A. trifoliata is an important oil crop. Most studies of the species have focused on active components, such as quinatic acid [21], triterpene saponins [22], and akebiaoside K [23]. Only a few studies have evaluated the biology of A. trifoliata. For example, Zou et al. [24] studied recurrent somatic embryogenesis and the development of somatic embryos. Niu et al [6] developed SSR markers via de novo transcriptome assembly and Zou et al [25] showed the effectiveness of recurrent selection in A. trifoliatabreeding. However, this approach is not conducive to the development of A. trifoliata as an oil crop.
In this study, the collected 955 A. trifoliata germplasms were not registered, and so the geographical origin of each germplasm resource was unclear, which limits our understanding of the genetic diversity of germplasm resources and the development of a core collection. However, SSR molecular marker technology is not affected by geographical origin and complex factors, such as collection organs, development period, and external environment, and results in high polymorphism, stable results, and good repeatability.
Progress in A. trifoliata breeding has been slow, in part because it is a perennial plant and new plants do not bear fruit for 4 years [26]. Therefore, the generation of new A. trifoliata varieties is time-consuming. Furthermore, little is known about the biological characteristics of A. trifoliata, making it difficult to choose good parents. A. trifoliata has many uses that may guide breeding. For example, the consumption of A. trifoliata fruits is limited by the thick skin and abundant seeds [27], suggesting that breeding for thin skin and fewer seeds will improve market value. Similarly, A. trifoliata can be cultivated for use as an oil crop by focusing on seed properties. Molecular genetic markers are widely used in plant breeding, and genetic diversity must be considered when identifying trait populations and choosing parental strains to ensure the success of breeding. The results obtained in this study deepen our understanding of the genetic diversity of germplasm resources and facilitate the rational utilization of germplasm resources.
In this study, an SSR analysis of 955 A. trifoliata germplasms was performed to evaluate genetic diversity. In a previous study, 49 pairs of SSR markers were used to analyze 88 A. trifoliata germplasms [6]; PIC and HO values were 0.43 and 0.2210, respectively, similar to those in our study (PIC = 0.41; HO = 0.2382), thereby verifying that the species is moderately polymorphic (0.25 < PIC < 0.5). Additionally, 14 pairs of EST–SSR markers have been used to evaluate polymorphisms in 106 individuals from four natural populations of Dysosma versipellis (Berberidaceae) [28], with average Na, HO, HE, and PIC values at 6.286, 0.296, 0.534, and 0.467, which were higher than the corresponding values in this study, but still demonstrated a moderate level of polymorphism. With respect to other oil plants, A. trifoliata polymorphism was similar to that in sesame (Sesamum indicum L.) [29] and peanut (Arachis hypogaea L.) [30], but lower than that estimated in maize (Zea mays L.) [31], soybean (G. max) [32], and sunflower (Helianthus annuus L.) [33].
Abundant crop germplasm resources are the basis of crop breeding. However, excessive germplasms have various limitations. For example, it is difficult to precisely and rapidly identify useful resources for plant breeders. The management and preservation of germplasm resources is expensive and time-consuming; a core collection can effectively resolve these issues [34]. This study demonstrates the feasibility of establishing a core germplasm collection in perennial oil crops and is the first core collection established in A. trifoliata. Although core collections have been reported for some oil crops, most are not perennial crops. The core germplasm represented 17.1% of all accessions, which is higher than the range of 5–10% recommended by Brown [35] as well as the values reported in other plants, e.g., sesame (Sesamum indicum L.) (28/277) [8], maize (Zea mays L.) (951/13521) [9], and soybean (G. max) [10], whereas they are slightly less than those for the rubber tree (Hevea brasiliensis) (128/505) [36], ramie (Boehmeria nivea L.) (22/105) [37], and Gympie messmate (Eucalyptus cloeziana F. Muell., family Myrtaceae Juss.) (247/707) [38]. However, if we apply one additional filter, the Na and HO are reduced to 82.1% and 84.2% of those for the full population, and the core collection is reduced to eight genetic individuals. The maintenance of the vast majority of germplasm diversity should be a priority for guiding the determination of an optimal fraction; accordingly, we did not aim for a low rate of germplasm retention.
To the best of our knowledge, this study is the first to apply SSR markers to a large number of A. trifoliata germplasms. Estimates of genetic diversity and genetic structure can provide a foundation for future A. trifoliata breeding. The core collection can reduce the management cost and improve the protection of germplasm resources. However, the establishment of a core germplasm collection is a dynamic process and subsequent studies are needed to continuously improve the core collection of A. trifoliata.