3.1 GBS analysis and SNP characterization
For the 39 Bupleurum accessions, GBS sequencing produced 1.3 billion clean reads, and more than 1.9 million reads per sample were retained for alignment. The number of clean reads ranged from 1,961,510 (HB2) to 6,702,434 (BJ2), with an average of 3,379207. The information for each of 39 accession reads was aligned to the simulated reference genome of BQ2, which has the highest tag number (5,192,250) (Table S1). In the 39 Bupleurum samples, an average of 2,657,398 (78.1%) reads were mapped to the reference genome. Among them, BJ3 showed the highest mapping rate (87.97%), and DY2 showed the lowest mapping rate (72.16%) (Table S2). After successful mapping, a total of 85,251 GBS SNPs were recognized, and 83,898 SNPs were obtained after filtering out multiple allele loci using Beagle software.
3.2 Population structure analysis
The K-value was calculated to evaluate the different clusters of these 39 Bupleurum samples based on 83,898 genome-wide SNPs with high-quality data. The cross-validation (CV) error was estimated to test the optimal K-value. From 2 to 11, the best K-value showed four groups with the highest probability of clustering according to the genetic background information instead of the sample collection location (Fig. 1). The four groups of 39 accessions accounted for 30.77%, 28.21%, 25.64% and 15.38%, respectively. Cluster Ⅰ contained 12 accessions of B. chinense, mainly from Hebei Province and Beijing, except for one sample from Heilongjiang Province (HB1, HB2, HB3, HB4, HB5, HB6, BJ2, BJ3, BJ4, BJ5, BJ7, MH1). Cluster Ⅱ included 11 accessions of B. chinense mainly from Shaanxi, Heilongjiang and Gansu Provinces (HRB1, HRB2, HRB3, DF4, DF5, DF6, GS1, GS2, GS3, GS4, GS6). Cluster Ⅲ included all 10 resources from Heilongjiang Province. However, these samples belong to different species. Three accessions were B. chinense (BQ1, BQ3, ZZ1), 3 samples were B. sibiricum Vest. (Q1, Q2, Q3), 3 accessions were B. longiradiatum Turcz. (DY1, DY2, DY3) and one sample was B. scorzonerifolium Willd. (MH3). Cluster Ⅳ contained the remaining 6 B. chinense resources from Heilongjiang Province (ZZ2, ZZ3, DQ1, DQ2, DQ3, BQ2).
Neighbour-joining (NJ) tree analysis was performed based on 83,898 SNPs and classified the 39 Bupleurum into three major groups: (1) B. chinense samples from Shaanxi Province; (2) B. chinense accessions from Gansu, Hebei Province and Beijing; and (3) B. chinense, B. scorzonerifolium, B. sibiricum and B. longiradiatum resources from Heilongjiang Province. Group Ⅱ was split into 2 subgroups on the basis of the population structure results. The first subgroup mainly corresponded to samples from Gansu Province, and the second subgroup corresponded to samples from Hebei Province and the city of Beijing (Fig. 2).
The relationship between the 39 Bupleurum accessions was explored based on the broad geographical regions using PCA. The samples showed a relatively tight distribution in the PCA results (Fig. 3). The majority of the B. chinense accessions from Hebei Province and the city of Beijing were distributed in group Ⅰ, resources of B. chinense from Gansu and Shaanxi Provinces were clustered in group Ⅱ. Group Ⅲ contained the other three “Chai-hu” species from Heilongjiang Province, and group Ⅳ clustered accessions of B. chinense from Heilongjiang Province.
3.3 Genetic diversity analysis
The tested Bupleurum materials were divided into 11 populations according to geographical origin and species (Table 1). The observed heterozygosity (Ho) ranged from 0.21 to 0.27 with an average of 0.24. The expected heterozygosity (He) value ranged from 0.15 to 0.19 with an average of 0.17. The polymorphic loci number (PLN) ranged from 30,724 to 45,778, and the average was 37,358. The percentage of polymorphic loci (PPL) ranged from 38.97 to 58.07, and the average PPL was 47.38. The mean heterozygosity, Ho, was greater than the expected heterozygosity, He, indicating that the genetic diversity of Bupleurum was narrow (Table 2).
Table 2
Genetic diversity of Bupleurum from different geographical resources and species
Origin | Species | Number | Ho | He | PLN | PPL/% |
Hebei | B. chinense DC. | 6 | 0.2427 | 0.1718 | 41196 | 52.2533 |
Beijing | 5 | 0.2082 | 0.1731 | 43285 | 54.903 |
Gansu | 5 | 0.2419 | 0.1862 | 45778 | 58.0652 |
Harbin | 3 | 0.2485 | 0.1774 | 38116 | 48.3466 |
Daqing | 3 | 0.2452 | 0.1766 | 38409 | 48.7183 |
Zhaozhou | 3 | 0.2157 | 0.1538 | 32949 | 41.7928 |
Qiqihar | 3 | 0.2701 | 0.1919 | 41160 | 52.2077 |
Shaanxi | 3 | 0.2262 | 0.1566 | 33302 | 42.2405 |
Heilongjiang | B. scorzonerifolium Willd. | 2 | 0.2359 | 0.1599 | 30724 | 38.9706 |
Heilongjiang | B. sibiricum Vest. | 3 | 0.2299 | 0.157 | 32587 | 41.3336 |
Heilongjiang | B. longiradiatum Turcz. | 3 | 0.2719 | 0.1711 | 33439 | 42.4143 |
Average | | | 0.24 | 0.17 | 37358.63 | 47.38 |
The genetic distance (GD) value between 11 populations from different geographical resources and species was -0.3916 to 0.2845, indicating that the genetic distance between populations was small and that the genetic basis was relatively narrow. Among these different species, B. scorzonerifolium Willd. showed the maximum genetic distance from B. chinense DC. from Heilongjiang Province (0.068), indicating that these resources possess different genetic background information but have the same effect as the Chinese traditional medicine “Chai-hu” (Table 3). The nucleotide diversity analysis of each population found that the mean nucleotide diversity (π) of the total loci was 0.2019, indicating that polymorphisms within populations were relatively rare. The AMOVA results revealed that the genetic variation within populations (23.36%) was greater than that among populations (2.46%), indicating that relatively high gene information exchange occurred within populations and low differentiation occurred among populations (Table 4).
Table 3
Genetic distance matrix of Bupleurum samples from different geographical and species resources
| HB | BJ | GS | BQ | HRB | DQ | MH | ZZ | Q | DY | DF |
HB | 0 | | | | | | | | | | |
BJ | 0.0821 | 0 | | | | | | | | | |
GS | 0.2138 | 0.1201 | 0 | | | | | | | | |
BQ | 0.1817 | 0.0212 | 0.1724 | 0 | | | | | | | |
HRB | 0.2854 | 0.1422 | 0.2380 | 0.1409 | 0 | | | | | | |
DQ | 0.1899 | 0.1932 | 0.1028 | 0.2958 | 0.1436 | 0 | | | | | |
MH | 0.4505 | 0.2377 | 0.1085 | 0.6804 | 0.3124 | 0.3964 | 0 | | | | |
ZZ | 0.1506 | 0.2650 | 0.0173 | 0.2029 | 0.2369 | 0.4547 | 0.4427 | 0 | | | |
Q | 0.4181 | 0.2036 | 0.0861 | 0.3977 | 0.0866 | 0.3710 | 0.5262 | 0.4284 | 0 | | |
DY | 0.3040 | 0.0640 | 0.1581 | 0.2257 | 0.0289 | 0.1311 | 0.3916 | 0.2587 | 0.4742 | 0 | |
DF | 0.2559 | 0.1151 | 0.2566 | 0.2600 | 0.0259 | 0.4367 | 0.3370 | 0.2132 | 0.5405 | 0.4465 | 0 |
Table 4
Analysis of molecular variance (AMOVA) for 39 Bupleurum accessions
Source of variation | df | Sum of squares | Variance components | Percentage of variation |
Among populations | 10 | 71131.032 | 191.35821 Va | 2.46 |
Among individuals within populations | 28 | 161578.25 | -1815.00744 Vb | -23.34 |
Within individuals | 39 | 366626 | 9400.66667 Vc | 120.88 |
Total | 77 | 599335.282 | 7777.01743 | |
ZZ2 | 2603018 | 1741851 | 66.92% | 27.39 | 28.65% | 17.28% |
ZZ3 | 4030106 | 3343969 | 82.97% | 40.52 | 36.55% | 22.55% |