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16 May 2013 Primers for Low-Copy Nuclear Genes in the Hawaiian Endemic Clermontia (Campanulaceae) and Cross-Amplification in Lobelioideae
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Although phylogenetic studies of plants rely heavily on plastid and nuclear ribosomal loci, the limitations of these loci are well known. Plastid loci are uniparentally inherited and susceptible to chloroplast capture (Rieseberg and Soltis, 1991). Furthermore, their low variation is particularly problematic for investigations of recent radiations such as those in the Hawaiian Islands, and indeed these markers have provided only limited resolution there. Nuclear ribosomal loci are subject to complex concerted evolution (Alvarez and Wendel, 2003), which can be incomplete so that multiple ancient alleles are maintained within a single individual, or rapid such that traces of hybridization are quickly erased. As a result, both plastid and ribosomal markers provide only incomplete information when hybridization is common.

The genus Clermontia Gaudich. (Campanulaceae, subfamily Lobelioideae) comprises 22 species endemic to Hawai‘i, 13 of which are on the IUCN Red List of Threatened Species. Species identification in the field is often difficult, particularly in the absence of flowers, and apparent hybrids can be common. Clermontia and other Hawaiian lobeliads (six genera total) form a monophyletic clade that represents the largest plant radiation in Hawai‘i (Givnish et al., 2009). The members of this clade, as well as the members of clade 4 of Antonelli (2008), in which the Hawaiian clade is nested, are suspected paleotetraploids (Lammers, 1988). To find genetic markers that will be useful for phylogenetics and DNA barcoding within Clermontia, we used 454 data of partial cDNA libraries to design primers for single-exon nuclear loci in Clermontia and then tested their crossamplification in Campanulaceae.

METHODS and RESULTS

We obtained a pooled, partial transcriptome library from leaf and floral buds (fixed in the field in RNAlater [QIAGEN, Gaithersburg, Maryland, USA] and stored at −80°C) of seven taxa: Clermontia arborescens (H. Mann) Hillebr., C. clermontioides (Gaudich.) A. Heller, C. fauriei H. Lév., C. kakeana Meyen, C. kohalae Rock, C. parviflora Gaudich. ex A. Gray, and C. peleana Rock. RNA isolation, cDNA synthesis, and 454 sequencing were done at the University of Arizona Genetics Core Laboratory. The 454 run provided 1.4 million reads with an average length of 395 bp. 454 adapters, ribosomal RNA, and low-quality and low-complexity sequences were removed/trimmed using SeqClean ( http://compbio.dfci.harvard.edu/tgi/software/), and each taxon was assembled separately by the TGI Clustering tools (TGICL; Pertea et al., 2003), using default settings. We conducted BLAST searches of the 400 most highly expressed genes in Arabidopsis (C. Fizames, personal communication) against our data in CLC DNA Workbench (CLC bio, Aarhus, Denmark) to identify a set of genes with high coverage within each of all or most of the species. We selected loci (generally only a small portion of a gene) that comprised a single, long exon (200 bp) with matches in multiple species, and designed primers with FastPCR (PrimerDigital Ltd., Helsinki, Finland;  http://www.primerdigital.com/fastpcr.html) for their amplification using default settings. The presence of introns was tested by comparison with genomic and cDNA sequences in the Arabidopsis Information Resource database ( www.arabidopsis.org). Avoiding introns allowed the direct sequencing of accessions even in the case of gene duplications; introns often contained indels, which often resulted in alleles of different lengths in heterozygotes or among copies of duplicated genes. Twelve exon regions were identified (Table 1, Appendix 1) and were tested on seven accessions: C. fauriei (the earliest diverging species within the genus), C. arborescens, C. kakeana, Cyanea asplenifolia Hillebr. (Cyanea is a Hawaiian endemic genus and putative sister group of Clermontia; Givnish et al., 2009), Hippobroma longiflora (L.) G. Don (belonging to a different major clade of Lobelioideae and a likely tetraploid; Antonelli, 2008), Lobelia erinus L. (one of the earliest diverging Lobelioideae; Antonelli, 2008), and Campanula persicifolia L. (Campanuloideae). Leaf material was collected in the field and dried in silica gel, and genomic DNA was extracted using the Nucleospin Plant II Kit (Macherey-Nagel, Düren, Germany). The nuclear regions were amplified using the following mix: 12.3 µL of H2O, 4 µL of GoTaq 5× Buffer (Promega Corporation, Madison, Wisconsin, USA), 2 µL of MgCl2 25 mM, 0.4 µL of dNTP 1.25 µM, 0.2 µL of each primer 10 µM, 0.1 µL of GoTaq Flexi DNA polymerase 5 U/µL (Promega Corporation), and 0.8 µL of DNA template. The following amplification program was used: 2 min at 94°C; 38 cycles of 1 min at 94°C, 1 min at 63°C, and 1 min at 72°C; and a final extension of 5 min at 72°C. PCR products were sequenced directly at the Core Genetics Laboratory at the University of Hawai‘i Hilo. The identity of each amplified gene was validated through BLAST or tBLASTx (Clerm4, Clerm10) searches in GenBank. In every case, the 10 best matches (identities >80%) were either the same gene from a different species or a gene that was not yet annotated.

TABLE 1.

Identity of the 12 intron-less, low-copy nuclear genes identified in this study, with primer sequences, results from cross-amplification tests, and inference of putative gene duplication. a

t01_01.gif

All 12 nuclear regions were successfully amplified and sequenced in Clermontia, Cyanea, and Hippobroma; a single gene was not amplified in Lobelia erinus, and three could not be sequenced in Campanula (Table 1). A high number of ambiguous bases were found consistently in the forward and reverse sequences of some accessions, suggesting the presence of multiple gene copies. In five genes (Clerm1, Clerm6, Clerm10, Clerm11, Clerm12), ambiguous sites were identical across the three Clermontia species and Cyanea but absent in the other genera (example in Fig. 1). To confirm the hypothesis of gene duplication, we separated alleles computationally from the direct sequences using PHASE (default settings) within the software DnaSP (Librado and Rozas, 2009), and built a neighborjoining tree of the alleles in Sea View (Gouy et al., 2010) with default settings. In each of these five cases, we recovered two clades, each comprising a single allele from each of the four Hawaiian lobeliad species examined (example in  Appendix S1 (APPS_1200450_AppendixS1.docx)). This pattern strongly suggests a genome duplication event that predates the divergence of Clermontia and Cyanea. Clerm5 was duplicated in Clermontia but apparently not in Cyanea. This is the only gene for which direct sequences turned out to be difficult to read due to the divergence of the two gene copies, which may be due to the presence of an intron not present in Arabidopsis. Five genes (Clerm2, Clerm4, Clerm7, Clerm8, and Clerm9) behaved as single-copy genes in Clermontia. No recombination was detected in those genes using genetic algorithms for recombination detection (GARD; Kosakovsky Pond et al., 2006;  http://www.datamonkey.org/). The percentage of variable sites within each of these genes was comparable to those of the plastid loci rbcL, matK, and psbA-trnH and the nuclear ribosomal ITS and ETS (Table 2). Genotyping of a broader taxonomic sample of Clermontia revealed a much greater number of variants at these newly described nuclear genes and a different pattern of evolution compared to plastid genes (Pillon et al., 2013).

Fig. 1.

Example of electropherograms for Clerm12 comparing Clermontia fauriei, Cyanea asplenifolia, and Hippobroma longiflora. Only the forward sequence is shown; asterisks indicate ambiguous sites. The matching positions of multiple ambiguous sites in Clermontia and Cyanea implicate an ancestral gene duplication prior to the divergence of these two genera rather than heterozygosity. Note: black = G; green = A; red = T; blue = C.

f01_01.jpg

TABLE 2.

Comparison of variation of the five apparently nonduplicated nuclear genes with three plastid and two nuclear ribosomal loci.a

t02_01.gif

CONCLUSIONS

The selection of intron-less regions proved successful for the amplification and direct sequencing of several nuclear loci and the detection of duplicated genes. The large number of gene duplications shared between Clermontia and Cyanea strongly supports the hypothesis of whole-genome duplication that predates the diversification of the lobeliads in Hawai‘i. Whole genome duplication has similarly been demonstrated in Hawaiian silverswords (Barrier et al., 1999). Despite the genome duplication, we nevertheless identified a number of apparently single-copy genes, whether due to the loss of one copy in each case or the selectivity of our primers for one copy. Geographic and taxonomic patterns of variations of two of these markers within Clermontia are examined in a study of their potential use as DNA barcodes (Pillon et al., 2013).

LITERATURE CITED

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Appendices

APPENDIX 1.

Location information, voucher specimens, and GenBank accession numbers for Clerm1, Clerm2, Clerm3, Clerm4, Clerm5, Clerm6, Clerm7, Clerm8, Clerm9, Clerm10, Clerm11, Clerm12, ETS, ITS, matK, psbA - trnH, and rbcL. For duplicated genes, only cDNA sequences were submitted, when available. Voucher specimens were deposited at the Herbarium of the University of Hawai‘i (HAW). The vouchers for Clermontia arborescens and Campanula persicifolia have been lost, and vouchers were not collected for Cyanea asplenifolia because it is an endangered plant. — signifies that no sequence is available for the particular locus for that accession.

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[1] The authors thank the following for facilitating the collection of plant specimens: Hawaii's Department of Land and Natural Resources—Division of Forestry and Wildlife, Maui Land and Pineapple (R. Bartlett), The Nature Conservancy (E. Naboa and P. Bily), and the Volcano Rare Plant Facility (P. Moriyasu and J. Enoka). The authors thank H. Issar and A. Veillet for technical assistance, and M. Lebrun and C. Fizames for information on nuclear genes. Funding was provided by the Gordon and Betty Moore Foundation.

Yohan Pillon, Jennifer Johansen, Tomoko Sakishima, Srikar Chamala, W. Brad Barbazuk, and Elizabeth A. Stacy "Primers for Low-Copy Nuclear Genes in the Hawaiian Endemic Clermontia (Campanulaceae) and Cross-Amplification in Lobelioideae," Applications in Plant Sciences 1(6), (16 May 2013). https://doi.org/10.3732/apps.1200450
Received: 27 August 2012; Accepted: 1 November 2012; Published: 16 May 2013
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