The USDA cucumber (Cucumis sativus L.) collection: genetic diversity, population structure, genome-wide association studies, and core collection development

Germplasm collections are a crucial resource to conserve natural genetic diversity and provide a source of novel traits essential for sustained crop improvement.Optimal collection,preservation and utilization of these materials depends upon knowledge of the genetic variation present within the collection.Here we use the high-throughput genotyping-by-sequencing(GBS)technology to characterize the United States National Plant Germplasm System(NPGS)collection of cucumber(Cucumis sativus L.).The GBS data,derived from 1234 cucumber accessions,provided more than 23 K high-quality single-nucleotide polymorphisms(SNPs)that are well distributed at high density in the genome(~1 SNP/10.6 kb).The SNP markers were used to characterize genetic diversity,population structure,phylogenetic relationships,linkage disequilibrium,and population differentiation of the NPGS cucumber collection.These results,providing detailed genetic analysis of the U.S.cucumber collection,complement NPGS descriptive information regarding geographic origin and phenotypic characterization.We also identified genome regions significantly associated with 13 horticulturally important traits through genome-wide association studies(GWAS).Finally,we developed a molecularly informed,publicly accessible core collection of 395 accessions that represents at least 96%of the genetic variation present in the NPGS.Collectively,the information obtained from the GBS data enabled deep insight into the diversity present and genetic relationships among accessions within the collection,and will provide a valuable resource for genetic analyses,gene discovery,crop improvement,and germplasm preservation.

Genetic characterization of melon accessions in the U.S. National Plant Germplasm System and construction of a melon core collection

Melon(C.melo L.)is an economically important vegetable crop cultivated worldwide.The melon collection in the U.S.National Plant Germplasm System(NPGS)is a valuable resource to conserve natural genetic diversity and provide novel traits for melon breeding.Here we use the genotyping-by-sequencing(GBS)technology to characterize 2083 melon accessions in the NPGS collected from major melon production areas as well as regions where primitive melons exist.Population structure and genetic diversity analyses suggested that C.melo ssp.melo was firstly introduced from the centers of origin,Indian and Pakistan,to Central and West Asia,and then brought to Europe and Americas.C.melo ssp.melo from East Asia was likely derived from C.melo ssp.agrestis in India and Pakistan and displayed a distinct genetic background compared to the rest of ssp.melo accessions from other geographic regions.We developed a core collection of 383 accessions capturing more than 98%of genetic variation in the germplasm,providing a publicly accessible collection for future research and genomics-assisted breeding of melon.Thirty-five morphological characters investigated in the core collection indicated high variability of these characters across accessions in the collection.Genome-wide association studies using the core collection panel identified potentially associated genome regions related to fruit quality and other horticultural traits.This study provides insights into melon origin and domestication,and the constructed core collection and identified genome loci potentially associated with important traits provide valuable resources for future melon research and breeding.