Submitted to: Genetic Resources and Crop Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 7, 2012
Publication Date: July 19, 2012
Citation: Bassil, N.V., Boccacci, P., Botta, R., Postman, J.D., Mehlenbacher, S. 2012. Nuclear and chloroplast microsatellite markers to assess genetic diversity and evolution in hazelnut species, hybrids and cultivars. Genetic Resources and Crop Evolution. 60:543-568. DOI:10.1007/s10722-012-9857-z. Interpretive Summary: The U.S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository in Corvallis, Oregon, preserves more than 800 hazelnut trees including cultivars and representatives of 10 other recognized shrub and tree types. Characterization and study of genetic diversity in this collection requires DNA-based information obtained from 'markers' that are conserved across this diverse hazelnut collection. Two such types of DNA markers were developed and used to evaluate 114 hazelnut plants from 11 species or types and 44 hybrid individuals derived from crosses between different species or types. Data analysis agreed with previous taxonomic analysis. Detected relationships among the different hazelnut types support Asia as a refugium where several hazelnut lineages survived during glaciation. These hazelnuts reappeared only later after the dispersal event from Asia through the Mediterranean to Europe, and across the Atlantic to North America.
Technical Abstract: The U.S. Department of Agriculture (USDA), Agricultural Research Service (ARS), National Clonal Germplasm Repository (NCGR) in Corvallis, Oregon, preserves more than 800 accessions of hazelnut (Corylus) including C. avellana cultivars and representatives of 10 other recognized shrub and tree species. Characterization and study of genetic diversity in this collection requires cross-transferable markers such as trinucleotide microsatellite or simple sequence repeat (SSR) markers and universal chloroplast SSR markers. We developed new SSR markers, and evaluated 114 Corylus accessions representing 11 species and 44 interspecific hybrids. Eight of 23 SSRs generated easy-to-score alleles in all species and seven were highly polymorphic. For the seven, the average heterozygosity was moderate at 0.49 while allele number, genetic diversity and PIC were high at 11.71, 0.79 and 0.76, respectively. The three most polymorphic SSRs were CaC-C008, CaC-C040 and CaC-C118. Neighbor joining (NJ) clustering and structure analysis agreed with taxonomic analysis and supported inclusion of C. maxima in the large polymorphic species C. avellana. Analysis also indicated that C. californica is a distinct species rather than a botanical variety of C. cornuta. Six universal cpSSRs were polymorphic in Corylus and generated an average of 3 alleles per locus and 21 chlorotypes. Diversity at these cpSSRs was high and ranged from 0.33 to 0.64, with an average of 0.69. Incongruence in NJ topologies between the nuclear and chloroplast markers could be attributed to chloroplast capture during the ancestral diversification of the genus, or homoplasy. The phylogeographical relationships among the 21 chlorotypes in the 11 Corylus species support Asia as a refugium where several hazelnut lineages survived during glaciation and from which they reappeared only later after the dispersal event from Asia through the Mediterranean to Europe, and across the Atlantic to North America.