Morphological to Molecular Markers: Plant Genetic Diversity Studies in Walnut (Juglans regia L.)—A Review

Persian walnut populations have tremendous morphological and allelic diversity in their germplasm due to heavy outcrossing and years of seed multiplication. These variations are assessed by morphological, cytological, biochemical, and molecular markers. Various researchers have used different tree, foliage, flower, nut, and kernel traits to evaluate morphological/phenotypic diversity. In walnut, morphological indices are first taken into consideration to describe and classify the germplasm, but the environment influences them. In comparison, DNA-based markers can detect genetic diversity at any stage of plant development and have been shown to be a potential tool for assessing variation at the DNA level and deciphering genetic relationships within and between species. Microsatellites are very powerful and informative among DNA-based markers in studying genetic relationships and genetic identity at different levels. They are neutral, highly frequent, uniformly distributed, hypervariable, codominant, highly reproducible, produce many alleles per locus, and require a small amount of DNA for analysis. Current breeding objectives can be achieved by selecting superior genotypes from the germplasm, supplemented by molecular characterization in the selection of parents for each breeding program. Therefore, the use of morphological and molecular markers is recommended for efficient exploration and utilization of germplasm resources and to improve diversity among genetic resources. The published literature on morphological and molecular markers, especially simple sequence repeats (SSRs), is presented in this review to provide current insights into the level of genetic diversity in walnut.