Relationships of walnut cultivars in a germplasm collection: Comparative analysis of phenotypic and molecular data

Abstract

Assessment of the genetic relatedness of walnut cultivars with phenotypic data and molecular markers allows progress in conservation and management of the genetic resources, breeding and protection of breeders’ intellectual property. In the present study we used 24 morphological traits, 25 random amplified polymorphic DNAs (RAPDs) and 7 simple sequence repeats (SSRs), to study genetic diversity and relationships in walnut cultivars at different levels. The 20 analyzed accessions represent some of the most important Romanian and international walnut cultivars and typify a genetic diversity characteristic of a germplasm collection. The distances based on morphological and SSR markers were significantly correlated. The two DNA marker systems were uncorrelated and RAPD markers failed to describe the pattern of molecular similarity. All marker systems detected polymorphisms that were adequate for the discrimination of all cultivars. Morphological- and SSR-based genetic distances were related to a certain point and differed from RAPD-based genetic distances. Our data indicate that the type and the number of phenotypic traits evaluated can considerably alter the result of the analysis and combination of qualitative and quantitative data needs caution. Moreover, the data imply that the two molecular marker systems are useful for cultivar characterization, but SSR markers are more advisable to investigate genetic relationships. Also, they can be employed to complete and aid the traditional registration of varieties. We propose that, since the information provided by morphological and SSR marker systems in walnut is similar, they should serve for cultivar characterization and assessment in genebanks.