SNP loci selected in present study
The genetic background of vegetable soybean is narrow due to excessive usage of limited elite germplasms in breeding
(Dong et al., 2014). It is difficult in germplasm identification and breeding selection by first and second-generation markers, including SSR, ISSR and RFLP markers, because of their low polymorphism
(Zhang et al., 2013). With the fast development of high-throughput sequencing, SNPs are more suitable for faster and cheaper genotyping compared to other markers
(Han et al., 2012). In present study, a total of 101 SNPs, including 6 for Gm01 and Gm18, 4 for Gm11 and 5 for each of the other chromosomes, was chosen and successfully converted into HRM based SNP markers (Table 1 and Table 2, Fig 1). Due to the complexity of soybean genome and the higher requirements for primer design, usually more than one SNP in a selected chromosome region were tested before the final generation of the optimal primer pairs. Taking into account the actual size differences of each chromosomes, the physical intervals between SNP markers varied from 7,164,318 bp to 11,196,952 bp on each chromosome with an overall average of 9,927,838 bp. The expected amplicons ranged from 60 bp to 100 bp with an average of 81.7 bp in length (Table 2). The probe, microarray, Sanger sequencing, the next generation sequencing (NGS) and so on. High-resolution melting curve (HRM) analysis has been proved a fast, accurate and inexpensive method to analyze SNPs
(Liew et al., 2004; Lopez et al., 2008). HRM-based SNP genotyping method was broadly used to analyze germplasm diversity of plants, including
Capsicum (Jeong et al., 2010), Medicago sativa (Han et al., 2012), Prunus avium (Ganopoulos et al., 2013) and
Taihangia rupestris (Li et al., 2018). In present study, HRM analysis was also used for genotyping SNPs in vegetable soybean germplasms. Theoretically, there could be three kinds of genotypes for a HRM-based SNP analysis, i.e., allele I, allele II and heterozygous. However, only two scenarios were discovered in this study due to the highly genomic homozygosity of these two soybean varieties (Fig 2).
In scenarios I, no difference was found between ‘Williams 82’ and ‘Zhexian No.9’ at the analyzed SNP locus, as shown by Melting Peaks (Fig 2A), Normalized Melting Curves (Fig 2B) and Normalized and Temp-Shifted Difference Plots (Fig 2C). Thus, the genotype of ‘Zhexian No.9’ could be deduced to be the same as that of the reference variety ‘Williams 82’ (Fig 2D). In scenarios II, two obvious groups of plots were observed in the Melting Peaks (Fig 2E), Normalized Melting Curves (Fig 2F), as well as in Normalized and Temp-Shifted Difference Plots (Fig 2G). It was significant different between ‘Williams 82’ and ‘Zhexian No.9’ at the analyzed SNP locus (Fig 2H). In consideration of the known SNP alleles in the public database, the genotype of ‘Zhexian No.9’ should be the alternative one from that of the ‘Williams 82’.
Verification of the HRM based SNP genotyping by sequencing
The accuracy of the HRM based SNP genotyping is the precondition for the subsequent application of this methodology. For this purpose, we sequenced the PCR products from 9 SNP markers randomly to verify the accuracy of HRM method. Consistencies between HRM based SNP genotyping and Sanger sequencing were observed for all those sequenced SNP loci, suggesting that the HRM based SNP analysis system established in this study is accurate and could be used in soybean SNP genotyping (Fig 3).
SNP fingerprinting of soybean variety ‘Zhexian No.9’
All of those 101 evenly distributed HRM based SNP markers were subsequently used in the genotyping of vegetable soybean variety ‘Zhexian No.9’. Among these SNP markers, 33.7% (34 of 101) SNP loci were polymorphic between ‘Williams 82’ and ‘Zhexian No.9’. When the number of assessed germplasms increased to 32 (31 vegetable soybean varieties/inbred lines plus the reference variety ‘Williams 82’), the polymorphic rate soared to 61.4% (62/101, data not shown). The polymorphic rate of SNP markers were higher than that of SSR markers with 34.38% in vegetable soybean
(Zhang et al., 2013), as well as 9.83% and 32.35% in grain soybean
(Hisano et al., 2007; Li et al., 2010). All of these results indicated that the 101 new developed HRM-based SNP markers are suiTable for soybean DNA fingerprinting and the related researches. The methodology described in this study can also be used for reference in other crops with known genomic information.
Polymorphism between ‘Williams 82’ and ‘Zhexian No.9’ was observed on 85% (17 of 20) of the soybean chromosomes (Fig 4). Thus, the SNP fingerprint of ‘Zhexian No.9’ could be described as “TCAACT AGCCA CTGCT CCTAG ATGAC CGCGA TGGGG TGGGC CACCA TCACT TCTT CTGGT TACCA TACCG TGGCG AGGTC TAAAC GATCTC CTCTG CTACT” with letters listed in the order of located chromosomes and physical positions of each SNPs.