their high genetic diversity values to be used in further genetic studies in walnut. The novel SSR markers developed in this study could be used in future studies for constructing a genetic linkage map, analyzing population genetics, identifying parents, and conducting marker-assisted breeding, fingerprinting, and germplasm characterization in walnut.
Highlights
Juglans regia L. is a monoecious tree crop with a haploid chromosome number of n = 16 and it belongs to the family Juglandaceae (Forde and Griggs, 1972)
Genetic diversity values of polymorphic Simple sequence repeats (SSRs) loci A total of 2696 alleles were produced by the 551 polymorphic SSR loci in the 16 walnut cultivars, ranging from 2 to 14 alleles per locus (Na) with an average number of 4.9 (Supplementary File 2)
Fifty-seven polymorphic SSR loci produced only two alleles, whereas the JRHR216759 locus amplified the highest number of alleles (14 alleles) in the 16 walnut cultivars
Summary
About 20 Juglans species are included in the genus, J. regia is considered most important owing to its edible nuts (Manning, 1978). Walnut is pollinated by wind (Rom and Carlos, 1987) and its origin is believed to be in Central Asia and neighboring regions (Browicz, 1976). In Europe, J. regia is the only commonly cultivated species of the genus. Turkey is considered as one of the origins of the species with a high production of nuts (212,140 t/year), which is fourth after China, the United States, and Iran (http://www.fao.org). Walnuts have high nutritional value and contain proteins, fats, vitamins, and minerals and they are considered important for human nutrition (Gandev, 2007)
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