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

Abstract

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 considered the first 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 SSRs, is presented in this review to provide current insights into the level of genetic diversity in walnut.

Molecular Characterization of Local Walnut (Juglans regia) Genotypes in the North-East Parnon Mountain Region of Greece

“…Nevertheless, the outcomes of our study are consistent with prior investigations into genetic diversity among walnut genotypes using SSR markers. It is important to acknowledge that the genetic and phylogenetic distances among the genotypes can also influence the level of polymorphism detected by SSR markers, particularly owing to the outcrossing nature of walnuts [26].…”

“…Also, we observed that the average PIC value was higher when compared to the previously mentioned studies. These differences in PIC values among studies may arise from the utilization of distinct SSR markers or from the origins of samples, which could encompass considerably more diverse regions, potentially spanning different continents [26]. Notably, the Eurasia region, which includes Greece, has been recognized for its notably high levels of genetic diversity in walnuts [57].…”

“…Given their property of co-dominant inheritance, SSR markers can effectively differentiate heterozygous individuals (bearing two differing alleles) from homozygous individuals (possessing two identical alleles), a characteristic that contributes to precise genotype identification and the comprehensive exploration of genetic diversity [25]. Moreover, the variable count of repeat units within microsatellites enables discrimination among closely related individuals, an assessment of genetic diversity, and differentiation between various cultivars or breeding lines [26].…”