Distribution of Chinese sweet potato landraces
132 sweet potato landraces were from different provinces in China. As shown in Fig. 1, the largest number of materials came from Guangdong Province, with a total of 87 materials, 15 materials from Hainan Province, 6 materials each from Guangxi Zhuang Autonomous Region and Guizhou Province, 5 materials from Yunnan Province, 4 materials each from Fujian Province and Sichuan Province, 1 material each from Jiangsu Province, Taiwan Province, Zhejiang Province, Hunan Province, and Anhui Province.
Diversity Analysis of the phenotypic traits
Most traits of sweet potato landraces showed significant phenotypic and physiological differences under investigation. The genetic diversity of 20 phenotypic traits of 132 Chinese sweet potato landraces was analyzed, and different phenotypic traits showed different diversity. As shown in Table 1, the color of top leaf, the color of top bud, and the leaf color accounted for the largest proportion of green, especially the leaf color, with a frequency as high as 79.55%, indicating that the overall color of plants was mainly green. The shape of top leaf and the shape of leaf were mostly incised. The main vein pigmentation color and the pigmentation of basic leaf vein were mostly purple, the side vein pigmentation color was mainly green, and the leaf apex shape was mainly acute. The petiole predominant color and the pigmentation of basic petiole were mainly investigated. Green was the dominant color of the petiole predominant color, with a frequency of 80.30%, while the pigmentation of basic petiole was mostly green and purple. Four traits of the vine were investigated, including the color of vine, the predominant color of vine, the secondary color of vine, and the vine tip pubescence. Among them, green was the highest in the color of vine and the predominant color of vine, both frequencies were close to 80%. The secondary color of vine was mainly purple with a small amount of none. The vine tip pubescence distribution was relatively uniform, with the highest frequency of none, followed by little. There were mainly two types of plant types, semi-erect and prostrate, mainly prostrate. Three traits were investigated for the storage root: the storage root shape, the skin color of storage root, and the predominant flesh color. The storage root shape was mainly elliptic, and the other storage root shapes were distributed except curve. The skin color of storage root was mauve, followed by white. The dominant color of predominant flesh color was white, followed by light yellow.
The coefficient of variation of 20 traits ranged from 92.25% to 17.83%, with a mean value of 45.84%. The highest coefficient of variation (92.25%) was found for stem end velvet and the lowest coefficient of variation (17.83%) was found for plant type. The Shannon diversity index of 20 traits ranged from 2.05 to 0.61, with an average value of 1.16. 8 traits were exceeding the average value, among which the Shannon diversity index of the skin color of storage root was the highest (2.05). Although mauve was the main color, other colors were also distributed, indicating that the genetic diversity of this trait was high. The Shannon diversity index of the leaf apex shape was the smallest (0.61), indicating that the genetic diversity of this trait was low.
Correlation analysis of the 20 phenotypic traits showed that out of 190 pairs, 43 pairs showed significant correlations (P<0.05) and 31 pairs showed highly significant correlations (P<0.01). There were 35 pairs of traits with significant positive correlations, including 24 pairs with highly significant positive correlations, and 8 pairs of traits with significant negative correlations, including 3 pairs with highly significant negative correlations. The highest correlation coefficient (0.83) was found for the shape of the terminal leaf and leaf traits, indicating a very high correlation between these two traits (Fig. 2).
Table 1 Phenotypic traits diversity of sweet potato landraces
Trait
|
Characteristic description (proportion of distribution, %)
|
Shannon diversity index(H')
|
CV (%)
|
0
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
Color of top leaf
|
|
Light green
|
Green
|
Purple green
|
Brown green
|
Light purple
|
Purple
|
Brown
|
Golden yellow
|
Red
|
|
1.68
|
57.92
|
|
11.36
|
41.67
|
3.79
|
11.36
|
5.30
|
18.18
|
7.58
|
0.76
|
0.00
|
|
Shape of top leaf
|
|
Round
|
Reniform
|
Cordate
|
Acuminate-cordate
|
Triangular
|
Incised
|
|
|
|
|
1.11
|
28.09
|
|
0.00
|
0.00
|
30.30
|
12.12
|
5.30
|
52.27
|
|
|
|
|
Color of top bud
|
|
Light green
|
Green
|
Light purple
|
Purple
|
Dark purple
|
Brown
|
|
|
|
|
1.45
|
53.55
|
|
15.15
|
50.76
|
7.58
|
13.64
|
5.30
|
7.58
|
|
|
|
|
Leaf color
|
|
Light green
|
Green
|
Purple green
|
Brown green
|
Light purple
|
Purple
|
Brown
|
Golden yellow
|
Red
|
|
0.75
|
42.65
|
|
9.85
|
79.55
|
6.82
|
0.76
|
0.00
|
2.27
|
0.00
|
0.76
|
0.00
|
|
Shape of leaf
|
|
Round
|
Reniform
|
Cordate
|
Acuminate-cordate
|
Triangular
|
Incised
|
|
|
|
|
0.99
|
27.83
|
|
0.00
|
0.76
|
28.79
|
6.82
|
3.03
|
60.61
|
|
|
|
|
Main vein pigmentataion
|
|
Light green
|
Green
|
Yellow
|
Light purple
|
Purple
|
Purple speckle
|
|
|
|
|
1.53
|
39.79
|
|
9.85
|
17.42
|
0.76
|
18.18
|
38.64
|
15.15
|
|
|
|
|
Side vein pigmentation
|
|
Light green
|
Green
|
Yellow
|
Light purple
|
Purple
|
Purple speckle
|
|
|
|
|
1.55
|
46.35
|
|
9.85
|
31.82
|
0.76
|
17.42
|
28.03
|
12.12
|
|
|
|
|
Pigmentation of basic leaf vein
|
|
Light green
|
Green
|
Light purple
|
Purple
|
Dark purple
|
|
|
|
|
|
1.16
|
24.68
|
|
0.00
|
16.67
|
13.64
|
56.82
|
12.88
|
|
|
|
|
|
Leaf apex shape
|
Absent
|
Acute
|
Blunt
|
|
|
|
|
|
|
|
|
0.61
|
35.27
|
0.00
|
69.70
|
30.30
|
|
|
|
|
|
|
|
|
Petiole predominant color
|
|
Light green
|
Green
|
Light purple
|
Purple
|
Dark purple
|
|
|
|
|
|
0.75
|
32.97
|
|
2.27
|
80.30
|
7.58
|
6.82
|
3.03
|
|
|
|
|
|
Pigmentation of basic petiole
|
|
Light green
|
Green
|
Light purple
|
Purple
|
Dark purple
|
|
|
|
|
|
1.27
|
33.22
|
|
0.76
|
38.64
|
17.42
|
35.61
|
7.58
|
|
|
|
|
|
Color of vine
|
|
Light green
|
Green
|
Brown
|
Light purple
|
Purple
|
Dark purple
|
|
|
|
|
0.78
|
48.51
|
|
1.52
|
78.79
|
5.30
|
0.00
|
8.33
|
6.06
|
|
|
|
|
Predominant color of vine
|
|
Light green
|
Green
|
Mauve
|
Light purple
|
Purple
|
Dark purple
|
Brown
|
|
|
|
0.82
|
52.34
|
|
1.52
|
77.27
|
2.27
|
0.00
|
12.88
|
3.79
|
2.27
|
|
|
|
Secondary color of vine
|
Absent
|
Green
|
Mauve
|
Purple
|
Brown
|
|
|
|
|
|
|
1.02
|
83.92
|
32.58
|
16.67
|
1.52
|
38.64
|
10.61
|
|
|
|
|
|
|
Vine tip pubescence
|
None
|
Little
|
Moderate
|
More
|
|
|
|
|
|
|
|
0.93
|
92.25
|
35.61
|
25.76
|
18.94
|
19.70
|
|
|
|
|
|
|
|
Plant type
|
|
Erect
|
Semi-erect
|
Prostrate
|
Scramble
|
|
|
|
|
|
|
0.64
|
17.83
|
|
0.00
|
34.09
|
65.91
|
0.00
|
|
|
|
|
|
|
Vigour of plant
|
|
Strong
|
Intermediate
|
Weak
|
|
|
|
|
|
|
|
0.95
|
38.85
|
|
43.18
|
46.97
|
9.85
|
|
|
|
|
|
|
|
Storage root shape
|
|
Rotundity
|
Short elliptic
|
Elliptic
|
Long elliptic
|
Obovate
|
Ovate
|
Rectangle
|
Curve
|
Anomaly
|
|
1.59
|
37.64
|
|
1.52
|
3.03
|
43.94
|
10.61
|
10.61
|
21.97
|
6.82
|
0.00
|
1.52
|
|
Skin color of storage root
|
|
White
|
Light yellow
|
Brown yellow
|
Yellow
|
Brown
|
Pink
|
Red
|
Mauve
|
Purple
|
Dark purple
|
2.05
|
54.05
|
|
18.94
|
6.82
|
4.55
|
9.85
|
4.55
|
6.82
|
12.12
|
26.52
|
9.09
|
0.76
|
Predominant flesh color
|
|
White
|
Light yellow
|
Brown yellow
|
Yellow
|
Brown
|
Pink
|
Red
|
Mauve
|
Purple
|
Dark purple
|
1.61
|
69.06
|
|
32.58
|
25.00
|
19.70
|
13.64
|
4.55
|
0.00
|
0.76
|
0.00
|
3.79
|
0.00
|
CV: the coefficient of variation.
Diversity of carotenoids
The content of 13 carotenoid monomers and the total content (the sum of 13 carotenoid monomers) of 132 sweet potato landraces in China were determined. The carotenoids in different sweet potato varieties were different. As shown in Table 2, 2 out of 13 carotenoid monomers were not detected in all materials and no carotenoids were detected in 38 materials. Among the 11 detected monomers, the average content of β-carotene was the highest, and the average content of β-cryptoxanthin was the lowest. The average total content of carotenoids was 6.33 μg/g, while the average content of β-carotene was 4.32 μg/g. The variety with the highest total carotenoid content (59.90 μg/g) also has very high β-carotene content (55.54 μg/g). After counting, the 55 materials had the highest total carotenoid content of β-carotene, indicating that β-carotene may be the predominant carotenoid in the sweet potato. Carotenoid monomer content and total content had a high coefficient of variation, indicating that there were significant differences in carotenoid content between Chinese sweet potato landraces, and the carotenoid content of landraces was rich in diversity.
Correlation analysis of 11 carotenoid monomers and total content showed that among the 66 pairs of combinations, 27 pairs of carotenoid monomers showed a significant positive correlation (P<0.05), of which xx pairs of carotenoid monomers showed a highly significant positive correlation (P<0.01); 10 carotenoid monomers showed a significant positive correlation between monomeric substances and total content, of which 9 carotenoid monomers showed a significant positive correlation between monomeric substances and The correlation between 10 carotenoid monomers and the total content was highly significant, and 9 carotenoid monomers and the total content were highly significant. The highest correlation coefficients among carotenoid monomer substances were lutein and α-cryptoxanthin (0.99), indicating a very high correlation between these two monomer substances; the highest correlation coefficient with the total content was β-carotene (0.99), indicating a very high correlation between β-carotene and the total content (Fig. 2).
Table 2 Carotenoid content diversity of sweet potato landraces
|
Mean
|
Standard Deviation
|
Max
|
Min
|
CV (%)
|
Violaxanthin
|
0.802
|
0.715
|
3.244
|
0.000
|
89.174
|
Neoxanthin
|
0.253
|
0.545
|
2.850
|
0.000
|
215.294
|
Antheraxanthin
|
0.884
|
0.988
|
4.140
|
0.000
|
111.734
|
Lutein
|
0.006
|
0.056
|
0.638
|
0.000
|
1009.910
|
Zeaxanthin
|
0.004
|
0.036
|
0.342
|
0.000
|
822.049
|
α-Cryptoxanthin
|
0.007
|
0.079
|
0.916
|
0.000
|
1107.100
|
ε-Carotene
|
0.016
|
0.082
|
0.515
|
0.000
|
518.563
|
β-Cryptoxanthin
|
0.002
|
0.011
|
0.096
|
0.000
|
683.719
|
α-Carotene
|
0.000
|
0.001
|
0.016
|
0.000
|
1144.552
|
β-Carotene
|
4.321
|
10.360
|
57.417
|
0.000
|
239.763
|
6R-δ-Carotene
|
ND
|
ND
|
ND
|
ND
|
ND
|
γ-Carotene
|
0.037
|
0.129
|
1.151
|
0.000
|
348.041
|
Lycopene
|
ND
|
ND
|
ND
|
ND
|
ND
|
All
|
6.331
|
11.470
|
59.904
|
0.000
|
181.170
|
CV: the coefficient of variation. ND: not detected.
SSR markers diversity
A total of 135 gene loci were amplified by PCR using 10 pairs of SSR primers. In all loci, the average number of alleles (Na) was 1.68, the average effective number of alleles (Ne) was 1.25, there were 92 polymorphic loci, the percentage of polymorphic loci was 68.15%, Nei's genetic diversity index (He) was 0.15, and Shannon's diversity index (I) was 0.24. Among single primers, the minimum value of Na was 1.44 (SPGS2) and the maximum value was 1.91 (GDAAS0922). The minimum value of Ne was 1.12 (SPGS2) and the maximum value was 1.42 (GDAAS0782). The minimum of H ' is 0.08 (SPGS2) and the maximum is 0.25 (GDAAS0782). The minimum value of I ' was 0.13 (SPGS2) and the maximum value was 0.38 (GDAAS0782) (Table 3).
Population structure analysis
According to the results of STRUCTURE, the highest value of ΔK was obtained at K=5 (Fig. 3a). Based on this result, 132 sweet potato landraces were divided into 5 sub-populations (Fig. 3c). Sub-population I consisted of 37 materials, accounting for 28.03 %, which were from Guangdong (23), Hainan (5), Sichuan (3), Guangxi (2), Guizhou (2), Fujian (1), and Yunnan (1). There are 27 materials in sub-population II, accounting for 20.45%, respectively from Guangdong (23), Hainan (3), and Guizhou (1). Guangdong materials account for the highest proportion (85.19%) in this sub-population Sub-population III had 14 materials, accounting for 10.61 %, respectively from Guangdong (9), Hainan (3), Yunnan (1), and Anhui (1). There were 17 materials in sub-population IV, accounting for 12.88 %, which were from Guangdong (12), Hainan (2), Guangxi (2), and Hunan (1). There were 37 materials in sub-population V, accounting for 28.03 %, which were from Guangdong (20), Guizhou (3), Fujian (3), Yunnan (3), Guangxi (2), Hainan (2), Zhejiang (1), Taiwan (1), Jiangsu (1) and Sichuan (1). As shown in the phylogenetic tree, the distance between sub-population I and sub-population V was the closest, indicating that sub-population I and sub-population V had a similar genetic relationship, and the distance between sub-population III and sub-population IV was the farthest, indicating that the genetic relationship between the two was very far (Fig. 3d).
The mean fixation index (Fst) of 5 sub-populations was determined. The Fst value of sub-population III was the highest (0.73), followed by sub-population IV (0.49), sub-population II (0.32), and sub-population V (0.21), and the Fst value of sub-population I was the lowest (0.01) (Table 4). The Fst value of sub-population I was between 0-0.05, indicating that its genetic differentiation was very small. The Fst value of sub-population V was between 0.05 and 0.25, indicating that there was moderate genetic differentiation. The Fst values of sub-population II, III, and IV were all greater than 0.25, indicating that these three sub-populations had great genetic differentiation. AMOVA analysis by GenALEx showed that the genetic variation in this species was mainly within populations, accounting for 84% of the total variation, and between populations accounted for 16% of the total variation (Fig. 3b).
Table 3 Genetic diversity analysis of 10 pairs of SSR molecular markers in sweet potato landraces
Primer name
|
Number of samples
|
Total number of strips
|
Number of polymorphic loci
|
The percentage of polymorphic loci (PIC)
|
The average number of alleles (Na)
|
The average number of effective alleles (Ne)
|
Nei's genetic diversity index (He)
|
Shannon's diversity index (I)
|
Mean
|
Standard Deviation
|
Mean
|
Standard Deviation
|
Mean
|
Standard Deviation
|
Mean
|
Standard Deviation
|
GDAAS0338
|
132
|
21
|
17
|
80.95%
|
1.81
|
0.39
|
1.20
|
0.33
|
0.12
|
0.18
|
0.19
|
0.25
|
GDAAS0694
|
132
|
11
|
5
|
45.45%
|
1.45
|
0.50
|
1.19
|
0.31
|
0.11
|
0.18
|
0.17
|
0.26
|
GDAAS0782
|
132
|
10
|
8
|
80.00%
|
1.80
|
0.40
|
1.42
|
0.36
|
0.25
|
0.19
|
0.38
|
0.26
|
GDAAS0819
|
132
|
11
|
7
|
63.64%
|
1.64
|
0.48
|
1.29
|
0.34
|
0.18
|
0.18
|
0.27
|
0.26
|
GDAAS0871
|
132
|
8
|
7
|
87.50%
|
1.88
|
0.33
|
1.26
|
0.34
|
0.16
|
0.19
|
0.25
|
0.26
|
GDAAS0911
|
132
|
18
|
13
|
72.22%
|
1.72
|
0.45
|
1.30
|
0.35
|
0.18
|
0.19
|
0.28
|
0.26
|
GDAAS0922
|
132
|
17
|
12
|
70.59%
|
1.71
|
0.46
|
1.29
|
0.36
|
0.17
|
0.20
|
0.26
|
0.27
|
GDAAS0940
|
132
|
11
|
10
|
90.91%
|
1.91
|
0.29
|
1.25
|
0.29
|
0.17
|
0.16
|
0.27
|
0.22
|
SPGS2
|
132
|
23
|
10
|
43.48%
|
1.43
|
0.50
|
1.11
|
0.21
|
0.08
|
0.13
|
0.13
|
0.20
|
SPGS3
|
132
|
5
|
3
|
60.00%
|
1.60
|
0.49
|
1.37
|
0.38
|
0.22
|
0.20
|
0.32
|
0.28
|
All
|
132
|
135
|
92
|
68.15%
|
1.68
|
0.47
|
1.25
|
0.33
|
0.15
|
0.18
|
0.23
|
0.26
|
Table 4 The mean fixation index (Fst) of 5 sub-populations
Sub-population
|
Fst
|
Ⅰ
|
0.01
|
Ⅱ
|
0.32
|
Ⅲ
|
0.73
|
Ⅳ
|
0.49
|
Ⅴ
|
0.21
|
Fst: the mean fixation index.