Orixa japonica Thunb. (Rutaceae) is a deciduous dioecious shrub known from China, Korea, and Japan; it is the only known species within the genus Orixa Thunb. The species propagates asexually by sprouting, layering, and root suckering and forms a large genet. Although O. japonica can also reproduce sexually via entomophilous flowers, its seedlings are scarce in the wild. The balance between sexual and asexual reproduction is an interesting theme related to the evolution of sexual reproduction (Obeso, 2002). In dioecious clonal plants, there are sexual differences in clonal growth, and these differences contribute to the sex ratio of the population (Escarre and Houssard, 1991). Orixa japonica is also an important plant for medicinal use (Kang et al., 2011). Identification and management of individual plants that are of medical benefit are important and require accurate clone identification. In this study, we developed 16 microsatellite markers to provide a useful tool for clone identification of O. japonica, to examine the species' clonal structure and true sex ratio in the wild, and to manage lineages within breeding programs examining the medicinal use of O. japonica.
METHODS AND RESULTS
Total genomic DNA of O. japonica was extracted from a fresh leaf collected from Yoro, Gifu, Japan, using the DNeasy Plant Mini Kit (QIAGEN, Hilden, Germany), and a voucher specimen of this sample was deposited in the Forestry and Forest Products Research Institute herbarium, Japan (accession no. TF-K11-0215) (Appendix 1). Multiplex identifier (MID) tags were used to multiplex the extracted DNA of O. japonica and other species. These samples were then pooled and pyrosequenced using a 454 GS Junior System (Roche, Basel, Switzerland). The raw data were demultiplexed and MID tags were removed from the reads. The de novo sequencing produced 73,267 reads with an average length of 427 bp. The identification of microsatellite regions and design of primer pairs from the sequence data were performed with the program QDD 2.1 (Meglécz et al., 2010). Microsatellite regions bordering sequences with more than five repeats of di- to hexanucleotide motifs and a minimum sequence length of 80 bp were selected. According to these criteria, 2846 reads contained microsatellite loci. To eliminate redundancy, the similarity of sequences containing microsatellite regions was detected by all-against-all BLAST searching. Subsequently, 1168 reads were selected from the whole set of sequences containing microsatellites. PCR primer pairs were designed using Primer3 (Rozen and Skaletsky, 1999) implemented in QDD 2.1 (Meglécz et al., 2010). Finally, 803 microsatellite primer pairs were designed.
Amplification and polymorphism tests were performed for 30 selected primer pairs; we selected these primer pairs on the basis of their having single repeat motifs of di- and trinucleotides, with 10–14 repeats. All forward primers were fluorescently labeled at the 5′-end with one of four different tail sequences (A to D) shown in Table 1, according to the method by Blacket et al. (2012). All reverse primers were attached to a 5′-GTT-3′ sequence at the 5′-end of the sequence to reduce stuttering due to the addition of nontemplated adenine base pairs by Taq DNA polymerase (Brownstein et al., 1996).
To evaluate polymorphisms in these markers using population samples, leaves were sampled from 106 individuals from four populations (Appendix 1). Because Orixa is a monotypic genus, to evaluate the cross-amplification potential of these markers, 16 individuals of Skimmia japonica Thunb., which is a consubfamilial species, were also sampled (Appendix 1). Although Skimmia has previously been classified into subfamily Toddalioideae, Toddalioideae is now merged into subfamily Rutoideae (Thorne, 2000), into which O. japonica is classified, and Skimmia forms a sister group to O. japonica on the phylogenetic trees constructed by internal transcribed spacer and/or chloroplast sequences (Poon et al., 2007; Salvo et al., 2008). Sampled leaves were dried with silica gel and stored at room temperature until DNA extraction. Leaves were pulverized using a mortar and pestle. Pulverized leaves were washed more than twice using HEPES buffer (Setoguchi and Ohba, 1995), and then the cetyltrimethylammonium bromide (CTAB) method was used to extract total DNA (Murray and Thompson, 1980). PCR was performed in a final volume of 5 µL, containing 2.5 µL of 2× Type-it Multiplex PCR Master Mix (QIAGEN), 0.1 µM forward primers, 0.2 µM reverse primers, 0.1 µM fluorescently tagged universal primers, and 10 ng DNA template. Reactions were performed with an initial denaturation at 95°C for 5 min; followed by 35 cycles of 95°C for 30 s, 57°C or 60°C for 90 s, and 72°C for 30 s; and finally 60°C for 30 min using a P × 2 Thermal Cycler (Thermo Fisher Scientific, Waltham, Massachusetts, USA). The PCR products were electrophoresed using an ABI PRISM 310 Genetic Analyzer (Applied Biosystems, Carlsbad, California, USA), and fragment sizes were determined using GeneMapper version 4.0 (Applied Biosystems).
Characteristics of 16 nuclear microsatellite loci isolated from Orixa japonica.
Sixteen loci out of 30 showed clear amplification, with a single band for each allele. For each of these 16 loci, the number of alleles (A), observed heterozygosity (Ho), expected heterozygosity (He), fixation index (FIS), and null allele frequency were calculated using INEst 1.1 (Chybicki and Burczyk, 2009). Deviation from Hardy–Weinberg equilibrium (HWE) was tested at each locus by a randomization test implemented in FSTAT 2.9.3 (Goudet, 1995). The HWE significance level was evaluated after Bonferroni correction for each population.
Among those 16 loci across the four populations, 132 alleles were detected. Excluding the Oj661 locus from the Mugi population, all other loci in each of the four populations showed polymorphism (Table 2). For these polymorphic loci, A ranged from two to 10, Ho from 0.000 to 0.767, He from 0.140 to 0.875, FIS from −0.260 to 1.000, and the null allele frequency from 0.029 to 0.637. Five loci (Oj125, Oj437, Oj509, Oj549, and Oj598) did not significantly deviate from HWE over all the populations. Seven out of 16 loci did not amplify in S. japonica (Table 2). Among the remaining nine loci, eight loci were monomorphic and thus only one locus, Oj598, showed polymorphism.
We developed the first set of microsatellite markers for O. japonica. These 16 microsatellite markers showed a high level of polymorphism and can be used to identify clones. Moreover, because five of 16 markers (Oj125, Oj437, Oj509, Oj549, and Oj598) did not significantly deviate from HWE and their null allele frequencies were relatively low, these markers can also be used for estimating mating systems or performing parentage analysis. These markers will help examine the clonal structure and true sex ratio in the wild and manage lineages in the breeding program for medicinal use of O. japonica.
Genetic properties of 16 polymorphic nuclear microsatellite loci in four populations of Orixa japonica and one population of Skimmia japonica.