Sacred lotus (Nelumbo nucifera Gaertn.) (2x = 2n = 16), an aquatic perennial plant in the family Nelumbonaceae, has been cultivated as an ornamental or vegetable plant for more than 7000 yr throughout Asia (Hu et al., 2012; Yang et al., 2015). Microsatellite (simple sequence repeat [SSR]) markers are sensitive tools for evaluating genetic diversity, population genetic structure, and intraspecific variation. Because microsatellites can be either intergenic or intragenic (Tóth et al., 2000), the variable length of repeat motifs at the SSR may be related to differential function or activity of the segments of chromosomes in which they reside. MicroRNAs (miRNAs) are ca. 21-nucleotide, noncoding, small RNAs that play an important role in gene expression under diverse stress conditions including various biotic as well as abiotic stresses (Bartel, 2004). miRNA-based SSR (miRNA-SSRs) markers are a novel type of functional marker exploited predominantly in animal sciences, but little reported in plants. In this study, we performed a genome-wide analysis of miRNA-SSRs in N. nucifera and validated 45 SSRs among the 36 genotypes. This is the first report of genome-wide identification and characterization of miRNA-SSRs in N. nucifera.
METHODS AND RESULTS
The 106 N. nucifera pre-miRNA sequences identified in our previous study (Pan et al., 2015) were used for the present investigation. The 1000-bp (500 bp upstream and 500 bp downstream of mature miRNA sequence) sequences were obtained from the sacred lotus reference genome (Ming et al., 2013). The miRNA-SSR loci distributed throughout the N. nucifera genome were screened using MISA (Thiel et al., 2003) with default parameters. SSRs were selected based on the length of the core repeat motif ≥10 nucleotides (e.g., five units of dinucleotide repeat motifs, four units of trinucleotide repeat motifs, or three units of tetranucleotide repeat motifs). A total of 485 miRNA-based SSRs were present in the genome of N. nucifera. Using the MISA output, primers of each of the SSR-containing sequences were designed using the program BatchPrimer3 ( http://probes.pw.usda.gov/batchprimer3) (You et al., 2008). The parameters of each primer were set using the following criteria: (1) primer size of 18–22 nucleotides in length; (2) GC content of 40–60%; (3) annealing temperature between 50°C and 60°C (55°C optimum); and (4) expected amplicon size of 100–300 bp. In total, 138 miRNA-SSR primer pairs of N. nucifera were designed, and 45 primer pairs were synthesized for further analysis (GenScript, Nanjing, China).
Thirty-six N. nucifera accessions were used in the current study (Appendix 1). Total genomic DNA was isolated from frozen young leaves using the modified cetyltrimethylammonium bromide (CTAB) method as described in Doyle and Doyle (1987). A preliminary study using 12 N. nucifera individuals from a population from Hubei (Appendix 1) resulted in the selection of 21 microsatellite loci (Table 1) that were polymorphic. The sequences of polymorphic microsatellite loci were deposited into GenBank (accession no. KT344795–KT344815; Table 1). PCR amplifications were performed in a 15-µL reaction containing 50–100 ng genomic DNA, 1.5 µL 10× PCR buffer, 0.4 µM for each primer, 1.5 mM MgCl2, 250 µM each dNTP, and 0.5 units Taq DNA polymerase (TianGen, Beijing, China). The thermocycling conditions were: 95°C for 3 min; 35 cycles of 94°C for 30 s, annealing temperature optimized for each primer for 30 s (Table 1), and 72°C for 40 s; and a final extension step at 72°C for 7 min. The amplified products were separated on 6% denaturing polyacrylamide sequencing gels in 0.5× TBE buffer and visualized by silver nitrate staining. The size of fragments was determined using a 20-bp marker of 20–500 bp (TaKaRa Biotechnology Co., Dalian, China).
Characteristics of 21 miRNA microsatellite loci and primer pairs developed in Nelumbo nucifera.
Genetic properties of 14 polymorphic miRNA-SSR markers in four populations of Nelumbo nuciferaa
Fourteen polymorphic SSR primers were used to genotype 36 individuals of N. nucifera collected from Jiangxi Province (N = 5; 1°17'N, 103°50′E), Hunan Province (N = 6; 26°54′N, 112°36′E), Fujian Province (N = 3; 26°15′N, 117°37′E), and Hubei Province (N = 22; 30°34′N, 116°16′E). Voucher specimens were deposited in the Wuhan National Field Observation and Research Station for Aquatic Vegetables (Appendix 1). Parameters of genetic diversity including number of alleles (A), observed heterozygosity (Ho) expected heterozygosity (He), and Hardy-Weinberg equilibrium (HWE) were determined by Arlequin version 184.108.40.206 (Excoffier et al., 2005). Each of the 14 loci exhibited two to five alleles among the 36 N. nucifera individuals, with H o and H e ranging from 0.000 to 1.000 and from 0.000 to 0.803, respectively (Table 2). A relatively high level of genetic diversity was found in the Hubei population (H o = 0.217, A = 3.14) compared with the other three populations. This may be due to the fact that we sampled more individuals from the Hubei population. Some loci showed significant deviation from HWE (Table 2) due to heterozygote deficiency.