Figures
Abstract
The advantages of using molecular markers in modern genebanks are well documented. They are commonly used to understand the distribution of genetic diversity in populations and among species which is crucial for efficient management and effective utilization of germplasm collections. We describe the development of two types of DArT molecular marker platforms for the new oilseed crop lesquerella (Physaria spp.), a member of the Brassicaceae family, to characterize a collection in the National Plant Germplasm System (NPGS) with relatively little known in regards to the genetic diversity and traits. The two types of platforms were developed using a subset of the germplasm conserved ex situ consisting of 87 Physaria and 2 Paysonia accessions. The microarray DArT revealed a total of 2,833 polymorphic markers with an average genotype call rate of 98.4% and a scoring reproducibility of 99.7%. On the other hand, the DArTseq platform developed for SNP and DArT markers from short sequence reads showed a total of 27,748 high quality markers. Cluster analysis and principal coordinate analysis indicated that the different accessions were successfully classified by both systems based on species, by geographical source, and breeding status. In the germplasm set analyzed, which represented more than 80% of the P. fendleri collection, we observed that a substantial amount of variation exists in the species collection. These markers will be valuable in germplasm management studies and lesquerella breeding, and augment the microsatellite markers previously developed on the taxa.
Citation: Cruz VMV, Kilian A, Dierig DA (2013) Development of DArT Marker Platforms and Genetic Diversity Assessment of the U.S. Collection of the New Oilseed Crop Lesquerella and Related Species. PLoS ONE 8(5): e64062. https://doi.org/10.1371/journal.pone.0064062
Editor: Matthew E. Hudson, University of Illinois, United States of America
Received: November 26, 2012; Accepted: April 8, 2013; Published: May 28, 2013
This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.
Funding: Funding for the study was provided by U.S. Department of Agriculture (USDA) - Agricultural Research Service Project No. 5402-21000-007-00. The acquisition of new germplasm in 2010 was supported by the NPGS Plant Exploration/Exchange Program. No additional external funding was received or used for this study. USDA determines the research priorities of all research that it funds. Publication of research is subject to review of USDA officials. Specific study design, data collection, data analysis, and preparation of the manuscript is under the control of the individual researchers.
Competing interests: The authors acknowledge the help and technical support from the employees of Diversity Arrays Technology Pty. Ltd. AK is an employee of Diversity Arrays Technology Pty Ltd, which offers genome profiling service using the technologies described in this report. This fact, however, has not interfered whatsoever with the full, objective, transparent and unbiased presentation of the research results described in the manuscript nor alters the authors’ adherence to all the PLOS ONE policies on sharing data and materials.
Introduction
Lesquerella (Physaria sp.), a member of the Brassicaceae, is a North American genus which has more than 90 member species thriving mostly in dry and arid habitats usually mixed with sparse vegetation [1], [2]. Commonly known as bladderpod, it has been identified by the U.S. Department of Agriculture and the U.S. Department of Energy as very promising species for the production of lubricants, engine oils, waxes, coatings and a source of natural estolides with valuable application in the automobile and biofuel industries [3], [4], [5]. There has been an increasing trend in research activities in the crop since its oil was first characterized for novel properties [6]. The fatty acid profile of lesquerella oil has been found to vary. Species found east of the Mississippi have mostly densipolic acid, those in the western U.S. mostly lesquerolic acid, and a species from Oklahoma predominantly with auricolic acid [7]. Lesquerella could be grown successfully as a winter annual in the southwest U.S. producing an average seed yield of 1.7 tons/ha [8]. At present, there are more than eight advanced breeding lines of P. fendleri ready for commercialization and a corresponding substantial germplasm collection has been assembled in the U.S. National Plant Germplasm System [9].
Molecular characterization of germplasm collections supplements phenotypic assessment of diversity and is important in the effective management of genetic resources. Molecular markers have been very useful in efforts to accurately identify gaps and redundancy within and among individual germplasm collections and have helped resolve important genebank management issues [10], [11], [12]. Examples of markers used in specific Brassicaceae collections include microsatellites for examining diversity in B. napus [13], [14], [15], lesquerella [16] and wild relatives such as Capsella, Crambe and Sinapis [17]; AFLPs in B. oleracea [18] and Lepidium [19]; and RAPD markers in Raphanus [20].
Compared to the previously mentioned molecular marker systems, Diversity Array Technology (DArT) markers are relatively new and were developed only in the early 2000. DArT markers overcame the difficulties in correlating bands on gels with allelic variants by utilizing hybridization-based methods and solid state surfaces [21]. They have been increasingly utilized as evidenced by the number of publications reporting successful implementation [22]. Among its advantages over other marker systems include high throughput capability allowing rapid germplasm characterization in a single experiment, independence of sequence data, and ability to detect single base changes and indels [23]. To date, DArT markers have been successfully applied in genetic diversity analysis, linkage mapping and in finding out population structure of collections in various crop species [24]. Their application in minor crops are likewise increasing due to their potential to accelerate gene discovery and initiate molecular breeding because of their whole genome coverage without relying on prior sequence data information [25], [26].
In lesquerella, allozymes have previously been used [27] and microsatellite markers have been developed [16]. But the small number of markers that are available presents a limitation in linkage mapping and in the study of genetic resources collections. At present, only fifteen microsatellites have demonstrated utility across different Physaria species. We present in this paper the development of two platforms of high density DArT markers for lesquerella and the results obtained after testing them to analyze the genetic diversity of Physaria and Paysonia national germplasm collection. This new molecular marker system for the new crop species will serve as an additional resource to augment the existing systems to assist crop improvement efforts, germplasm management activities and genetic studies.
Materials and Methods
Tissue Sampling
The lesquerella germplasm set was obtained from collections of the USDA-ARS National Arid Land Plant Genetic Resources Unit, Parlier, CA and the USDA-ARS Arid Land Agricultural Research Center, Maricopa, AZ. The samples used in this study were selected based on each accession’s geographic location, sampling within distinct counties in each State when possible. Seven advanced P. fendleri breeding lines, WCL-LH1, WCL-LO1, WCL-LO2, WCL-LO4, WCL-LY1, WCL-SL1 and WCL-YS1 were included among the samples. DNA was extracted from fresh leaf tissue obtained from five week old seedlings using Qiagen DNeasy 96 Plant Kits (Qiagen Inc., Valencia, CA). Two DArT platforms were developed for lesquerella as described below. The platform development utilized 86 Physaria accessions (see Table 1), representing 11 species and one Paysonia accession since the latter is a sister genus of Physaria and only recently was there a recognized taxonomic delineation between them [2].
Microarray DArT Platform Development
The microarray DArT was developed by first testing combinations of the rare-cutting restriction enzyme PstI with several restriction endonucleases that cut frequently on DNA samples from 8 representative accessions to determine the restriction enzyme combination that provided the best complexity reduction. Final genomic representations were prepared using PstI/BstNI combinations. Approximately 50 ng of genomic DNA was digested with PstI/BstNI combinations and the resulting fragments ligated to a PstI overhang compatible oligonucleotide adapter. A primer annealing to this adapter was used in PCR reaction to amplify complexity-reduced representation of a sample. Amplification products were either used for cloning a in marker development process or labeled with fluorescent dyes and hybridized to DArT array in the genotyping process. Library construction was subsequently performed using 80 P. fendleri accessions and 16 accessions of wild related Brassica species. The amplified PstI restriction fragments from all accessions were cloned into pCR2.1-TOPO vector (Invitrogen, Australia) as described by Jaccoud et al. [21] and four libraries were generated. The white colonies containing genomic fragments inserted into pCR2.1-TOPO vector were picked into individual wells of 384-well microtiter plates filled with ampicillin/kanamycin-supplemented freezing medium. A total of 3,456 clones from Physaria were obtained –1,920 from three libraries of P. fendleri and 1,536 from the related species. Inserts from these clones were amplified using M13F and M13R primers in 384 plate format, a subset of PCR products were assessed for quality (10% of 25 µl PCR reaction) through gel electrophoresis, and all remaining PCR products dried, washed and dissolved in a spotting buffer. A total of 6,144 clones were printed with spot duplication on SuperChip poly-L-lysine slides (Thermo Scientific, Australia) using a MicroGrid arrayer (Genomics Solutions, UK). The microarrays included 1,920 Brassica clones and 768 Arabidopsis clones in addition to the Physaria clones.
Each sample was assayed using methods described above for library construction. Genomic representations were assessed for quality through gel electrophoresis in 1.2% agarose and labeled with fluorescent dyes (Cy3 and Cy5). Labelled targets were then hybridized to printed DArT arrays for 16 hours at 62°C in a water bath. Slides were washed as described by Kilian et al. [28], dried initially by centrifugation at 500 × g for 7 min and later by a desiccator under vacuum for 30 min. The slides were scanned using Tecan LS300 scanner (Tecan Group Ltd, Männedorf, Switzerland) generating three images per array: one image scanned at 488 nm for reference signal measures the amount of DNA within the spot based on hybridization signal of FAM-labeled fragment of a TOPO vector multiple cloning site fragment and two images for “target” signal measurement: one scanned at 543 nM (for Cy3 labeled targets) and one at 633 nM (for Cy5 labeled targets). All DArT techniques applied for this work were recently described in much more detail by Kilian et al. [28].
DArTseq Platform Development
For the sequencing-based DArT genotyping, four complexity reduction methods optimized for several other plant species at DArT P/L were used. PstI-RE site specific adapter was tagged with 96 different barcodes enabling encoding a plate of DNA samples to run within a single lane on an Illumina Genome Analyzer IIx (Illumina Inc., San Diego, CA). PstI adapter included also a sequencing primer site, so that the tags generated were always reading into the genomic fragments from the PstI sites. After the sequencing run, the FASTQ files were quality filtered using the threshold of 90% confidence for at least 50% of the bases and in addition filtered more stringently for barcode sequences. Two lanes of GAIIx were run with all samples providing fully replicated sequencing data. The filtered data were split into their respective target (individual) data using barcode splitting script. Each sample had on average 500,000 counts per replicate. After producing various QC statistics and trimming of the barcode the sequences were aligned against the reference created from the tags identified in the sequence reads generated from all the samples. In addition the short sequence tags were aligned against Arabidopsis thaliana’s genome available in Genbank. Arabidopsis is a close relative of Physaria in the Brassicaceae [29]. The output files from alignment generated using Bowtie software [30] were processed using an analytical pipeline developed by DArT P/L to produce “DArT score” tables and “SNP” tables.
Genotyping
Both DArT on array platform and DArTseq use a set of quality parameters to select markers which are of use for a specific application. One of these parameters is reproducibility of markers in technical replicates for a subset of samples. In diversity analysis, the reproducibility parameter threshold is set usually at 97% which translates to average reproducibility of the dataset around 99.7%. A total of 87 common accessions were genotyped using both the microarray DArT and DArTseq platforms. A total of 11 Physaria species, 1 interspecific hybrid, and 2 Paysonia species were analyzed by microarray DArT. Additional accessions were genotyped using the DArTseq platform comprised of 17 Physaria and 7 Paysonia species. Overall, there was a total of 177 accessions represented by single plant samples genotyped using the two platforms, majority of which are P. fendleri (Tables 1 and 2). The marker sequences and genotype data will be stored in the U.S. Germplasm Resources Information Network (GRIN) database (http://www.ars-grin.gov) along with the accessions’ phenotypic observations and germplasm passport data curated by the U.S. National Plant Germplasm System.
Data Analysis
All the images from DArT platforms were analyzed using DArTsoft v.7.4.7 (DArT P/L, Canberra, Australia). The markers were scored as binary data (1/0), indicating presence or absence of a marker in genomic representation of each sample as described by Wenzl et al. [31]. For quality control, 30% of genotypes were genotyped in full technical replication. Clones with P>77%, a call rate >97% and >98% allele-calling consistency across the replicates were selected as markers. P value represents the allelic-states variance of the relative target hybridization intensity as a percentage of the total variance. The informativeness of the DArT markers was determined by calculating the polymorphism information content (PIC) within the panel of diverse accessions according to Anderson et al. [32]. The P. fendleri data were used in GenAlEx v.6.41 [33] to determine 2D spatial autocorrelation of the DArT microarray and Arlequin v.3.5.1.3 [34] to assess the amount of variation among the assigned regional groupings by AMOVA. To summarize the relationships among all examined accessions, cluster analysis was performed on Dice similarity values with the SAHN procedure using the unweighted pair-group method done using NTSYS-pc v. 2.21 m [35]. The Dice coefficient was preferred over simple matching coefficient because DArT is a dominant marker system and there were several non-Physaria species in the sample set [36], [37]. In addition, a Bayesian model-based clustering was performed on microarray DArT markers using STRUCTURE v.2.3.4 [38] testing 3 independent runs with K from 1 to 8, each run with a burn-in period of 50,000 iterations and 300,000 Monte Carlo Markov Chain (MCMC) iterations, assuming an admixture model and correlated allele frequencies. The STRUCTURE data was subsequently analyzed by HARVESTER v.06.92 [39]. Mantel tests were made to determine if there are significant correlations between the dendrogram representations and the distance matrices, between the matrices of geographic and genetic distances, and between the distance matrices from the two DArT platforms used in the P. fendleri. The missing geographic coordinates of collection sites of ten accessions (PI 293027, PI 293028, PI 337050, PI 345712, PI 355037, PI 355041, PI 355042, PI 275771, PI 283700 and PI 299412) in the GRIN database were estimated using Google Earth v. 6.1.0.5001 based on available locality description and collectors’ notes and the information included in the analysis. Map projections of the analyzed P. fendleri accessions were made using ArcGIS Explorer v.2.0.0.1750 (ESRI, Redlands, CA) and non-parametric correlation tests done using JMP v.9 (SAS Institute, Cary, NC).
Results
Relationships among Accessions from Microarray DArT Analysis
A total of 2,833 polymorphic markers were found using microarray DArT, with an average genotype call rate of 98.4% and a scoring reproducibility of 99.7%. The average PIC value was 0.21 and the median 0.19. About 20% of the markers have values in the range of 0.06 to 0.10, and almost an equal proportion of 12% of markers on the following PIC classes - 0.11 to 0.15, 0.16 to 0.20, and 0.21 to 0.25 (Figure 1). Overall, the distribution of PIC values was asymmetrical and skewed towards the lower values.
Cluster analysis and principal coordinate analysis (PC plot not shown) indicated that the different accessions were successfully classified by the marker system based on species, by geographical source, and breeding status (Figure 2), except for one new collection of P. gordonii, DDMC2010-6, which clustered with the P. fendleri accessions. The cophenetic correlation coefficient between the dendrogram and the distance matrix was highly significant (r = 0.98, t = 10.30, prob random Z<obs. Z = 1.00, 3000 permutations) indicating that the tree is a very good representation of the distance matrix. All P. fendleri accessions grouped in a separate cluster from the other species. The main cluster has two subgroups from Mexico, and a subgroup with a majority of accessions from Texas. There was no single group of P. fendleri accessions from Arizona and New Mexico. Most accessions from these States were associated with other accessions from Texas.
The labels denote the germplasm collection numbers and origin. Suffixes indicate respective species (PAR: P. argyraea, PAU: P. auriculata, PFE: P. fendleri, PGO: P. gordonii, PGF: P. grandiflora, PGR: P. gracilis, PKN: P. ‘kathryn’, PLI: P. lindheimeri, PPL: P. pallida, PRC: P. recurvata, PRT: P. rectipes, and PTH: P. thamnophila).
The four advanced P. fendleri breeding lines were partitioned into two clusters. The breeding lines WCL-SL1 and WCL-YS1 were found to be more genetically distant to WCL-LO2 and WCL-LO4. The last two lines were determined to be more similar to the rest of the P. fendleri from North America than WCL-SL1 and WCL-YS1.
Among the different species, the most similar to P. fendleri was determined to be P. argyraea while the least similar was P. thamnophila. The P. pallida accessions grouped in one cluster along with P. lindheimeri and other accessions representing the species, P. gracilis, P. gordonii, P. recurvata, and P. rectipes. The two accessions of Paysonia auriculata (3009) and Paysonia grandiflora (2243) grouped together in a separate cluster along with the interspecific hybrid swarm ‘Kathryn’ (4087) from five Paysonia species.
The analysis of molecular variance considering P. fendleri accessions only, showed that there was a much greater proportion of variation within groups (90%) than among groups (10%) in the species (Table 3). Among the unimproved germplasm set, the pairwise Fst values that showed the greatest differentiation was between Arizona accessions and those from New Mexico and the least amount of differentiation between Mexico and Arizona (Table 4). There was a significant correlation found between the computed genetic distance and geographic distance matrices from Mantel test (r = 0.33, t = 5.94, prob random Z<obs z = 1.00, 3000 permutations), indicating that distant accession pairs are more different genetically, supporting the previously mentioned result (Figure 3).
Relationships among Accessions from DArTseq Analysis
There was a total of 27,748 markers obtained using the Physaria DArTseq platform. The average genotype call rate was 98.8% and a scoring reproducibility of 99.7%. The average PIC value was 0.12 and the median was 0.09. About 57% of markers have PIC values in the range of 0.06 to 0.10, while 20% and 11% on PIC classes 0.11–0.15 and 0.01–0.05, respectively (Figure 1). The `distribution of the PIC values of DArTseq markers follows the same skewed pattern as the DArT microarray markers presented earlier.
Cluster analysis using the markers from this platform resulted in a relationship that follows that of the taxonomic groupings based on general morphological affinities presented by Rollins and Shaw [1]. The results did not deviate from those when the first platform with fewer markers used. Four accessions of P. fendleri (2258, 2274, 3083, and DDMC2010-8) clustered with the other Physaria species – P. gordonii and P. gracilis, indicating higher genetic similarity to representative accessions of these species than the rest of P. fendleri. These four P. fendleri accessions will be further examined for misidentification and for oil and morphological trait variation when verified.
The cophenetic correlation coefficient between the dendrogram and the distance matrix was highly significant (r = 0.94, t = 49.20, prob random Z<obs. Z = 1.00, 3,000 permutations) indicating that the tree is a very good representation of the distance matrix. The cluster of P. fendleri showed a distinct group of accessions from Mexico and a cluster comprised of all other germplasm from North America (Figure 4). The DArTseq platform indicated two separate clusters for the advanced lines. The breeding lines WCL-LH1, WCL-LO1, and WCL-LY1 are all in a group with greater similarity to accessions from Texas. WCL-LO2, WCL-LO4, WCL-SL1, and WCL-YS1 grouped together in one cluster indicating greater genetic similarity to accessions from Arizona and Mexico. WCL-LO2 was derived from the WCL-LO1 and the remaining three from WCL-LO2. It appears from these data that perhaps more genotypes from the Arizona accession were integrated into WCL-LO2.
The labels denote the germplasm collection numbers and origin. Suffixes indicate respective species (PAC: P. acutifolia, PAR: P. argyraea, PAU: P. auriculata, PDE: P. densipila, PDF: P. densiflora, PDO: P. douglasii, PFE: P. fendleri, PGO: P. gordonii, PGF: P. grandiflora, PGR: P. gracilis, PIF: P. inflata, PIN: P. intermedia, PKA: P. kaibabensis, PKN: P. ‘kathryn’, PYL: P. lasiocarpa, PLI: P. lindheimeri, PLT: P. lyrata, PLU: P. ludoviciana, PMC: P. mcvaughiana, PMX: P. mexicana, PPF: P. perforata, PPL: P. pallida, PRC: P. recurvata, PRT: P. rectipes, PST: P. stonensis, PTH: P. thamnophila and PVA: P. valida).
Similar to results from the microarray DArT, the accessions of the following species formed distinct groups consistent with the classification by Rollins and Shaw [1]: a) P. intermedia, P. valida, and P. rectipes, b) P. gordonii, P. gracilis, P. rectipes, and P. lindheimeri, and c) P. auriculata, P. densipila, P. lyrata, P. stonensis, P. grandiflora, and P. perforata. The two accessions of P. lasiocarpa (2217 and 2228) were most genetically similar to the accessions of P. grandiflora. This last group of species comprised of annual, auriculate-leaved types whose taxonomic nomenclature was segregated from Physaria and transferred to Paysonia based on leaf-trichome morphology, chromosome number, and molecular data from analyzing internal transcribed spacers of nuclear ribosomal DNA [40]. Apart from the accession of Physaria mexicana nested within the Paysonia cluster, the results of this genetic analysis using both microarray DArT and DArTseq platforms support the previous segregation of Physaria from Paysonia as proposed by O’Kane and Al-Shehbaz [40] indicating that the group of Paysonia species to be the least genetically similar to the P. fendleri and other Physaria accessions.
Results of the analysis of molecular variance when DArTseq markers were used correspond to that from microarray DArT. A greater proportion of variation within groups (93%) than among groups (7%) in the species was found (Table 3).
The average genetic similarity in the P. fendleri group when using microarray DArT was 0.86, while only 0.44 when DArTseq was used. This is in line with the assumption that more differences may be found when more markers are used because of the increased sensitivity and resolution to detect genetic distinctiveness [41]. The genetic similarity matrices of P. fendleri obtained using the two platform systems showed a good fit when compared by a Mantel test (r = 0.48, t = 11.28, prob random Z<obs z = 1.00, 3,000 permutations).
Population Structure Analysis of P. fendleri
Using results from microarray DArT, the population structure of the P. fendleri samples was determined. The plot of ΔK for each K value is shown in Figure 5a. It was estimated through the method of Evanno et. al. [42] that there are 4 groups contributing significant genetic information in the P. fendleri collection. The bar plot of the population assignment test when K = 4 is shown in Figure 5b. Three of the four accessions of P. fendleri breeding lines are shown to have mixed backgrounds. Of the other P. fendleri accessions, twelve (16%) have close to homogeneous genetic background (>98% probability) while 63 accessions (84%) are highly heterogeneous showing intermediate and/or highly mixed composition. A majority of the accessions from Texas are assigned to one cluster, while those from New Mexico have myriad cluster assignments suggesting the greater diversity of P. fendleri in this U.S. state. When the cluster assignments of the P. fendleri accessions were projected on a map, the segregation among clusters was evident in their geographic location (Figure 6). Further testing for association between the assigned clusters and available site elevation data by computing Spearman’s rank correlation coefficient showed a very weak positive correlation (Spearman ρ = 0.11, p = 0.33).
Discussion
The importance of understanding genetic diversity in germplasm collections is critical for the effective management of accessions in genebanks. Molecular characterization supplements morphological evaluation of germplasm and allows measurements to help resolve numerous operational, logistical, and biological questions that face genebank managers and conservation biologists [43], [44]. The Physaria collection in the U.S. NPGS has not been well characterized before for genetic diversity, though there have been preliminary studies on subsets of accessions using a limited number of microsatellite markers [16], and an extensive evaluation for diversity in oil characteristics and other morphological characters [45]. In this study, the new DArT platforms for Physaria were found both acceptable and provided robust information about the genetic variability of the collection.
The development and utilization of DArT markers allowed us to determine the genetic diversity of the Physaria collection. The 2,833 microarray DArT markers were found to be useful in providing a picture of genetic diversity in the Physaria germplasm collection using a large set of accessions. Overall, the average PIC of the Physaria and Paysonia microarray DArT markers was found to be lower than that observed in other species where similar markers were developed, like wheat (0.44) [46], cassava (0.42) [47], and sorghum (0.41) [48], but comparable to that observed in sugar beet (0.28) [49] and Asplenium fern (0.21) [50]. The average PIC of the DArTseq markers is much less than that of the microarray DArT. However, the more numerous DArTseq markers may have the capability of providing a better picture of diversity by sampling more points in the genome. The distribution of these almost 30,000 new DArT markers in the Physaria and Paysonia genome remains to be determined. However, based on the information from a large number of organisms in which DArT system was applied more broadly including genetic mapping and/or sequence-based physical mapping, we can assume that DArT marker from both platforms will be distributed throughout the genome with marker density highly correlated to gene density [28], [51]. Compared to microsatellite markers, DArT markers are very suitable for high-throughput work and previously have been determined to have clear advantages in cost and time aspects of genotyping as demonstrated in other crops [52]. Both microarray DArT and DArTseq platforms have the same development costs. However, the higher number of markers obtained in DArTseq resulted to an overall lower cost per datapoint than microarray DArT. This higher cost effectiveness of DArTseq is in parallel to other sequenced-based genotyping strategies which can provide substantial cost savings compared to microarrays when conducting genetic diversity studies [53], [54]. Importantly, when comparing effectiveness of the two platforms one has to keep in mind that it may vary according to specific application: in genetic ID and product quality testing (i.e. seed purity) modest number of array-based DArT markers may perform as well as DArTseq platform and currently for a better price. A further cost reduction of sequencing may however push even this balance towards DArTseq platform in the future.
The relationships found among the accessions are in line with the previously proposed evolution within the genus. Taxonomists assert that P. auriculata is the most primitive species due to its very distinct evolutionarily primitive characters such as large siliques, large number of ovules around the replum, and predominance of simple trichomes [1]. P. auriculata has been proposed to be closely related to P. grandiflora and this relationship is supported by the results of the molecular marker analysis between the accessions representative of these species (2243 and 3009) showing high genetic similarities. Likewise, P. gordonii and P. gracilis grouped in the same cluster with P. rectipes, P. recurvata, and P. thamnophila which is in agreement with their previously known taxonomic groupings based on very general morphological similarities. The high genetic similarity between the P. argyraea accessions (2212 and 319 b) to the P. fendleri group supports phylogenetic subsectional grouping based on pod morphology.
Overall, there was higher genetic similarity found among accessions of the other Physaria and Paysonia species than among accessions of P. fendleri. In particular, species that are included in the federal or state threatened and endangered species list, like P. pallida and P. stonensis [55], have very low genetic diversity as indicated by results of DArTseq markers. In P. pallida, the representative accessions (4091 and 4093) were found to be highly similar. The limited geographic range and the proximity of the collection sites of these two samples suggest that they might have been from just one population. Likewise for P. stonensis, there was a very high genetic similarity found on both of the representative accessions (3092 and 3347). Because only a limited number of accessions were included in these other species, a follow up study that includes additional accessions is recommended to validate if this is a general trend.
Physaria ‘Kathryn’ (4087) is a cultivar from interspecific hybridization developed by allowing five species (P. densipila, P. lescurii, P. lyrata, P. perforata, and P. stonensis) to intermate for twenty two generations [56]. P. ‘Kathryn’ was determined to be most closely related to P. auriculata in our analysis using microarray DArT. However, with the expanded set of accessions included in the DArTseq analysis, it grouped with the representative samples of its parent species – P. lyrata (3000 and 3370), P. perforata (3091), and P. stonensis (3092, 3347). These species were also found most genetically similar to P. mexicana (3344) which is the only perennial type in the species cluster. P. mexicana is a previously undescribed species in Mexico and is among the more recent species reported by Rollins [57].
DDMC2010-6 was from a more recent germplasm collecting trip and it was entered in the database as P. gordonii. However, based on results of DArT markers, it clustered with the P. fendleri accessions after using both DArT platforms, indicating the need to review its species assignment. The species identity of this particular accession will again be verified using its plant voucher specimen as well as in the NPGS site handling the germplasm when it is regenerated in the future.
The two clusters of P. fendleri breeding lines - WCL-LO2, WCL-LO4, WCL-SL1, and WCL-YS1 in one cluster, while the other set of breeding lines WCL-LO1, WCL-LH1, and WCL-LY1 in another, indicates the possibility of developing genetically differentiated lines for crop improvement and may have applications in future hybrid development work. The lines WCL-LH1, WCL-LO1 and WCL-LY1 are the first three germplasm lines that were publicly released in 1996. These were developed using recurrent selection on a population made by bulking seeds of one accession that came from Arizona and nine from Texas in 1986 [58]. The bulked seeds were also the starting material for other breeding lines. WCL-LO4 was derived from mass selection from WCL-LO3 and WCL-LY2 (both not included in this study) which has WCL-LY1 as the source population [59], [60]. The other breeding lines were developed through phenotypic selection: WCL-YS1 was a selection from PI 311165, one of the initial accession from Arizona that comprise the original bulk in 1986 [61], while WCL-SL1 came from plants that survived at the highest salinity levels during a salt tolerance screening study when seeds from the original bulked seeds were planted [62].
The accessions of P. fendleri from Mexico are genetically similar as indicated by the cluster analyses. The range of Physaria species has been reported as limited to the northeastern part of country and concentrated on mountain and high plains of Coahuila, Nuevo León, and Zacatecas [57]. This limited geographic distribution likely prevented their further genetic differentiation. A similar investigation focusing on the other Physaria species in this region could confirm this trend.
The array of P. fendleri germplasm consisted of 75 accessions analyzed using microarray DArT and 128 accessions using DArTseq. There is ample genetic variability in the P. fendleri collection found, as indicated by the cluster analysis as well as analysis of population structure. This is expected for a cross pollinating species that has not been fully domesticated [63]. However, the higher within group variation detected by AMOVA using data from both DArT platforms suggests that there is only a small amount of genetic differentiation among groups in the sample as a whole. Results from the Bayesian clustering approach when comparing the geographical sources of the accessions suggested that there is more variability in New Mexico than the other P. fendleri locations and that there is a spatial pattern evident in the microarray DArT results. This pattern of genetic differentiation occurs outward from central Texas, the region identified as the putative origin of Physaria as proposed earlier by Payson [64].
An increased population differentiation has been reported in many plant species between source populations and new ones when plants colonize new habitats [65]. The more dynamic nature of Physaria populations in distant locations from Texas has been reported by Payson [64] and he attributed part of the process as caused by barriers that separate populations, such as soil properties and moisture availability which are very important to survival of ephemeral populations of the taxon. In P. fendleri, Dierig et al. [66] stated that temperature and elevation effects can also account for significant differences in reproductive capacity. Selection patterns have also been investigated by past studies reporting that non-random mating, sexual selection and dormancy characteristics play a significant role in how traits evolved in the species [67], [68]. Seed dormancy characteristics in particular cause a persistent soil seed bank which may prevent genetic differentiation in the species because certain genotypes are reintroduced back during subsequent seasons [69], [70]. In terms of genetic resources conservation of P. fendleri germplasm, the DArT results suggest the existence of more variable genotypes in New Mexico and it is recommended that future collecting missions select this geographic area to possibly expand the genetic diversity in the species collection.
Conclusion
The availability of genetic diversity information of the P. fendleri collection will enable better germplasm management and conservation of the species. In this study we report the successful development of two DArT marker platforms that were utilized for genotyping Physaria and Paysonia accessions. This marker system complements the microsatellite markers developed previously for Physaria. The high number of DArT markers allows a greater resolution of genetic differences among accessions and enabled us to examine the extent of variation in the P. fendleri collection, as well as provide support to known taxonomic classification and recent nomenclatural changes of certain Physaria species to Paysonia. We intend to further utilize the DArT markers in developing a linkage map in Physaria to assist breeding efforts and for future genetic mapping studies.
Acknowledgments
USDA is an equal opportunity provider and employer. Mention of companies or commercial products does not imply recommendation or endorsement by the USDA over others not mentioned. USDA neither guarantees nor warrants the standard of any product mentioned. Product names are mentioned solely to report factually on available data and to provide specific information. We would like to thank David Ellis and Chris Richards for the valuable suggestions during the conduct of the study.
Author Contributions
Conceived and designed the experiments: VMVC AK DAD. Performed the experiments: VMVC AK DAD. Analyzed the data: VMVC AK. Wrote the paper: VMVC AK DAD. Participated in germplasm collection: VMVC DAD.
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