- Research article
- Open Access
Genome-wide identification and characterization of the bHLH gene family in tomato
© Sun et al.; licensee Biomed Central. 2015
- Received: 27 June 2014
- Accepted: 30 December 2014
- Published: 22 January 2015
The basic helix-loop-helix (bHLH) proteins are a large superfamily of transcription factors, and play a central role in a wide range of metabolic, physiological, and developmental processes in higher organisms. Tomato is an important vegetable crop, and its genome sequence has been published recently. However, the bHLH gene family of tomato has not been systematically identified and characterized yet.
In this study, we identified 159 bHLH protein-encoding genes (SlbHLH) in tomato genome and analyzed their structures. Although bHLH domains were conserved among the bHLH proteins between tomato and Arabidopsis, the intron sequences and distribution of tomato bHLH genes were extremely different compared with Arabidopsis. The gene duplication analysis showed that 58.5% and 6.3% of SlbHLH genes belonged to low-stringency and high-stringency duplication, respectively, indicating that the SlbHLH genes are mainly generated via short low-stringency region duplication in tomato. Subsequently, we classified the SlbHLH genes into 21 subfamilies by phylogenetic tree analysis, and predicted their possible functions by comparison with their homologous genes of Arabidopsis. Moreover, the expression profile analysis of SlbHLH genes from 10 different tissues showed that 21 SlbHLH genes exhibited tissue-specific expression. Further, we identified that 11 SlbHLH genes were associated with fruit development and ripening (eight of them associated with young fruit development and three with fruit ripening). The evolutionary analysis revealed that 92% SlbHLH genes might be evolved from ancestor(s) originated from early land plant, and 8% from algae.
In this work, we systematically identified SlbHLHs by analyzing the tomato genome sequence using a set of bioinformatics approaches, and characterized their chromosomal distribution, gene structures, duplication, phylogenetic relationship and expression profiles, as well predicted their possible biological functions via comparative analysis with bHLHs of Arabidopsis. The results and information provide a good basis for further investigation of the biological functions and evolution of tomato bHLH genes.
- Solanum lycopersicum
- bHLH gene family
- transcription factor
- fruit development
The basic helix–loop–helix (bHLH) proteins are a large superfamily of eukaryotic transcription factors, and play a central role in a wide range of metabolic, physiological, and developmental processes [1-3]. Their bHLH domain contains approximately 60 amino acids, including a basic region and a HLH region . The basic region, which consists of approximately 17 amino acids and is located at the N-terminus of the domain, is a DNA-binding region that allows HLH proteins to bind to a consensus hexanucleotide E-box (CANNTG) [5,6]. The HLH region is composed of two amphipathic helices consisting of hydrophobic residues linked by a divergent (both in length and primary sequence) loop, and functions as a dimerization domain [4,7]. The HLH domain promotes protein–protein interactions and allows for the formation of homodimeric or heterodimeric complexes . Excluding the conserved bHLH domain, the proteins showed considerable sequence divergence . Furthermore, groups of evolutionary and/or functionally related bHLH proteins shared additional motifs. Some of these motifs have been characterized in animals regarding specificity in the DNA-binding sequence recognition and dimerization activities responsible for the activation or repression of target genes or for binding to small molecules .
Previous classifications of animal bHLHs have led to the definition of six major functional and evolutionary lineages (groups A–F) [1,8], which have been further subdivided into several smaller orthologous subfamilies . Group A of the bHLH proteins can bind to the E-box sequence. In group B, several proteins such as Max, Myc, MITF, SREBP and USF have diverse functions and bind to the G-box sequence CACGTG [10-12]. Members of Group C contain an additional protein–protein interaction region and bind to sequences (NACGTG or NGCGTG) that do not resemble the E-box. Proteins in group D contain only the HLH region and can form heterodimers with bHLH proteins and thus are functionally related to typical bHLH proteins . Group E proteins contain Pro or Gly residues within the basic region and can bind preferentially to the CACGNG sequence [14,15]. Group F proteins contain divergent sequences compared with other groups and another domain for dimerization and DNA binding .
Studies on the bHLH gene family in various species will increase our understanding of their evolution and functions. However, systematic identification of the bHLH genes has been performed only in a few plants, such as Arabidopsis, rice, poplar, and moss [2,17-19]. Tomato (Solanum lycopersicum) is one of the most important vegetables in the world and is also a model plant for studying fruit development . Tomato genome sequencing was recently completed and published . But the bHLH gene family of tomato has still not been reported. In this study, we systematically identified and characterized the bHLH genes of tomato (SlbHLH) and compared them with the bHLHs of Arabidopsis thaliana. In addition, we also analyzed the expression profiles of SlbHLH genes in different tissues and at different stages of fruit development as well in response to iron-deficiency stress. Finally, we detected several genes associated with fruit development and ripening, and with iron-deficiency responses. In addition, we did the evolutionary analysis of SlbHLH genes by comparison of tomato bHLH genes with that of angiosperm, early land plants and algaes.
Identification and classification of tomato bHLH genes
bHLH domain consensus motif
Atchley et al. [ 5 ]
Toledo-Ortiz et al. [ 2 ]
Position in the alignment
Consensus motif amino acid frequency within the bHLH domain
Position in the alignment
Amino acid frequency within the Arabidopsis bHLH domains
Position in the alignment
Amino acid frequency within the tomato bHLH domains
R (61%), K (27%)
R (24%), K (22%)
K(28%), R(25%), N(11%)
R (77%), K (16%)
E (76%), A (10%)
R (81%), K (14%)
R (74%), K (14%)
I (35%), L (33%), V (23%)
I (52%), L (27%), M (12%)
I(53%), L(28%), M(17%)
N (51%), S (19%)
F (72%), L (14%), I (9%)
F (26%), L (26%), M (20%), I (14%)
L(28%), F(26%), M(19%), I(16%)
R (44%), K (35%)
Q (42%), R (35%)
K (58%), R (24%)
K (45%), T (13%)
I (74%), T (15%), V(7%)
M (33%), I (27%), V (16%), L (14%)
M(33%), I(28%), V(15%), L(14%)
L (76%), V (14%)
A (60%), I (16%), V (12%)
A(60%), I(18%), V(11%), T(10%)
I (31%), V (27%), T (23%)
I (63%), V (22%)
I (69%), L (16%), V (8%)
I (40%), V (33%), L (13%)
I(43%), V(38%), L(13%)
L (80%), M (7%)
The tomato ‘Heinz 1706’ genome, with a genome size of approximately 900 Mb, was 7.2 times larger than the A. thaliana genome. However, the number of SlbHLH genes was similar to Arabidopsis (162). Based on the total genes, the ratio to SlbHLH genes in the tomato genome was about 0.46%, which was similar to rice (0.44%) and poplar (0.40%) [18,19], but was less than Arabidopsis (0.59%) [2,17].
Conserved amino acid residues in the bHLH domains and DNA-binding ability
Predicted DNA-binding categories based on the bHLH domain
Number of AtbHLHs
Number of SlbHLHs
(Toledo-Ortiz et al. [ 2 ])
Intron distribution and phylogenetic analysis of SlbHLH genes
Up to now, the biological functions of the most SlbHLHs remain unclear. However, approximately 40% of Arabidopsis bHLH proteins have been functionally characterized. Hence, the clustering and comparison of tomato bHLH proteins with AtbHLHs can help to predict their functions via ortholog analysis. The phylogenetic analysis revealed that 43 SlbHLHs were tightly grouped with the AtbHLHs, in which their functions are known. These suggest that the 43 SlbHLH proteins may have the similar functions as their Arabidopsis ortholog (Additional file 7 and Additional file 8).
Expression pattern of SlbHLH genes among different tissues
To analyze the expression pattern of SlbHLH genes among ten different tissues, we used the Reads Per Kilobase per Million (RPKM) normalized data from RNA-seq . Additional file 9 showed the expression profiles of SlbHLH genes in the ten tomato tissues. Among 159 SlbHLHs genes, 122 expressed at least in one of the ten tissues with an RPKM value greater than 1.4, while the rest 37 exhibited a low expression (RPKM ≤ 1.4) in all ten tissues. Moreover, we found that 21 SlbHLHs displayed the tissue-specific expression preference (the expression intensity was more than 2 times higher in a particular tissue than that of other tissues). They include eleven genes (SlbHLH034, SlbHLH038, SlbHLH050, SlbHLH053, SlbHLH057, SlbHLH085, SlbHLH092, SlbHLH105, SlbHLH120, SlbHLH128, SlbHLH137) in root, three genes (SlbHLH024, SlbHLH041, SlbHLH094) in leaf, three genes (SlbHLH001, SlbHLH022, SlHLH059) in flower, one gene (SlbHLH089) in bud, and three genes (SlbHLH006, SlbHLH078, SlbHLH095) in fruit (Additional file 9), implying that they may have some functions in these tissues, respectively.
Fruit-development-related SlbHLH genes and their cis-element analysis
To investigate the regulation mechanisms of the 11 fruit-related SlbHLH genes, we analyzed their cis-elements from the transcriptional start site to the –1500-bp upstream region (Figure 5B). In the 11 SlbHLHs, 172 elements were detected using PLACE (http://www.dna.affrc.go.jp/PLACE/), of which 22 were mutual elements. Ethylene is known to be important for fruit development and ripening . Thus, we examined ethylene-regulated elements using bioinformatics tool. Ethylene-responsive elements (ERELEE4, AWTTCAAA) were found in seven genes (SlbHLH006, SlbHLH022, SlbHLH065, SlbHLH069, SlbHLH073, SlbHLH078, and SlbHLH108), which were involved in young fruit development, but were not detected in the three SlbHLH genes related to fruits ripening (SlbHLH025, SlbHLH095, and SlbHLH113). In previous study, the TGTCACA motif was shown to be an enhancer element required for fruit-specific expression of the cucumisin gene from melon, and the I-box-like sequence AGATATGATAAAA functions as a negative regulatory element . Among the fruit-specific bHLH genes of tomato, only SlbHLH095 contained the TGTCACA element in promoter. Although the I-box-like motif was not detected in the promoter region of the 11 SlbHLHs, the I-box core element (GATAA) and I-box (GATAAG) presented commonly. Considering the distribution of cis-elements in the promoter of these genes, we speculate that they may play some roles in regulating the expression of the corresponding genes for the fruit development and ripening.
Prediction of SlbHLHs involved in the regulation of iron deficiency responses and homeostasis
Evolutionary relationship of SlbHLH genes
Plant kingdom is divided into algae (including red algae, chlorophyta) and land plant (mosses, lycophytes, and angiosperms) . Its evolution is from algae to land plant. Recently, Pires and Dolan  defined the relationships of bHLH proteins in plant kingdom using the whole-genome sequences of nine species from algae and land plants. They showed that only few (less than 5) bHLH proteins were detected in the genomes of chlorophytes and red algae. In contrast, many bHLH proteins (100–170) are identified in the genomes of land plants (embryophytes). Phylogenetic analyses suggest that plant bHLH proteins are monophyletic and much of the bHLH protein diversity in plant kingdom was established in early land plants, over 440 million years ago .
In this work, we systematically analyzed the sequence of tomato genome, identified 159 bHLH genes, and characterized their structure, duplication, chromosomal distribution, phylogenetic tree, and expression patterns. Among the 159 SlbHLHs, the expression of 11 and 6 SlbHLHs was related to fruit development and ripening, and to response of iron deficiency, respectively. Further, we annotated the possible biological functions of 43 SlbHLHs by comparative analysis with bHLHs of Arabidopsis. The evolution analysis showed that all of SlbHLHs are highly conserved in plant evolution and much of the diversity of SlbHLH proteins was established in early land plants. Taken together, the results and information described in this work provide a good basis for further investigation of the biological functions and evolution of tomato bHLH genes.
Data set collection and identification of bHLH genes
Tomato genome sequence data were obtained from the Solanaceae Genomics Network (SGN) in May 2012 (http://solgenomics.net; ITAG Release 2.3) . The information and sequences of A. thaliana bHLHs (AtbHLHs) were retrieved from The Arabidopsis Information Resource (TAIR; http://www.arabidopsis.org/), Oryza sativa bHLHs (OsbHLHs) were obtained from Li et al. , and a data set of bHLH proteins from early land plant (Lycophyte - Selaginella moellendorffii, and Moss - Physcomitrella patens) and algaes (Chlorophyceae - Volvox carteri and Chlamydomonas reinhardtii, and Red algae - Cyanidioschyzon merolae) was retrieved from Pires and Dolan . The bHLH proteins of tomato (SlbHLHs) were predicted using the HLH hidden Markov model (HMM) profile obtained from Pfam (http://pfam.xfam.org, PF00010) and used as queries to search the bHLH proteins from tomato sequences with HMMER software (http://hmmer.janelia.org). In addition, the previously known AtbHLH sequences were applied as input to build a bHLH consensus domain profile with MEME software (http://meme.nbcr.net/meme/). The profile was then used as queries to identify bHLH proteins using the MAST program (http://meme.nbcr.net/meme/) with tomato sequences. To further confirm and filter uncertain bHLH proteins, the predicted bHLH domains were examined with the SMART tool (http://smart.embl-heidelberg.de). The all of the bHLH protein sequences used in this study were showed in Additional file 11.
Alignment and phylogenetic analysis
Multiple domain alignments were performed using Multalin (http://multalin.toulouse.inra.fr) and Clustal-omega tool (v 1.2, http://www.clustal.org). To visualize the conserved motifs, the sequences were analyzed with WEBLOGO programs (http://weblogo.berkeley.edu). Phylogenetic tree was constructed using MEGA5 (http://www.megasoftware.net) with the neighbor-joining method and the following parameters: pairwise deletion option, 1000 replicates of bootstrap and Poisson correction distance. The consensus tree showed only branches with a bootstrap consensus > 50. The maximum likelihood (ML) analyses were done with the program PhyML version 3.0 (http://www.atgc-montpellier.fr/phyml) using the JTT model of amino acid substitution, the radial tree was drawn using FigTree v1.3.1 (http://tree.bio.ed.ac.uk/software/figtree).
Gene duplication pattern and location of SlbHLH genes on chromosomes
Gene duplication was classified into two groups: low-stringency duplication (protein pairs with ≥ 30% identity and covering ≥ 70% protein length) and high-stringency duplication (protein pairs with ≥ 50% identity and covering ≥ 90% protein length) . TAGs were defined if they belonged to the same superfamily and were either physically adjacent or separated by a specific number of nonhomologous intervening “spacer” genes . Segmental duplications (length ≥ 3 kb; identity ≥ 90%) were identified using MUMmer (http://sourceforge.net/projects/mummer) in the whole genome sequences of tomato. The SlbHLH genes were mapped onto the corresponding chromosomes by identifying their chromosomal positions provided in the SGN. The distribution of SlbHLH genes on chromosomes were drawn using MapChart software (http://www.wageningenur.nl/en/show/Mapchart.htm).
Gene expression analysis and cis-element prediction
The expression pattern of the genes in different tissues was drawn using R script based on an average of normalized expression (RPKM mapped reads) of tomato bHLH genes from RNA-seq data . The cis-element was predicted by PLACE (http://www.dna.affrc.go.jp/PLACE/).
After germination for 5 days, the tomato seedlings (Heinz) were cultured in a modified half-strength Hoagland nutrient solution  for 4 days. The nutrient solution contained 3.0 mM KNO3, 2.0 mM Ca(NO3)2·4H2O, 1.0 mM NH4H2PO4, 0.5 mM MgSO4·7H2O, 1.0 μM KCl, 25.0 μM H3BO3, 2.0 μM MnSO4·4H2O, 2.0 μM ZnSO4·7H2O, 0.1 μM CuSO4·5H2O, 0.1 μM (NH4)6Mo7O24·4H2O, 2.0 mM MES, and 20.0 μM Fe(Na)–EDTA, and the pH of the solution was adjusted to 5.5 with KOH. The 5-days-old seedlings were transferred into the one-half strength modified Hoagland solution with or without iron supply, and cultivated for 3 days. Subsequently, the roots and shoots were separately harvested, and their total RNAs were extracted. After eliminating genomic DNA contamination by DNase I (Fermentas, Waltham, MA, USA), about 2.0 μg of total RNA were used for the synthesis of first-strand complementary DNA (cDNA) with the cDNA Synthesis Kit (Invitrogen, Carlsbad, CA, USA) and subjected to qRT-PCR analysis using the LightCycler (Roche Diagnostics, Indianapolis, IN, USA). The relative expression level for each candidate gene was calculated using the 2–ΔΔCT method with LeEF-1α as an internal reference gene. The primers used for qRT-PCR reactions are listed in the Additional file 12.
Availability of supporting data
The data sets supporting the results of this article are included within the article and its additional files.
This work was supported by the Ministry of Science and Technology of China (grant No. 2011CB100304), the Ministry of Agriculture of China (Grant No. 2011ZX08009-003-005).
- Ledent V, Vervoort M. The basic helix-loop-helix protein family: comparative genomics and phylogenetic analysis. Genome Res. 2001;11:754–70.PubMed CentralPubMedView ArticleGoogle Scholar
- Toledo-Ortiz G, Huq E, Quail PH. The Arabidopsis basic/helix-loop-helix transcription factor family. Plant Cell. 2003;15:1749–70.PubMed CentralPubMedView ArticleGoogle Scholar
- Sonnenfeld MJ, Delvecchio C, Sun X. Analysis of the transcriptional activation domain of the Drosophila tango bHLH-PAS transcription factor. Dev Genes Evol. 2005;215:221–9.PubMedView ArticleGoogle Scholar
- Murre C, McCaw PS, Baltimore D. A new DNA binding and dimerization motif in immunoglobulin enhancer binding, daughterless, MyoD, and myc proteins. Cell. 1989;56:777–83.PubMedView ArticleGoogle Scholar
- Atchley WR, Terhalle W, Dress A. Positional dependence, cliques, and predictive motifs in the bHLH protein domain. J Mol Evol. 1999;48:501–16.PubMedView ArticleGoogle Scholar
- Massari ME, Murre C. Helix-loop-helix proteins: regulators of transcription in eucaryotic organisms. Mol Cell Biol. 2000;20:429–40.PubMed CentralPubMedView ArticleGoogle Scholar
- Ferre-D’Amare AR, Pognonec P, Roeder RG, Burley SK. Structure and function of the b/HLH/Z domain of USF. EMBO J. 1994;13:180–9.PubMed CentralPubMedGoogle Scholar
- Atchley WR, Fitch WM. A natural classification of the basic helix-loop-helix class of transcription factors. Proc Natl Acad Sci U S A. 1997;94:5172–6.PubMed CentralPubMedView ArticleGoogle Scholar
- Simionato E, Ledent V, Richards G, Thomas-Chollier M, Kerner P, Coornaert D, et al. Origin and diversification of the basic helix-loop-helix gene family in metazoans: insights from comparative genomics. BMC Evol Biol. 2007;7(33):1–18.Google Scholar
- Henriksson M, Luscher B. Proteins of the Myc network: essential regulators of cell growth and differentiation. Adv Cancer Res. 1996;68:109–82.PubMedView ArticleGoogle Scholar
- Facchini LM, Penn LZ. The molecular role of Myc in growth and transformation: recent discoveries lead to new insights. FASEB J. 1998;12:633–51.PubMedGoogle Scholar
- Goding CR. Mitf from neural crest to melanoma: signal transduction and transcription in the melanocyte lineage. Genes Dev. 2000;14:1712–28.PubMedGoogle Scholar
- Sun XH, Copeland NG, Jenkins NA, Baltimore D. Id proteins Id1 and Id2 selectively inhibit DNA binding by one class of helix-loop-helix proteins. Mol Cell Biol. 1991;11:5603–11.PubMed CentralPubMedGoogle Scholar
- Fisher A, Caudy M. The function of hairy-related bHLH repressor proteins in cell fate decisions. Bioessays. 1998;20:298–306.PubMedView ArticleGoogle Scholar
- Steidl C, Leimeister C, Klamt B, Maier M, Nanda I, Dixon M, et al. Characterization of the human and mouse HEY1, HEY2, and HEYL genes: cloning, mapping, and mutation screening of a new bHLH gene family. Genomics. 2000;66:195–203.PubMedView ArticleGoogle Scholar
- Crozatier M, Valle D, Dubois L, Ibnsouda S, Vincent A. Collier, a novel regulator of Drosophila head development, is expressed in a single mitotic domain. Curr Biol. 1996;6:707–18.PubMedView ArticleGoogle Scholar
- Heim MA, Jakoby M, Werber M, Martin C, Weisshaar B, Bailey PC. The basic helix-loop-helix transcription factor family in plants: a genome-wide study of protein structure and functional diversity. Mol Biol Evol. 2003;20:735–47.PubMedView ArticleGoogle Scholar
- Li X, Duan X, Jiang H, Sun Y, Tang Y, Yuan Z, et al. Genome-wide analysis of basic/helix-loop-helix transcription factor family in rice and Arabidopsis. Plant Physiol. 2006;141:1167–84.PubMed CentralPubMedView ArticleGoogle Scholar
- Carretero-Paulet L, Galstyan A, Roig-Villanova I, Martinez-Garcia JF, Bilbao-Castro JR, Robertson DL. Genome-wide classification and evolutionary analysis of the bHLH family of transcription factors in Arabidopsis, poplar, rice, moss, and algae. Plant Physiol. 2010;153:1398–412.PubMed CentralPubMedView ArticleGoogle Scholar
- Klee HJ, Giovannoni JJ. Genetics and control of tomato fruit ripening and quality attributes. Annu Rev Genet. 2011;45:41–59.PubMedView ArticleGoogle Scholar
- Consortium TTG. The tomato genome sequence provides insights into fleshy fruit evolution. Nature. 2012;485:635–41.View ArticleGoogle Scholar
- Shimizu T, Toumoto A, Ihara K, Shimizu M, Kyogoku Y, Ogawa N, et al. Crystal structure of PHO4 bHLH domain-DNA complex: flanking base recognition. EMBO J. 1997;16:4689–97.PubMed CentralPubMedView ArticleGoogle Scholar
- Fujisawa M, Nakano T, Shima Y, Ito Y. A Large-scale identification of direct targets of the tomato MADS box transcription factor RIPENING INHIBITOR reveals the regulation of fruit ripening. Plant Cell. 2013;25:371–86.PubMed CentralPubMedView ArticleGoogle Scholar
- Yamagata H, Yonesu K, Hirata A, Aizono Y. TGTCACA motif is a novel cis-regulatory enhancer element involved in fruit-specific expression of the cucumisin gene. J Biol Chem. 2002;277:11582–90.PubMedView ArticleGoogle Scholar
- Yuan YX, Zhang J, Wang DW, Ling HQ. AtbHLH29 of Arabidopsis thaliana is a functional ortholog of tomato FER involved in controlling iron acquisition in strategy I plants. Cell Res. 2005;15:613–21.PubMedView ArticleGoogle Scholar
- Yuan Y, Wu H, Wang N, Li J, Zhao W, Du J, et al. FIT interacts with AtbHLH38 and AtbHLH39 in regulating iron uptake gene expression for iron homeostasis in Arabidopsis. Cell Res. 2008;18:385–97.PubMedView ArticleGoogle Scholar
- Long TA, Tsukagoshi H, Busch W, Lahner B, Salt DE, Benfey PN. The bHLH transcription factor POPEYE regulates response to iron deficiency in Arabidopsis roots. Plant Cell. 2010;22:2219–36.PubMed CentralPubMedView ArticleGoogle Scholar
- Wang N, Cui Y, Liu Y, Fan H, Du J, Huang Z, et al. Requirement and functional redundancy of Ib subgroup bHLH proteins for iron deficiency responses and uptake in Arabidopsis thaliana. Mol Plant. 2013;6:503–13.PubMedView ArticleGoogle Scholar
- Ling HQ, Bauer P, Bereczky Z, Keller B, Ganal M. The tomato fer gene encoding a bHLH protein controls iron-uptake responses in roots. Proc Natl Acad Sci U S A. 2002;99:13938–43.PubMed CentralPubMedView ArticleGoogle Scholar
- Pires ND, Dolan L. Morphological evolution in land plants: new designs with old genes. Philos Trans R Soc Lond B Biol Sci. 2012;367:508–18.PubMed CentralPubMedView ArticleGoogle Scholar
- Pires N, Dolan L. Origin and diversification of basic-helix-loop-helix proteins in plants. Mol Biol Evol. 2010;27:862–74.PubMed CentralPubMedView ArticleGoogle Scholar
- Rodriguez-Ezpeleta N, Brinkmann H, Burey SC, Roure B, Burger G, Loffelhardt W, et al. Monophyly of primary photosynthetic eukaryotes: green plants, red algae, and glaucophytes. Curr Biol. 2005;15:1325–30.PubMedView ArticleGoogle Scholar
- Steemans P, Herisse AL, Melvin J, Miller MA, Paris F, Verniers J, et al. Origin and radiation of the earliest vascular land plants. Science. 2009;324:353.PubMedView ArticleGoogle Scholar
- Rizzon C, Ponger L, Gaut BS. Striking similarities in the genomic distribution of tandemly arrayed genes in Arabidopsis and rice. PLoS Comput Biol. 2006;2:989–1000.View ArticleGoogle Scholar
- Schat H, Vooijs R, Kuiper E. Identical major gene loci for heavy metal tolerances that have independently evolved in different local populations and subspecies of Silene vulgaris. Evolution. 1996;50:1888–95.View ArticleGoogle Scholar
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