Sesame (Sesamum indicum L.) has been cultivated in Asia for over 5000 years. In China, sesame is one of the four major oil crops, along with rapeseed, soybean, and peanut. On average (from 2001 to 2010), over 627,000 hectares of sesame are harvested annually, producing over 663,000 tons of sesame seeds, representing about 20% of the world’s production . Furthermore, China has been identified as one of the five sesame diversity centers in classical studies [2, 3]. As of 2012, the national gene bank of China has collected, reproduced, and preserved 5550 accessions of sesame.
Abundant plant germplasm resources provide a broad genetic foundation for plant breeding and genetic research. However, large germplasm resources are also difficult to preserve, evaluate, and use . Establishing a core collection (CC) is a favored approach for the efficient exploration and utilization of novel variation in genetic resources [5, 6]. The concept of a CC was first proposed by Frankel  and later developed by Brown . It involves the selection of a subset from the whole germplasm by certain methods in order to capture the maximum genetic diversity of the whole collection while minimizing accessions and redundancy. To date, CC have been established for many plant species around the world, including peanut [9, 10], barley , ryegrass , soybean [13, 14], safflower , rice [6, 16], olive [17, 18], Brassica rapa, Cornus officinalis, Arabidopsis thaliana, Medicago truncatula, and Vitis vinifera. To increase the usefulness of CC, genetic information must be clearly identified and documented .
To further reduce the duplication of some accessions in a CC, a ‘mini-core collection’ (MC) can serve as a small, representative subset of the CC. MC have been developed and evaluated for chickpea , peanut [26, 27], pigeon pea , maize , sorghum , rice [6, 31, 32], and other crops, promoting the utilization of genetic resources for these plants. For example, Upadhyaya  investigated the variability in drought resistance-related traits in the 184 entries of a MC for peanut. The results suggested certain accessions that can be used in peanut improvement programs to develop cultivars with a broad genetic base. Chamberlin et al.  evaluated a U.S. peanut MC using a molecular marker for resistance to Sclerotinia minor Jagger. They identified 39 accessions as new potential sources for resistance and targets for further evaluation. Using association analysis, Li et al.  mapped quantitative trait loci (QTLs) for improving grain yield using the USDA rice MC. Wang et al.  conducted association analysis of seed quality traits in a U.S. peanut (Arachis hypogaea L.) MC. In addition, Sharma et al.  identified new sources of resistance to Fusarium wilt and sterility mosaic disease using a pigeon pea MC and found that the diverse accessions with resistance would be useful in pigeon pea resistance breeding programs.
India, China, and Korea are the world’s leading countries for sesame germplasm collection and preservation, as well as research on sesame CC establishment. Bisht et al.  investigated 19 phenotypic and agronomic traits in 3129 sesame accessions from seven eco-geographical regions in India and established a sesame CC consisting of 362 accessions in India. Kang et al.  investigated 12 agronomic traits in 2246 sesame accessions from ten agro-climate zones preserved in the Rural Development Administration (RDA) Genebank in Korea and established a sesame CC of 475 accessions. In China, a systematic study of technical methods for the establishment of a sesame CC was conducted in cooperation with the International Plant Genetic Resources Institute (IPGRI). A sesame CC containing 453 accessions was established from the basic collection (BC) of 4251 accessions collected in China and 15 other countries using Ward’s clustering method and a stratified sampling strategy based on data for 14 phenotypic traits . The major objective for establishing sesame germplasm CC in China, Korea, and India has been to utilize germplasm more effectively. However, up until now, no comprehensive study of sesame CC genetic diversity has been carried out at either a phenotypic level or molecular level.
This study examined the genetic diversity of accessions from the sesame CC in China. The objectives of this investigation were to comprehensively explore the characteristics of genetic diversity at both a phenotypic level and a molecular level, and then to provide a theoretical foundation for effectively protecting and utilizing sesame genetic resources. Furthermore, a MC was extracted using an advanced maximization strategy based on both phenotypic and molecular data with the aim of promoting reasonable and efficient applications of sesame accessions in breeding and genotypic biological studies.