Author for correspondence: David A. Baum Tel: +1 608 265 5305 Email: email@example.com
•Transgenomics is the process of introducing genomic clones from a donor species into a recipient species and then screening the resultant transgenic lines for phenotypes of interest. This method might allow us to find genes involved in the evolution of phenotypic differences between species as well as genes that have the potential to contribute to reproductive isolation: potential speciation genes.
•More than 1100 20-kbp genomic clones from Leavenworthia alabamica were moved into Arabidopsis thaliana by transformation. After screening a single primary transformant for each line, clones associated with mutant phenotypes were tested for repeatability and co-segregation.
•We found 84 clones with possible phenotypic effects, of which eight were repeatedly associated with the same phenotype. One clone, 11_11B, co-segregated with a short fruit phenotype. Further study showed that 11_11B affects seed development, with as much as one-third of the seeds aborted in some fruit.
•Transgenomics is a viable strategy for discovering genes of evolutionary interest. We identify methods to reduce false positives and false negatives in the future. 11_11B can be viewed as a potential speciation gene, illustrating the value of transgenomics for studying the molecular basis of reproductive isolation.
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A second commonly used approach for identifying species difference genes begins with quantitative trait locus (QTL) mapping and proceeds to clone genes using a variety of strategies (e.g. Bradshaw & Schemske, 2003; Briggs et al., 2007; Hovav et al., 2007; Lowry et al., 2009). QTL approaches are not biased against uncharacterized genes, but they are limited to species pairs that can be hybridized to yield fertile offspring. Additionally, it often proves impractical to clone the causal gene underlying a QTL, especially when working with nonmodel species (Bradshaw & Schemske, 2003; Bowman, 2006).
Here, we utilized a third strategy, transgenomics, that has the potential to identify genes that have contributed to the phenotypic differentiation and/or reproductive incompatibility of even quite divergent species pairs. This method involves transforming plants of a recipient species with random genomic clones from a donor species and then screening the transformants for phenotypic effects. Transgenomics has complementary strengths and weaknesses to candidate gene and QTL methods. Like QTL analysis, it can identify causal genomic regions that would not be predicted based on previous work in model species. At the same time, like a candidate gene approach, transgenomics is not restricted to crossable species and leads directly to experimentation at the molecular level. Transgenomics therefore has the potential to be an important addition to the toolbox of the evolutionary developmental geneticist.
Transgenomics was proposed as a theoretical possibility almost a decade ago (Baum, 2002), but has not yet been implemented because of the technical challenge of conducting even a pilot screen. In this study, we assessed the viability of the approach by introducing large numbers of genomic DNA clones from Alabama gladecress (Leavenworthia alabamica) into Arabidopsis thaliana. We found eight clones that associate with repeatable phenotypic effects, representing diverse aspects of plant form and development. One of these clones, 11_11B, was shown through co-segregation analysis to cause seed abortion and shortened fruit. Based on this analysis, we conclude that 11_11B has a dominant negative effect on plant fitness in an A. thaliana background. This shows that the causal gene within 11_11B has the potential to contribute to reproductive isolation between incipient species.
Materials and Methods
Donor species library construction
We partially digested total high-molecular-weight DNA from Leavenworthia alabamica Rollins with TaqI and fractionated the products on a 10–40% sucrose gradient. Approx. 20–25-kb fragments were ligated into the ClaI site of the binary cosmid vector pCLD04541 (Jones et al., 1992). This vector (28 kb) is based upon pRK290 and carries cos from bacteriophage lambda which allows highly efficient in vitro packaging. Additionally, it carries a polylinker with blue/white selection, T-DNA border sequences for insertion in the plant genome, and a kanamycin resistance gene (neomycin phosphotransferase II; NPTII) for selection of transgenic plants. We packaged ligated products with the Gigapack III kit (Stratagene, Santa Clara, CA, USA) and introduced the cosmid DNA library into XL1-Blue MR Escherichia coli host cells. This library was determined to contain c. 8.5 × 105 colony-forming units (CFUs), which translates to c. 34.5-fold coverage of the c. 500-Mb L. alabamica genome. Aliquots of this library are available upon request to researchers interested in additional transgenomic screening.
Ninety-six-well plate archives
We plated L. alabamica clones in E. coli on 2X Yeast Extract Tryptone media with tetracycline selection (10 μg ml−1). We picked single colonies of various sizes to reduce bias against clones that slow bacterial growth. Each colony was transferred into a different well of a 96-well plate containing 150 μl of 2X YT media with 10 μg ml−1 tetracycline, and was grown overnight. Then 1.3 μl of each culture was used to inoculate l.3 ml of 2X YT in a 96-square-well block format. The remaining culture was mixed with 100 μl of 60% glycerol and frozen for long-term storage. We used the R.E.A.L. Prep 96 kit (Qiagen, Valencia, CA, USA) to isolate cosmid DNA from cultures grown in the 96-square-well format. We froze 16 μl of the purified cosmid for long-term storage in a 96-well plate, and used 4 μl of the DNA to transform 20 μl of freshly grown Agrobacterium tumefaciens strain GV3101::pMP90 using a high-throughput freeze-thaw transformation protocol (Weigel & Glazebrook, 2006) in 96-well format. Immediately following transformations, we mixed each A. tumefaciens sample with 96 μl of liquid Lysogeny Broth (LB) media without selection and allowed 4 h of recovery with gentle rocking. We then further grew each A. tumefaciens sample with an additional 60 μl of liquid LB with final concentrations of 75 μg ml−1 kanamycin, 50 μg ml−1 rifampicin, and 25 μg ml−1 gentamycin. We used kanamycin instead of tetracycline for selection because GV3101::pMP90 is capable of evolving spontaneous tetracycline resistance. Following growth, we mixed each A. tumefaciens culture with 70 μl of 60% glycerol and froze it for long-term storage. We named each L. alabamica clone according to the well and 96-well archive in which it was stored. For example, the L. alabamica clone stored in the well in column 8 and row H of the fifth 96-well archive was given the name ‘05_08H’. Replicates of the fourteen 96-well plates of arrayed E. coli clones are available from the authors upon request.
We grew three to five wild-type A. thaliana (L.) Columbia (T0) plants in a single 3.5-inch SVD pot (T.O. Plastics, Clearwater, MN, USA). We transformed up to 192 T0 pots (two plates’ worth) per week. Upon bolting of T0 plants, we clipped their primary shoots to induce early flowering of lateral shoots. We transferred 40 μl of each A. tumefaciens culture from a 96-well archive to a 50-ml conical tube containing 10 ml of liquid LB with 75 μg ml−1 kanamycin, 50 μg ml−1 rifampicin, and 25 μg ml−1 gentamycin. These 10-ml A. tumefaciens cultures were grown for 2 d at 28°C with 200 rpm shaking. Cells were pelleted and resuspended in 8 ml of infiltration medium (50 g l−1 sucrose, 2 g l−1 MS salt, 0.4 g l−1 MES, 1.3 M KOH and 200 μl l−1 silwet). We then dripped infiltration cultures onto the inflorescences of T0 plants. This protocol is based on that of Clough & Bent (1998), but is better suited to transgenomics because of the small volume of A. tumefaciens cultures and the relative speed of dripping bacteria onto plants rather than dipping plants into bacterial cultures. After inoculation, we shaded T0 plants for 16 h and watered them for 3 wk before allowing them to dry. We harvested the seeds of T0 plants in one pot and pooled the seeds to form a T1 seed stock made with a single L. alabamica clone. All plants were grown under standard glasshouse conditions (16 h light at 22.5°C : 8 h dark at 18°C, 300–500 mmol m−2 s−1, and air-conditioning) at the University of Wisconsin-Madison Biotron Facility.
Phenotypic screening of primary transformants
We applied a portion of each T1 seed stock to one kanamycin medium plate (75 mg l−1 kanamycin, 2.2 g l−1 MS salt, 0.4 g l−1 MES, 8.5 g l−1 agar and 0.8 mM KOH). We incubated plates at 4°C for 3 d to synchronize germination. Following cold treatment, we grew plates for 11 d in standard glasshouse conditions under shade cloth (50–100 mmol m−2 s−1) to avoid excessive humidity inside plates. At the end of this period, we transferred one or two T1 seedlings per plate to a 3.25-inch SVD pot of soil with the goal of allowing one T1 plant to develop. T1 plants were visually screened during development and at maturity for various morphological phenotypes that were not seen in untransformed plants grown in parallel. We screened T1 plants for changes in plant architecture, stature, rosette and cauline leaf morphology, flowering time (indicated by the total number of leaves on each plant), and reproductive traits such as flower and fruit morphology, including cases of low fertility, as indicated by few or stunted fruit on plants. We screened flowers in the mornings when petals were open and observed the other traits throughout the day. Seeds were harvested from all T1 plants (regardless of their phenotype) and those that were not used up in subsequent experiments are available from the authors upon request.
For each plant in which we identified a phenotypic abnormality, we grew four additional independent T1s and screened them for the phenotype. If phenotypic effects recurred in at least one of the four additional T1s, then further T1 plants were isolated to see if each phenotype continued to repeat. A ‘repeatable phenotype’ was defined as one that recurred in at least three T1 plants.
To infer the number of insert loci for each T1 line, we sowed c. 500 T2 seeds on kanamycin medium. Seedlings were allowed to grow with 16 h light (c. 100 mmol m−2 s−1) at 22.5°C : 8 h dark at 18°C. After 2 wk, we counted numbers of kanamycin-resistant and -sensitive offspring on each plate. A 99% confidence interval of the observed kanamycin resistant-to-sensitive (KRS) ratio was used to assess whether it was consistent with Mendelian expectations for one or two segregating loci. For lines with significantly fewer kanamycin-resistant seedlings than expected for one transgene locus (KRS significantly <3), we also assessed segregation in T2 plants genotyped by PCR amplification of the NPTII gene using the primers (5′–3′) ATCCCATGG CTGATGCAATGCG and CCATGATATTCGGCAAGCAGGCAT.
For each repeatable clone, we selected a line whose segregation ratio suggested a single insert locus, namely, a line for which the 99% confidence interval around the observed KRS included 3 : 1 (see Supporting Information Table S1). When there were multiple such lines for a clone, we chose the line with the clearest phenotypic effect in the T1 generation. For one clone (06_05C), all lines screened deviated significantly from a 3 : 1 ratio so we picked the line that was closest to a 3 : 1 ratio.
Approximately 50 T2 plants from each selected line were grown in individual 3.25-inch pots. Each T2 family was grown to maturity for 42–56 d in standard glasshouse conditions. We then scored families for phenotypic variation. The length of fruit valves or entire fruit was determined by scanning them at 4000 dpi on an Epson perfection V700 Photo scanner and using ImageJ 1.43u software (Rasband, 1997–2011) to measure the length.
To obtain T2 genotypes, we harvested T3 seeds produced by each T2 plant. Approximately 100 seeds of each T3 plant were screened on kanamycin plates. The KRS ratios of T3 plates were then used to infer the transgene genotype of each T2 plant. In the family for clone 06_05C, we could not confidently genotype T2 plants because of deviations from Mendelian ratios, suggesting trans-gene silencing. Therefore, we scored T2 plants as ‘transgene present’ if they produced any resistant seedlings and ‘transgene absent’ if they produced only sensitive T3 seedlings.
For continuous phenotypes, we used ANOVA to assess whether there was a significant association between transgene genotype and phenotype. We used a one-way ANOVA for the initial round of experiments that tested one line per clone. When studying the four independent lines from clone 11_11B, we used a mixed-effects ANOVA model that explained fruit length variation with fixed effects for individual line, genotype, an interaction between them, location of fruit within the inflorescence (categorized with separate categories for the lowest five locations and one category for all other locations), an interaction between fruit location and line, and a random effect for individual plant. Categorization of fruit location was based on graphical data exploration. We considered models with additional interaction terms, but found none that significantly improved the fit to the data. We used the lme4 package in R to conduct the mixed-effects ANOVA analysis. ANOVA P-values < 0.05 were taken to indicate significant co-segregation. For clone 06_05C, co-segregation was assessed with a two-tailed homoscedastic Student’s t-test.
Identifying appropriate model species for transgenomics
Transgenomics depends on being able to easily move donor species’ genomic DNA into a recipient species. While reliable transformation methods exist for many species, we chose the model plant A. thaliana because of its great transformation efficiency and the immense body of available genetic and genomic data. An appropriate donor species would be phylogenetically close (i.e. within Brassicaceae), yet divergent from A. thaliana for as many distinct phenotypes as possible. Leavenworthia is a member of the Cardamineae clade, which is within ‘Lineage I’ of Brassicaceae, the same lineage that includes Arabidopsis (Beilstein et al., 2006). The radiation of Lineage I Brassicaceae has been dated at 8–14 Ma (Franzke et al., 2009), 18–36 Ma (Couvreur et al., 2010) or 35 ± 6 Ma (Beilstein et al., 2010). Leavenworthia and Arabidopsis differ in almost all visible traits that show discrete variation within Brassicaceae (Fig. 1), including leaf shape (pinnately compound vs simple), trichomes (absent vs present), shoot architecture (rosette flowering vs inflorescence flowering), flower size (> 1 cm diameter vs < 3 mm diameter), fruit shape (strongly flattened vs terete), seed number per fruit (c. 9–10 vs 50–70) and seed size (c. 3.1 vs 0.5 mm). Leavenworthia alabamica has a modest genome size of c. 500 Mbp (M. A. Lysak & P. Bures, unpublished), has 11 chromosomes (Lysak et al., 2009), and is the target of an ongoing genome-sequencing project (http://biology.mcgill.ca/vegi/index.html).
Arabidopsis thaliana can be transformed with L. alabamica genomic clones in a high-throughput manner
As a first step in the transgenomic screen, we generated a plant transformation-competent genomic library of L. alabamica, with inserts of 20–25 kb. We conducted pilot analyses of alternative screening strategies and concluded that the optimal was a high-throughput clone-by-clone screening approach (Fig. 2). This method has the advantage that one can readily determine whether a clone generates the same abnormal phenotype in multiple independent transformants. Because each independent transformant is likely to have a unique insertion site, one can avoid wasting effort following up phenotypes that result from insertional mutagenesis or other artifacts of transformation.
We used this clone-by-clone transgenomic pipeline to conduct an initial screen of 14 96-well archives. We found that this workflow could be completed by a small team, comprising one graduate student and two to three undergraduates, in < 6 months with a peak rate of plant transformation of c. 160 clones (grown from two 96-well plates) per week. Eighty-five per cent of the E. coli clones were successfully transferred to A. tumefaciens and 99% of those were successfully introduced into plants.
Analysis of primary transformants yielded clones associated with repeatable phenoypic abnormalities
Because A. tumefaciens-mediated transformation of A. thaliana occurs in the female gametophyte, and because T-DNA integration is nonhomologous, T1 plants are hemizygous for the transgenic fragment. A transgene is only expected to manifest a visible phenotype in the T1 generation if it is trans-dominant, meaning that its effects are not masked by endogenous homologs in the genome. Our aim was to screen for clones associated with such trans-dominant phenotypic effects.
We used a small portion of each T1 seed stock to isolate one kanamycin-resistant T1 plant per L. alabamica clone (Fig. 2c). Of 1147 A. tumefaciens clones, 1134 were successfully introduced into A. thaliana plants. For each clone, we visually screened at least one T1 individual during development and at maturity (c. 6 wk of age) for morphological phenotypes that differed from those of untransformed plants. Eighty-four clones (7.4%) yielded initial T1 plants that were judged to deviate from wild type (Table 1).
Table 1. Effects of 1147 Leavenworthia alabamica clones
No visible effect
An abnormal phenotype in a single T1 plant could result from a trans-dominant effect of an L. alabamica clone, insertional mutagenesis, background mutation, perturbation caused by screening on kanamycin plates and transplanting, or environmental variability. Of these, only effects that are attributable to inserted L. alabamica DNA are of interest. Because independent transformants of the same clone are likely to have integrated into the genome at unique insertion sites, phenotypes that appear repeatably are most likely to be attributable to the L. alabamica insert. Therefore, we excluded 70 clones that failed to repeat the abnormal phenotype after screening four additional, independent T1 plants for the same clone (Fig. 2c). We then screened additional T1 plants and excluded two further clones because of a failure to generate additional abnormal T1 plants.
We discarded four clones that were found, based on end-sequencing, to include Lambda viral DNA, probably introduced during library construction. These four clones were associated with low repeatability and involved phenotypes that occur not infrequently in wild-type plants.
After this winnowing process, we were able to identify eight clones containing L. alabamica genomic DNA that were associated with repeatable, trans-dominant phenotypes in A. thaliana (Table 2). The level of repeatability, defined as the number of T1 plants with the phenotype divided by the number of T1s screened, varied from 16 to 60%, with an average of c. 42% (Table 2). This collection of clones was associated with a broad range of phenotypes in A. thaliana: two were flagged primarily because of a change in fruit size/shape, three for plant architecture/stature, one for flower form, one for a leaf defect, and one for abnormal leaf and flower development (Figs 3, S1, S2).
Table 2. Clones assigned with repeatable phenotypes
Repeatability (T1s with phenotype/T1s screened)
Short plants withreduced seed set
Petals unevenly spaced
Rosette leaves twisted
Fruit cluster with very reduced internodes
Cauline leaves decurrent to stem
For 10 independent T1 plants from clone 05_01C and nine from 11_11B, we measured valve length for 20–40 fruit. This confirmed that they do, indeed, tend to have shorter fruit than wild type (Table 3). Because data were only collected for transgenic lines that had been visually flagged as having short fruit, meaningful statistical comparisons could not be made until later generations. However, the dramatic differences between wild type and several T1 plants for clones 05_01C and 11_11B are consistent with these clones having a transdominant effect on fruit length.
Table 3. Fruit valve lengths
Number of T1 plants
Number of valves per plant
Valve length (mm) (mean ± SE)
Plant with shortest fruit
Plant with longest fruit
11.3 ± 1.0
14.2 ± 0.7
12.7 ± 0.2
7.4 ± 2.5
13.5 ± 0.8
10.9 ± 4.5
4.9 ± 0.6
12.5 ± 0.5
9.3 ± 0.3
To identify lines with one transgene locus, we screened T2 seeds for four to eight lines per clone (Table S1). For seven of the eight clones, we found at least two lines whose KRS ratio was consistent with a single transgene locus. Of the 44 lines screened, 21 were inferred to have one transgene locus, and two to have two unlinked transgene loci (Table S1). Of the remaining 21 lines, 12 had a deficit of resistant plants beyond that expected even for a single transgene locus. To help assess the causes of non-Mendelian segregation patterns, we used PCR amplification of NPTII to genotype 23–40 T2 plants from those lines that showed a deficit of kanamycin-resistant seedlings (Table S2). In two of 12 lines (12_03E_1; 12_06G_1) we detected a significant deficit of NPTII-containing offspring. This could indicate an effect of the L. alabamica transgene on gamete, gametophyte or embryo development, or it could be attributable to insertional mutagenesis or chromosomal rearrangements during transformation (e.g. see Ray et al., 1997). In three lines (11_11B_2, 11_11B_15 and 12_06G_2) we found a significant difference between the KRS and NPTII ratios, where the latter was not significantly different from 3 : 1. This is most easily explained by Mendelian segregation of the transgene coupled with silencing of NPTII in many plants. In the remaining lines the ratio determined by PCR was consistent with both the KRS ratio and 3 : 1.
We selected one candidate line from each L. alabamica clone, choosing the line with the strongest T1 phenotype that was inferred to contain a single transgene locus. Because we found no lines for clone 06_05C that showed a 3 : 1 ratio, we selected a line (06_05C_03) that produced about half sensitive and half resistant T2 seedlings.
For each selected line, we grew a T2 family on soil so that we would be able to score plant phenotypes while being blind to genotype. We allowed T2 plants to self and set T3 seed. A portion of T3 seeds from each T2 plant were tested on kanamycin medium to infer its transgene genotype (see the Materials and Methods section).
ANOVA could not reject the null hypothesis that variation in the scored phenotype was independent of transgene genotype in six of eight T2 families (Fig. 4). In the T2 family for clone 06_05C, a two-tailed homoscedastic Student’s t-test did not detect a significant difference in fruit length between transgene-present and transgene-absent T2 plants. However, only 22 T2 plants were scored in this case. Combined with the distorted segregation ratios seen in this line, we judge this co-segregation test to be noninformative.
For clone 11_11B, transgene-hemizygous T2 plants were found to have shorter fruit, while transgene-homozygous and transgene-absent T2 plants had fruit of normal length (one-way ANOVA, P =0.00046; Fig. 4e). This result suggests that the transgene has a trans-overdominant effect: shortening fruit only when the transgene locus is hemizygous, and not when it is homozygous.
Co-segregation analysis suggests that 11_11B causes short fruit and ovule/seed abortion
The conclusion that 11_11B co-segregates with fruit length variation came from a single T1 line. To determine if this result could be replicated, we conducted a more thorough study of the same line (11_11B_1) plus three additional, independent 11_11B lines (5, 6, and 10). All lines had been scored with KRS ratios that were close to 3 : 1 (Table S1). We scanned the inflorescences and used image analysis software to measure fruit length for all mature fruits along the main axis. Genotypes were then inferred by sowing batches of T3 seeds on kanamycin plates.
Controlling for variation that was attributable to line, individual plant, individual fruit, and location of fruit in the inflorescence, the null hypothesis that genotype has no effect on fruit length was rejected. Once again, a significant transgene overdominance effect on fruit length was observed (Fig. 5). However, this effect was only detectable in lines 1, 5, and 6. Within these three lines transgene-absent and transgene-homozygous plants had indistinguishable fruit sizes, whereas transgene-hemizygous fruit were, on average, 1.6 ± 0.7, 3.1 ± 0.8, or 3.7 ± 0.7 mm shorter than the average transgene-absent fruit, respectively. The mean fruit length in line 10 hemizygotes was indistinguishable from that of wild type.
As it is well established that normal fruit elongation requires the production of signals from developing seeds (Gillaspy et al., 1993; Chaudhury et al., 1997; Vivian-Smith et al., 2001), the fruit-length effect could be caused by defects during ovule/megagametophyte development, fertilization, or early embryogenesis. We observed aborted seeds in a subset of T2 plants. Aborted seeds were recognizable as short funicles with a knob-like ending, which persisted even after seed dispersal, whereas normal funicles were long and tapering after seed abscission. We scored one to four mature, dehisced fruit per plant from at least five individual plants for each genotypic class, for lines 1, 5, 6, and 10, all while blind to plant genotype.
As summarized in Table 4, plants carrying the 11_11B transgene showed significantly higher levels of seed abortion than wild-type plants in the same three lines (1, 5, and 6) that showed a fruit-length effect. Whereas < 5% of seeds typically aborted in wild-type lines, the mean abortion rate in line 1, 5, and 6 hemizygotes was 12, 34, and 31%, respectively. However, in contrast to the complete overdominance seen for fruit length, significantly elevated seed abortion was also seen in some transgene homozygotes. Examination of abortion rates for individual plants from lines 1, 5, and 6 suggested that among the transgene-containing lines there was an extensive variation in seed abortion rate (Fig. S3).
Table 4. Proportion of seed aborted (mean ± SD) in the three genotypic classes in four independent 11_11B lines
0.03 ± 0.03
0.12 ± 0.04
0.10 ± 0.07
0.03 ± 0.02
0.34 ± 0.14
0.27 ± 0.19
0.05 ± 0.03
0.31 ± 0.09
0.27 ± 0.15
0.04 ± 0.02
0.03 ± 0.04
0.06 ± 0.02
The aborted seeds seen in 11_11B-containing plants could be a result of disruption of processes such as ovule development and pollen maturation in parental tissues. Alternatively, the transgene could act directly on gametophytes or embryos. In the latter case, we might expect the seeds of hemizygotes to show a deficit of transgene homozygotes and hemizygotes because they would abort at a higher rate than wild-type gametophytes/embryos. However, for all four lines tested for co-segregation, the ratio of inferred wild-type, transgene-hemizygous, and transgene-homozygous T2 plants did not deviate significantly from 1 : 2 : 1. This supports the conclusion that these four T1 lines each has a single transgene locus and suggests that embryos carrying the transgene do not have an elevated abortion rate.
Transgenomic screens are feasible and can be improved to reduce the false negative and false positive rate
Our study succeeded in introducing 1147 clones from an L. alabamica genomic library into A. tumefaciens. For all but the 13 clones that did not yield transformant plants, we examined at least one primary transformant. Assuming a genome size for L. alabamica of 500 Mb, and an average insert size of 20 kb, our study achieved just over 4.5% coverage of the L. alabamica genome. While this is still a small fraction of the donor genome, considering that the initial screen took < 6 months and was conducted by one graduate student with two to three undergraduate assistants, it is possible for a transgenomic screen to achieve 1X or higher coverage of the donor genome. Even with a team twice the size of ours, and provided that the efforts were well coordinated and that growth space was not limited, it should be possible to generate transformants for 2000 clones per month. With potential technical improvements, such as larger clone inserts, it could become feasible for even small research groups to undertake complete transgenomic screens of various donor species in an A. thaliana genetic background. Further, as transformation technology improves for many species, including more and more cases of drip/dip transformation (e.g. Bartholmes et al., 2008), it may become possible to use recipient species other than A. thaliana.
In addition to generating a large number of transgenomic lines that can be investigated further, our research provides a baseline from which to optimize future transgenomic screens. While this study illustrates the feasibility of transgenomics, it also highlights some challenges. For example, we found a large number of false positives. Whereas 7.4% of the initial transgenic plants showed some deviation from wild-type morphology, the great majority did not reproduce that phenotype in additional lines. Most of these anomalous phenotypes are presumed to be caused by insertional mutagenesis, stress during kanamycin screening and transplantation, fluctuations in growth conditions, or disruption of T1 seedling development by A. tumefaciens-mediated drip transformation of T0 plants.
We believe that growing a larger initial number of T1 plants per clone would be likely to reduce the number of false positives (and false negatives). This would enhance the ability to quickly identify repeatable phenotypes and would not greatly slow down the screen. Thus, we predict that a significant reduction in the false positive rate can be achieved.
Of the 84 clones tested for repeatability, eight were capable of generating four or more independent T1 plants that shared a particular morphological effect (Table 2). However, these clones also yielded as many or more normal T1 plants (Table 2). If L. alabamica clones were responsible for the phenotypes, then potential explanations for low repeatability include sensitivity to position of insertion and/or transgene silencing. We demonstrated silencing of the selectable marker gene NPTII in some lines (Table S2), so it is reasonable to assume that silencing could also affect a phenotype-altering transgene derived from the donor species. Alternatively, repeatable phenotypes could reflect parental background mutations or nongenetic effects that recurred in plants in similar microenvironments.
The low repeatability associated with the eight clones suggests that, by screening just one T1 plant per clone, we were likely to have experienced a considerable false negative rate. The one clone that was confirmed as inducing a phenotypic effect had a T1 repeatability of 56%. Assuming that this is typical, we should expect to miss almost half of the phenotype-altering clones when conducting a screen based on only one initial T1 plant. The simplest way to reduce this false negative rate would be to screen several T1 plants per clone rather than just one.
Seven out of eight repeatable phenotypes are likely to be the result of background mutations in T0 plants or to reflect environmental variability
Our finding that six of the eight repeatable phenotypes did not co-segregate with the transgene does not necessarily rule out the possibility that the transgenes do explain the observed T1 phenotypes. The lack of co-segregation could be caused by a confounding factor such as post-transcriptional transgene silencing (Morel et al., 2000) or environmental effects that prevented phenotype manifestation in T2 plants. To be sure that no lines are prematurely discarded, future co-segregation analyses should consider additional T2 lines from the same clones.
One possible explanation for the non-co-segregation of repeatable clones is that the repeatable phenotypes reflect genetic abnormalities of the T0 parental plants. Supposing that one or two of the T0 plants carried a background mutation or had a genotype that rendered it and its offspring prone to express a phenotypic abnormality, one could expect the phenotype to recur in T1 plants at a greater frequency than in the population as a whole. If one allows for uneven contributions of T0 plants to the T1 seed stock, then such an effect can readily explain traits occurring at a sufficient frequency to pass our test for repeatability (Table 2).
Understanding the potential sources of false positives, we can suggest methodological refinements for future transgenomic screens. For example, a phenotype caused by a homozygous background mutation in a T0 plant should be inherited by all offspring of that plant, while the L. alabamica transgene can only occur in T1 plants that are resistant to kanamycin. Therefore, transgenomic screens should be able to distinguish potentially transgene-induced phenotypes from background efffects by growing several T1s from the same clone – half taken from a kanamycin plate and half from a kanamycin-lacking plate (the vast majority of plants from the latter plate will lack the transgene). Alternatively, T0 plants could be harvested separately and several T0 plants could be transformed. This would allow identification of repeatable phenotypes that are independent on T0 parent and, therefore, likely to be caused by the transgene.
Leavenworthia alabamica clone 11_11B insert contains a transgene with overdominant effects on A. thaliana reproductive development
For three of the four 11_11B lines, the transgene genotype correlated with an overdominant phenotypic effect. Hemizygous fruit were significantly shorter than wild type, whereas transgene-homozygous fruit resembled wild type (Fig. 4e, 5). Likewise, mean seed abortion was highest in hemizygotes (Table 4). Especially in lines 5 and 6, transgene-homozygous plants showed elevated levels of seed abortion that were closer to those of hemizygous plants than to those of wild type (Table 4, Fig. S3).
Overdominance is not uncommon in transgenic experiments (Nap et al., 1997; Damgaard & Borksted, 2004). The most plausible hypothesis to explain overdominance of 11_11B is that the transgene (but not NPTII) is silenced more frequently in homozygous lines than in hemizygous lines. This is suggested by the seed abortion data, which appear to show a bimodal distribution of seed abortion rates in the presence of the transgene (Fig. S3). The distribution of seed abortion rates (Fig. S3) suggests that the difference in the mean seed abortion rate between transgene-homozygous and -hemizygous plants may be attributable to differences in the probability of gene silencing rather than a difference in the severity of the phenotype when the transgene escapes silencing.
Taken together, our data show that the 11_11B insert contains a gene that has a phenotypic effect when introduced as an extra copy into the genome of A. thaliana. There are two main ways that a transgene from one species can cause a phenotype when added to a foreign genome. First, if the causal transgene is similar to an endogenous gene in the recipient genome, the addition of an extra one or two allelic copies could cause a phenotypic abnormality by altering gene dosage. For example, the presence of the extra gene copy could elevate the expression level of a gene product, resulting in a gain-of-function phenotype, or, conversely, it could trigger silencing of the endogenous homologs, resulting in a loss-of-function phenotype. Secondly, coding or cis-regulatory sequences may have diverged from the orthologous gene in the recipient genome, resulting in novel biochemical functions or expression patterns. The genetic pattern of overdominance in 11_11B provides circumstantial evidence against an effect that is entirely attributable to gene dosage. Plausible dosage-based models do not predict overdominance, but either additivity, if each extra gene copy incrementally increases the effect, or simple dominance, if there is a threshold beyond which extra copies have no additional effect. This leads us to believe that our preliminary transgenomic screen has succeeded in identifying at least one phenotype-altering gene whose effect is a result of sequence evolution since A. thaliana and L. alabamica last shared a common ancestor.
The 11_11B clone causes a reduction in seed set of A. thaliana and this phenotypic effect is unlikely to be a dosage effect. Thus, it is reasonable to suppose that, if an F1 hybrid were formed between a plant carrying the causal L. alabamica gene and a plant carrying interacting genes from A. thaliana, then these F1 hybrids would show reduced fertility. Thus, the causal sequence underlying the effect of 11_11B is a candidate for being a potential speciation gene, where a potential speciation gene is one that has the potential to undergo mutations that could contribute to hybrid infertility between the incipient species. We are not claiming that the 11_11B causal sequences played a role in the speciation event that gave rise (eventually) to the Arabidopsis and Leavenworthia lineages. Given how long ago the common ancestor lived, this is actually quite unlikely. Rather, we are suggesting that, if the analogous changes were to occur in incipient species, the gene could contribute to their hybrid infertility. This shows that, in the same way that we can learn about the molecular basis of reproductive isolation by studying genes that cause reproductive isolation between genotypes within a single species (Bomblies & Weigel, 2007, 2010; Bomblies et al., 2007), transgenomics can help identify genes that have the potential to participate in speciation.
We thank Kandis Elliot, Jeremy Berg, Thomas Coolidge, Caitlin Heaton, Pa Yiar Khang, Emily Kief, Kevin Miller, Evan Nondorf, Brittany Ota, Rebecka Pralle, C. R. Pulikesi, Tanjina Shabu, Jamie Shier, Colt Skenandore, Jacob Smith, Bohkyeong Suh, Daniel Wear, Brandon Weathersby and Arielle Woods for their contributions to this project. Much useful advice was provided by Philip Anderson, John Doebley, Donna Fernandez, Patrick Krysan, and Marek Sliwinski. We gratefully acknowledge the UW-Madison Biotron facility and its staff. This work was funded by NSF (IOB-0641428 and IOS-1021930) and the UW-Madison Graduate School. R.C. was supported on NIH training grant T32 GM007133.