1. Introduction
Yam is considered the common name for plants, with over 600 species ofDioscorea genus. The species of yams widely cultivated in the world are D. bulbifera , D. panthaica , D. esculenta ,D. japonica , D. trifida , D. pentaphylla , andD. rotundata (Lebot, 2009). Yams are also one of the 10 most important edible tuber and root plants in the world and next to potato, cassava, and sweet potato in yield (Shewry, 2003). They play an important role in sustaining many livelihoods in the tropics and subtropics (Tschannen et al., 2005; Wu et al., 2019). In China, yam is known as a medicinal and edible crop with high nutritional and medicinal value; its tuber is rich in starch, protein, and medicinal ingredients (e.g., allinogenin, diosgenin, and dehydroepiandrosterone; Lebot, et al., 2019).
In China, yam resources are extremely rich, with a total of 93 species.D. opposita , D. alata ,D. persimilis , D. fordii , and D. quinquelaba are widely cultivated in China nowadays (Huang, 2011). Yams are cultivated in other provinces, except Qinghai and Tibet, including a large number of local varieties. However, they have long been regarded as “orphan” or “neglected” crops despite their considerable edible and medicinal value and received little attention or investment from researchers (Tamiru et al. , 2017). Moreover,Dioscorea is mainly dioecious, rarely flowering, and has difficult forming mature seeds (Bressan et al. , 2011). The development of medicinal ingredients from a few species of yam has long been emphasized in China (Li et al., 2018; Lebot et al., 2018; Cheng et al., 2020), but the analysis of the resource types and genetic diversity of yam is insufficient. Moreover, several studies only focused on D. alata , and the method of resource evaluation is relatively simple (Siqueira et al., 2014; Arnau et al., 2017; Wu et al., 2019). In the long-term cultivation and domestication process, the variation types of yam are complex, and a single classification method is difficult to identify, thereby causing confusion in various records and nomenclatures and even in the classification of some species. These factors seriously hinder the resource conservation and further utilization of yam.
Therefore, studying the genetic diversity, genetic variation, and population structure of yam is considerably important to its origin, distribution, resource utilization, parental selection, and development (Mignouna et al., 2003). So far, phenotypic traits, isozyme, karyotype analysis and DNA diversity have been used to describe the genetic diversity of germplasm (Sartie et al., 2012; Nemorin et al. , 2013; Kouam et al., 2018; Ghorbani et al., 2020; Cao et al., 2020). However, research on the genetic diversity of yams has focused on a few species in Africa and the Americas. Anokye et al. (2014) used morphological descriptors to dissect the phenotypic diversity ofD. alata from Ghana and Nigeria and divided these materials into different clusters independent of geographic origin. Mignouna et al. (2002) evaluated the genetic variation of 45 cultivated yams (D .cayenensis /D. rotundata complex) from Cameroon by using morphology and isozyme, and the clustering results of the two were relatively consistent. Silva et al. (2016) used 12 microsatellites and four morphological markers to analyze the genetic diversity and structure of 89 yam (D . alata ) in Brazil; a high diversity is found in Brazilian yam, but it is not related to geographical location. At present, despite some reports available on the genetic diversity of Chinese yam local varieties, the research methods were too single, the number of molecular markers was small, and the representation of germplasm source regions was poor.
In the present study, large-scale yam resources were collected in 21 provinces, including the main species used in cultivation to fully identify and evaluate the genetic information of yams in China. The genetic diversity, genetic relationship, and population structure of yams were evaluated using the combination analysis of phenotypic traits and two molecular markers (SRAP and SSR). The results could provide a basis for yam classification, breeding, germplasm innovation, utilization, and variety protection.