Transferability of common bean EST-SSRs in hyacinth bean, pea and soybean
In hyacinth bean, 19 (27.14%) of the 70 EST-SSRs were functional, among which, 8 (42.11%) EST-SSRs displayed polymorphism. In addition, fourteen (20.00%) among the 70 EST-SSRs were functional in pea, with 12 (85.71%) primer pairs showing polymorphism. Fifteen (21.43%) EST-SSRs were functional and all of them showed clear polymorphic patterns in soybean (Fig 1A).
Among the 70 EST-SSRs, 40 loci yielded fragments from one or more species. However, among those40 functional primers, only 5 displayed transferability to two among those three tested legume species. And successful amplification of fragment was only observed in 2 EST-SSRs among those three tested legume species (Fig 1B).
Polymorphism of common bean EST-SSRs in hyacinth bean, pea and soybean
Among the 8 polymorphic EST-SSRs, altogether 19 alleles were discovered among the 20 hyacinth bean landraces. In addition, the values of H
O and H
E were 0.000-0.350 and 0.051-0.710, separately, whereas the PIC value was 0.046-0.658 (mean, 0.232) (Table 1). Among the 12 polymorphic EST-SSRs, altogether 33 alleles were identified among the 20 pea landraces. The value of H
O was 0.000-0.870 (mean, 0.239) and that of H
E was 0.087-0.662 (mean, 0.353). The value of PIC was between 0.082 (PV019 and PV062) and 0.601 (PV061) (mean, 0.313) (Table 2). In soybean, 15 EST-SSRs exhibited polymorphism and totally 85 alleles (mean, 5.7) were identified. The value of PIC was 0.136-0.835 (mean, 0.576), while the value of H
O was 0.000-0.500 (mean, 0.175) and the value of H
E was 0.141-0.852 (mean, 0.615) (Table 3).
Application of common bean EST-SSRs on genetic diversity analysis of hyacinth bean, pea and soybean
PCoA was conducted based on the EST-SSR data in three legume species. The first and second PCoA components in hyacinth bean took up 37.14% and 19.30%, respectively, of total variations, revealing three diverse clusters with regard to germplasm types among the 21 hyacinth bean landraces (Fig 3). Meanwhile, the first and second PCoA components in pea occupied 29.34% and 19.01%, respectively, of total variations. Those 20 pea landraces showed the highest distributions among three areas in the plot, which were consistent with the respective distribution in natural regions. The first and second PCoA components in soybean took up 22.71% and 20.14%, respectively, of total variations. Those 21 soybean landraces were classified into three diverse clusters in terms of germplasm types (Fig 3).
The EST-SSR markers are widely used for genetic research of economic crops, such as, sweet potato
(Wang et al., 2011), maize
(Galvao et al., 2015) and soybeans
(He et al., 2015). However, mining for the EST-SSR markers is restricted to plants with genome databases and this limits EST-SSR marker mining and use for species with currently known genomic information
(Wang et al., 2013). Some study has been carried out on EST-SSR transferability among legumes crops.
Dutta et al., (2011) developed numerous functional EST-SSR markers based on those assembled transcriptome sequences for pigeon pea. In the present study, altogether 70 EST-SSR markers were screened from common bean, which were identified as the efficient markers of hyacinth bean, pea and soybean. Among these EST-SSR markers, a total of 19 loci were identified to be functional within hyacinth bean. Additionally, in pea and soybeans, we obtained 14 and 15 primer pairs that had clear amplification products, respectively. Among the 70 EST-SSRs, 40 loci yielded fragments from one or more species. It was suggested that the EST-SSRs of common bean can be highly transferable into hyacinth bean, pea as well as soybean, which may serve as the possible source to develop these legume species EST-SSR markers.
The amplified SSRs number within a specific variety is found to show positive correlation with the phylogenetic relatedness for that specific variety, as well as the variety to design marker
(Pan et al., 2018). Thus, we explored the phylogenetic relationships among 10 common cash crops included in the legume family. As we expected, the common bean and hyacinth bean showed the closest relationship in ten tested species, the soybean was identified as the sister group of hyacinth bean and common bean, they all belonged to Phaseoleae. While
Pisum sativum and
Viciafaba belonged to the different tribe Vicieae (Fig 2). The close phylogenetic relationship between common bean and hyacinth bean seems like one reason why hyacinth bean showed a significant higher transferability rate of common bean EST-SSRs than those of soybean and pea. The transferability rates of common bean EST-SSRs in hyacinth bean, pea and soybean were positively correlated with the phylogenetic relatedness among these legumes crops. It was suggested that the transferability of EST-SSRs markers of common beans reflected the sequence conservation and genetic distance of the tested species.
Polymorphism is one of the key indexes for evaluating the EST-SSR markers. In general, He, Ho, PIC and Na represent the vital indexes for the EST-SSRs polymorphism. This work calculated the polymorphism ratios of common bean EST-SSRs markers in hyacinth bean, pea and soybean to be 42.11%, 85.71% and 100%, respectively. Meanwhile, the average PIC values of polymorphic markers were 0.232, 0.313 and 0.576 and the average Na of polymorphic markers were 2.4, 2.8 and 5.7, respectively, indicating the significantly higher population diversity of soybean than those in hyacinth bean and pea. This might be related to the relatively large number of selected soybean. In addition, the polymorphism of molecular markers has a certain relationship with the sequence position.
The random genomic DNA markers are widely adopted to investigate economic crop genetic diversity, including SAMPL
(Roy et al., 2002), AFLP
(Sun et al., 2017), along with SSRs
(Suman et al., 2019) markers. While the genetic diversity identified through the above markers may be unable to stand for the real genetic diversity. On the contrary, the functional markers, including EST-SSRs, can detect the polymorphism related to the genomic coding regions
(Thiel et al., 2003). Several studies on EST-SSRs amplification among different species have been reported. Zhang
et al., (2006) reported that the transferable EST-SSRs from bread wheat might be adopted in phylogenetic research across various
Triticeae species. Moreover, 30 faba bean germplasms were classified into two major clusters with Pea EST-SSRs
(Gong et al., 2011). In present study, common bean EST-SSR markers were used to analyze genetic diversity in hyacinth bean, pea and soybean. Based on PCoA, varieties that had identical origin were more likely to be clustered, which indicated that such newly developed markers were effective on the analysis of hyacinth bean, pea and soybean accessions. In addition, the genetic relationships among hyacinth bean, soybean and pea accessions were greatly associated with the respective geographic distribution and such results were consistent with those obtained in wheat
(Li et al., 2008).