MBE Advance Access originally published online on October 19, 2006
Molecular Biology and Evolution 2007 24(1):217-227; doi:10.1093/molbev/msl151
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Research Articles |
Independent Wheat B and G Genome Origins in Outcrossing Aegilops Progenitor Haplotypes
* Max Planck Institute for Plant Breeding Research, Köln, Germany
Institute of Botany III, Heinrich-Heine-Universität Düsseldorf, Germany
Department of Field Crops, Faculty of Agriculture, University of Cukurova, Adana, Turkey
Istituto Sperimentale per la Cerealicoltura—CRA, S. Angelo Lodigiano, Italy
|| Institute of Bioinformatics, Heinrich-Heine-Universität Düsseldorf, Germany
¶ Fondazione Parco Tecnologico Padano, Via Einstein, Lodi, Italy
E-mail: francesco.salamini{at}tecnoparco.org.
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Abstract |
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The origin of modern wheats involved alloploidization among related genomes. To determine if Aegilops speltoides was the donor of the B and G genomes in AABB and AAGG tetraploids, we used a 3-tiered approach. Using 70 amplified fragment length polymorphism (AFLP) loci, we sampled molecular diversity among 480 wheat lines from their natural habitats encompassing all S genome Aegilops, the putative progenitors of wheat B and G genomes. Fifty-nine Aegilops representatives for S genome diversity were compared at 375 AFLP loci with diploid, tetraploid, and 11 nulli–tetrasomic Triticum aestivum wheat lines. B genome–specific markers allowed pinning the origin of the B genome to S chromosomes of A. speltoides, while excluding other lineages. The outbreeding nature of A. speltoides influences its molecular diversity and bears upon inferences of B and G genome origins. Haplotypes at nuclear and chloroplast loci ACC1, G6PDH, GPT, PGK1, Q, VRN1, and ndhF for
70 Aegilops and Triticum lines (0.73 Mb sequenced) reveal both B and G genomes of polyploid wheats as unique samples of A. speltoides haplotype diversity. These have been sequestered by the AABB Triticum dicoccoides and AAGG Triticum araraticum lineages during their independent origins.
Key Words: molecular evolution • Triticum • Aegilops • hybridization • alloploidization • AFLPs
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Introduction |
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TOPAbstractIntroductionMethodsResultsDiscussionSupplementary MaterialAcknowledgementsReferences |
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Bread wheat, Triticum aestivum, has no direct hexaploid wild progenitor (Morris and Sears 1967
; Kimber and Feldman 1987). The species possesses 3 sets of homologous chromosomes, designated as AABBDD, whose origins have differing degrees of certainty. The D chromosomes stem from the wild diploid Aegilops tauschii (Kihara 1944) through alloploidization with the wild AABB tetraploid Triticum dicoccoides. The A and B chromosomes of that tetraploid derive from an earlier hybridization between the wild AA diploid Triticum urartu (Dvorak et al. 1993) and a wild diploid B genome donor: the ultimate source of this B genome is still discussed. A related conundrum is the origin of AAGG Triticum araraticum, whose A genome also stems from T. urartu, whereby the wild G progenitor is frequently reported to be Aegilops speltoides (Rodriguez, Maestra et al. 2000). The B donor is traditionally sought in the Sitopsis section of the genus Aegilops (Sarkar and Stebbins 1956; Kerby and Kuspira 1988). Previous molecular analyses of single-gene loci for a few accessions are not inconsistent with the view that both the B genome of T. dicoccoides (AABB) and the G genome of T. araraticum (AAGG) might trace to the Sitopsis section, in genetic proximity to wild A. speltoides (Blake et al. 1999; Rodriguez, Maestra et al. 2000; Zhang et al. 2002). However, there are caveats.
First, nuclear and cytoplasmically inherited markers yield contrasting results on the issue of B genome origin (discussed in Wang et al. 1997). In addition, ancient allelic diversity among wild ancestors, compounded by the possibility of unrecognized hybridization events, renders inferences of the B progenitor questionable (Huang et al. 2002) in the absence of genome-wide surveys for many loci and accessions. Furthermore, the outcrossing nature of A. speltoides (Kimber and Feldman 1987) renders introgression for individual loci difficult to exclude in the absence of extensive lineage sampling. Importantly, cytogenetic evidence does not support the view that A. speltoides was the donor of B or G genome, even though such suggestions can be found (Maestra and Naranjo 1998). When synthetic SSAA genomes (S contributed by A. speltoides and A by Triticum) are crossed to Triticum durum (AABB, the domesticated form of T. dicoccoides), sterility is observed, pointing to differences between S and B genomes; the same is reported for S and G genomes (Dvorak 1972; Kimber and Athwal 1972). Moreover, B–S pairing in wheat/A. speltoides hybrids is comparable to that noted for wheat/Aegilops longissima and wheat/Aegilops sharonensis hybrids (Fernandez-Calvin and Orellana 1994), suggesting that B chromosomes of polyploid wheats do not pair preferentially to those of A. speltoides.
Understanding hexaploid wheat origin would further its genetic improvement (Salamini et al. 2002; Chantret et al. 2005). Here, we report a comprehensive amplified fragment length polymorphism (AFLP) survey of genomic diversity among 1372 individuals from 480 wild B genome progenitor candidates. Through the analysis of Sears's (1954) nulli–tetrasomic (AADDDD) lines, B genome–specific AFLPs were identified. For 70 domesticated and progenitor lines representing the breadth of wild genomic diversity, haplotypes at nuclear loci ACC1, G6PDH, GPT, PGK1, Q, VRN1 and of the chloroplast locus ndhF were determined. Comparisons to haplotypes from AA Triticum boeoticum, Triticum monococcum, and T. urartu identified haplotypes specific to the B genome to allow comparison to Sitopsis accessions. The data circumscribe molecular diversity among Sitopsis Aegilops species and specify the nature of wheat B and G genome origins.
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Methods |
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AFLP Analysis
The 480 Aegilops lines used in this study are listed in Supplementary table S1 (Supplementary Material online). DNA was isolated from freeze-dried or silica-dried leaves of 1372 plants (Supplementary table S2, Supplementary Material online), using the Qiagen (Hilden) DNeasy Kit, and amplified as described by Zabeau and Vos (1993)
using the primer combinations EACC/MACA, EACC/MAGC, and EACC/MAGG. The AFLP bands were scored as 1 or 0 (present or absent). Jaccard (1908) similarities of the 850 individuals with different AFLP patterns (Supplementary table S3, Supplementary Material online) were computed using DistAFLP (Mougel et al. 2002), and Neighbor-Joining (NJ) bootstrap trees were inferred with PHYLIP 3.6 (Felsenstein 2002). DNA from 94 selected Aegilops, T. boeoticum, T. urartu, T. dicoccoides, T. araraticum, and T. aestivum accessions (Supplementary table S4, Supplementary Material online), along with T. aestivum Chinese Spring aneuploids: 6 nulliB–tetraD (N1BT1D, N2BT2D, N3BT3D, N4BT4D, N5BT5D, N6BT6D) and 5 nulliB–tetraA (N1BT1A, N2BT2A, N3BT3A, N5BT5A, N7BT7A) (Sears 1954), was amplified using primer combinations EACC/MAGC, EACC/MAGG, EACG/MACC, EACG/MACT, EACG/MAGG, EACG/MAGT, EAGC/MAGC, and EAGC/MATA. NeighborNet (NNet) planar graphs (Bryant and Moulton 2004) of AFLP Hamming distances between individuals were constructed with SplitsTree 4.1 (Huson and Bryant 2006).
Haplotype Analysis
Genes and accessions considered for haplotype analysis are recorded in Supplementary tables S5–S7 (Supplementary Material online). Sixty-seven lines were common to all loci—T. dicoccoides (34), T. dicoccum (5), T. durum (1), T. araraticum (5), Triticum timopheevii (6), Aegilops bicornis (2), A. longissima (2), Aegilops searsii (2), A. sharonensis (2), A. speltoides (7), and A. tauschii (1). Other sequences from additional lines of those same species, from T. boeoticum, T. monococcum, T. urartu, T. araraticum, and T. timopheevii (table 2), as well as available published sequences were included. DNA was isolated as described above. Primers (Supplementary table S8, Supplementary Material online) were designed with Primer3 against sequences for ACC1 (Huang et al. 2002), G6PDH (Nemoto et al. 1999), GPT (GenBank AF548741), PGK1 (Huang et al. 2002), Q (Faris et al. 2003), VRN1 (Sherman et al. 2004; Yan et al. 2004), and ndhF (Ogihara et al. 2002). Some accessions of A. speltoides have 2 copies each of the genes ACC1 and PGK1 (Huang et al. 2002); primers for these 2 genes were used allowing the amplification of the same gene in all A. speltoides. DNA amplifications were performed in 25 µl containing 100 ng of leaf DNA, 0.4 µM of each primer, 125 µM of each deoxynucleoside triphosphate (AB gene, Surrey, UK), 3 mM MgCl2, 4% dimethyl sulfoxide, and 1 unit of Taq DNA polymerase incubated in a PTC-225 Tetrad Thermal Cycler (MJ Research) as follows: 94 °C for 3 min, 28–33 cycles of 30 s at 94 °C, 20–40 s at 59–65 °C, 50–95 s at 72 °C, and a final extension step of 6 min at 72 °C. Polymerase chain reaction products were sequenced on both strands (Supplementary table S9, Supplementary Material online). Sequences (Supplementary table S10, Supplementary Material online) were processed with Applied Biosystems DNA Sequencing Analysis Software 5.1.1 and manually inspected with BioEdit version 7.0.1 (Hall 1999). The alignments were generated with ClustalW, and the haplotypes were scored manually and with DnaSP (Rozas et al. 2003). For homozygous loci, only one haplotype per line was included in the alignment, both haplotypes for heterozygous loci. Median-joining (MJ) networks (Bandelt et al. 1999) were constructed with the Network 4.1.1.2
[EC]
program (Fluxus Technology Ltd, Clare, Suffolk, UK). Total number of substitutions per site between populations, Da (equation 10.21; Nei 1987), using the Jukes–Cantor method was calculated with DnaSP (Rozas et al. 2003).
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To obtain A, B, and G genome–specific haplotype sequences for polyploid wheats, three approaches were used. When B genome–specific sequence differences were available, 1) primer combinations were designed and used for haplotype-specific amplification and sequencing; 2) amplification products from A, B, and G genomes were obtained with the A genome primers, but sequenced using genome-specific primers; and 3) in the remaining cases, amplification products for A, B, and G genomes were obtained using nondiscriminating A genome primers, cloned in Escherichia coli, and at least 3 sequences per clone obtained, until both haplotypes were identified by comparison to existing A and B genome data for the locus.
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Results |
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Genomic Diversity within the Sitopsis Section of Aegilops
To survey the molecular diversity among candidate B genome donors, we studied 2–3 plants each from a total of 501 accessions spanning all 5 Sitopsis Aegilops species—A. searsii, A. bicornis, A. sharonensis, A. longissima, and A. speltoides—collected along the Eastern Mediterranean (fig. 1). (We follow Dorofeev et al. [1979]
for Triticum binomial nomenclature and Van Slageren [1994] for Aegilops). Accessions were grown in 2003 and morphologically reidentified, whereby 21 misassigned lines or interspecific hybrids were discarded. DNA was collected from 1372 plants: A. bicornis (39 accessions, 105 individuals), A. longissima (81, 227), A. searsii (97, 285), A. sharonensis (112, 327), A. speltoides (149, 422) and from the D genome outgroup species A. tauschii (2, 6).
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A screen using 3 AFLP primer combinations uncovered a total of 70 polymorphic bands across all plants. This revealed 850 individuals with different AFLP patterns: A. bicornis (36 accessions, 44 individuals), A. longissima (80, 165), A. searsii (54, 77), A. sharonensis (101, 176), A. speltoides (147, 386), and A. tauschii (2, 2). The NJ tree of Jaccard (1908)
distances is shown in figure 2 and provides an overview of Sitopsis genome diversity. The primary screen revealed the breadth of divergence within each species, helping to choose accessions for subsequent fine-scale analyses. In addition, two notable results emerged. First, in all intraspecific pairwise comparisons, AFLP-based genetic similarity was lower in the outbreeder A. speltoides than in any of the other members of the section which are inbreeders (Kimber and Feldman 1987) (fig. 2). Second, figure 1 shows that the Sitopsis section (with one exception, a plant of A. bicornis mapping outside but near the major cluster of the species) does not include other genetically distinct major groups, besides those represented by A. bicornis, A. searsii, A. speltoides, and the A. sharonensis–A. longissima cluster (the two last species, however, mapped to some degree separately when using more AFLP primer combinations, like in fig. 3A and B). It is concluded that the Sitopsis section does not contain cryptic species molecularly distinct from those currently recognized (Kimber and Feldman 1987), such that our sample appears to span the full breadth and depth of molecular diversity within the section. The inability to distinguish A. sharonensis from A. longissima individuals in the coarse screen is irrelevant here, as are the low bootstrap proportions for branches separating species at these 70 loci.
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Higher Resolution among Wild and Domesticated Genomes
A reduced set of 59 Aegilops plants representative for molecular diversity within the section was considered for further AFLP studies, carried out with a higher number of primer combinations. The selected plants were A. bicornis (11 individuals), A. longissima (8), A. searsii (13), A. sharonensis (13), and A. speltoides (14). Selection of plants within species was carried out, maximizing the average genetic distances among selected plants. Two plants from the A. tauschii (D genome) outgroup were included, as were 5 T. aestivum cultivars (AABBDD), 3 wild T. dicoccoides (AABB), 4 wild T. araraticum (AAGG, the progenitor of the domesticated T. timopheevii), 9 wild T. boeoticum (AbAb, the progenitor of the domesticated T. monococcum, AmAm), and 12 wild T. urartu (AA). The choice of the 9 and 12 accessions, respectively, of T. boeoticum and T. urartu considered the criterion specified above, based on published and unpublished molecular data of the authors. Other Triticum accessions were chosen as representative of molecular diversity among the species considered in experiments previously published by the authors (references in Salamini et al. 2002
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Eleven AFLP primer combinations amplified 375 polymorphic bands across these 94 lines, from which NNets were constructed (fig. 3A). NNet splits graphs can be interpreted like trees in that they contain splits (branches) with weights (lengths). Parallel lines identify the same split or branch. Boxes indicate support for 2 competing patterns of taxon relationship. NNet splits graphs highlight the predominant phylogenetic signals in the data and the extent to which these signals may or may not be tree-like (Huson and Bryant 2006). In cases of reticulate evolutionary history, hybrid taxa are suggested by the occurrence of incompatible splits (which appear as boxes), often with hybrid taxa being linked by splits to their potential parents. NNet split graphs only display the contradictory splits that can be visualized in a single plane and should not be considered an explicit model of reticulate evolutionary history. Nevertheless, they provide an implicit representation of evolutionary history (Huson and Bryant 2006) and one that is useful for identifying and exploring different signals and their meaning. For allopolyploid species, NNet has an advantage for data visualization over tree-building methods, which assume that the data have evolved on a single bifurcating tree. At this level of genome-wide comparison, the only Sitopsis member that shared a split with the AABB, AAGG, and AABBDD polyploids was A. speltoides. That split reflects a higher proportion of shared AFLP bands between polyploid wheats and A. speltoides as compared with other SS genomes. A second split divides T. urartu (AA) from all diploids sampled but excludes T. boeticum (AbAb), and indeed T. urartu is the A genome donor (Dvorak et al. 1993). No split links A. tauschii (DD) to the hexaploid AABBDD genome. However, this might be expected because the NNet method can only represent incompatible splits projected onto 2 dimensions (Huson and Bryant 2006). With the D genome absent in the 7 tetraploids sampled, signals from A, B, G, and S genomes will override the weaker signal linking AABBDD and DD genomes. A strong split linking the AA diploids with T. araraticum (AAGG) to the exclusion of T. dicoccoides (AABB) is also observed, indicating that the AABB and AAGG genomes, both involving T. urartu, are the result of independent polyploidization events.
The B Genome
Six nulliB–tetraD and 5 nulliB–tetraA lines of the hexaploid cultivar Chinese Spring (Sears 1954) were included in the AFLP analysis. They identified 65 bands that reside specifically on the B genome. If the split that links A. speltoides to polyploids is a historical component of genome similarity, it should become more prominent in the NNet based on the 65 B genome–specific bands. This is observed in figure 3B, where the split linking A. speltoides to polyploid wheats is highlighted in blue. These B bands were selected by virtue of their occurrence in hexaploid wheat, not by virtue of their character state among tetraploids or diploids. Hence, they do not skew the locus sample systematically toward any potential B genome donor. They represent B genome–specific signals with regard to polyploid origins; competing A- and D-specific signals are diminished, but not abolished, because A, B, G, and D genomes are still related at these loci.
Figure 3B reveals that the A. speltoides genome is most similar to the B and G genomes of polyploid wheats. And because we have extensively sampled genome diversity across the Sitopsis (fig. 2), this indicates that the A. speltoides S genome is the extant version of B and G genomes of polyploid wheats. Identical B genome–specific AFLP bands shared between tetraploid wheats and the species of the Sitopsis section using the Jaccard (1908) similarity (fig. 3C) further support that conclusion.
The NNet shows a strong split linking hexaploid wheat with T. dicoccoides to the exclusion of T. araraticum, highlighted in green in figure 3B. This corresponds to the well-known participation of T. dicoccoides in bread wheat origin (Dvorak et al. 1993). Evidence for additional hybridization events was uncovered, namely the strong component of similarity linking few T. urartu (AA) accessions to the T. boeticum complex (AbAb), probably reflecting interspecific crosses. However, even in the B genome–specific data, no A. speltoides genome sampled was specifically more similar to all polyploids sampled. Nonetheless, if the B and G genomes stem from within A. speltoides, then genome-specific haplotypes from polyploids should provide more detailed evidence for that origin.
Congruent Evidence from Haplotypes
Haplotypes recognized in DNA fragments for the nuclear genes ACC1, G6PDH, GPT, PGK1, Q, and VRN1 and in a 719-bp region of the chloroplast gene ndhF were determined for tetraploids, Sitopsis members, and AA diploids. In total, 0.73 Mb of sequence data were obtained and combined with 80 000 bp from previous studies (Supplementary tables S5–S7, Supplementary Material online) for analysis. At all nuclear loci investigated, the number of net nucleotide substitutions per site between populations (Nei 1987), Da, revealed that T. dicoccoides B genome haplotypes were always more similar to those in A. speltoides than those in any other species. The same was true for comparisons of the T. araraticum G genome haplotypes (table 1), providing additional evidence for an origin of both B and G genomes from a A. speltoides donor. The same was evident for the cytoplasmically inherited ndhF gene (table 1).
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MJ networks for these loci (fig. 4) reveal higher levels of haplotype diversity within the outbreeder A. speltoides than in other wheats. Furthermore, B and G genome haplotypes of the tetraploids were consistently more closely related to A. speltoides than to other sources. At G6PDH, 8 A. speltoides haplotypes were observed: SPE-I is the closest relative of B and G haplotypes, which are monomorphic for T. dicoccoides and dimorphic for T. araraticum, whereas other Sitopsis or A genome haplotypes are distinct by
20 substitutions. At ACC1, A. speltoides revealed 7 haplotypes: SPE-I and -II are identical to those found in G genome, SPE-III is the closest relative of the major T. dicoccoides B haplotype; no ACC1 haplotypes are shared between A. speltoides and remaining Sitopsis species. A genome haplotypes at ACC1 were much less diverse than B genome homologues. At GPT, only one A. speltoides haplotype was observed, which is identical to the major T. araraticum haplotype and shows only 2 nucleotide differences to the main T. dicoccoides B haplotype; other Sitopsis or A genome haplotypes were clearly distinct. Q was by far the most polymorphic locus sampled: the closest progenitor to the main T. dicoccoides B genome haplotype was SPE-I, different by 7 nucleotides to the T. araraticum haplotype and by 17 nucleotides from the major T. dicoccoides B haplotype; the other haplotypes were more distant to B and G genomes. At PGK1, 13 A. speltoides haplotypes were found: SPE-I differed by 2 nucleotides from the main B haplotype of T. dicoccoides; the rare SPE-II is not more closely related to the single T. araraticum G haplotype than haplotypes found among other Sitopsis; for this gene, a greater diversity of A. speltoides haplotypes relative to other Sitopsis was particularly evident. VRN1 (Supplementary fig. S1, Supplementary Material online) did not reveal a closer relationship for either A. speltoides or other Sitopsis to B or G genome. For this gene, the simplest interpretation is that our present lineage sample at VRN1 did not uncover A. speltoides B genome progenitor haplotypes: only a clear distinction between A and B/G genome–specific haplotypes was evident.
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The main A. speltoides ndhF haplotype was identical with that of tetraploids and hexaploids (fig. 4). The network, while excluding the progenitors of A. bicornis, A. longissima, A. sharonensis, and A. searsii as the B female recipients in the cross with the A genome, provides evidence that female gametes of A. speltoides generated the AABB and AAGG genomes.
In summary, the MJ networks uncover no Sitopsis haplotypes that are more similar to B or G genome than A. speltoides haplotypes are, indicating that the A. speltoides gene pool participated in the synthesis of AABB and AAGG genomes. Furthermore, loci that are highly polymorphic in A. speltoides, such as PGK1, underscore the need to sample many lineages to uncover B genome progenitor alleles.
The 2 distinct G genome haplotypes at ACC1 differing by 4 substitutions, each identical to haplotypes occurring in A. speltoides, could, at face value, suggest 2 independent origins for T. araraticum. However, for all loci at which the B genome donor was heterozygous—for instance, in unreduced gametes—both alleles should persist in modern tetraploids. This problem is related to the outcrossing nature of A. speltoides (table 2): of the 39 loci investigated, 76% were heterozygous as compared with 7.4% for remaining Aegilops species, all predominantly inbreeders (Kimber and Feldman 1987). The distinctness of B and G genome haplotypes at all nuclear loci, and the proximity of A. speltoides progenitors in most cases, clearly indicates independent alloploidization events underlying T. araraticum and T. dicoccoides origins, consistent with their divergent positions in the analysis of B genome–specific AFLPs (fig. 3B). The results of table 3, which underscore the close relationship of B and G genomes relative to A genome, support this conclusion.
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Discussion |
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Domestication of wheats commenced about 10 000 years ago (Salamini et al. 2002
), but the events that gave rise to wild polyploids are older. Estimates for the age of T. dicoccoides origin range from >0.5 MYA (Huang et al. 2002), 0.25–1.3 MYA (Mori et al. 1995; Huang et al. 2002), or 0.36 MYA (Dvorak and Akhunov 2005) but are heavily subject to haplotype sampling variance, as our data underscore, for which reason the B genome donor has remained in issue. Though the source of the B genome has been sought among Sitopsis (Sarkar and Stebbins 1956; Kerby and Kuspira 1988), with a focus on A. speltoides (Dvorak and Zhang 1990; Huang et al. 2002), genetic data have been equivocal (Dvorak 1972; Kimber and Athwal 1972; Fernandez-Calvin and Orellana 1994) in the absence of studies sampling many loci and accessions. Our data indicate an origin of the B genome from within A. speltoides. First, the analysis of 375 AFLP loci links specifically polyploid wheats with A. speltoides to the exclusion of other Sitopsis species (fig. 3A). In addition, 65 AFLPs specific to the T. aestivum B genome link A. speltoides even more closely to the B and G genomes (fig. 3B). Second, with the exception of VRN1, haplotypes from chloroplast and nuclear loci show that A. speltoides shares the highest average sequence identity with the B and G genomes (table 3) and reveals specific progenitor-descendant relationships in the MJ networks (fig. 4). These findings can be incorporated into a broader scheme of wheat genome evolution (Supplementary fig. S2, Supplementary Material online) with resolved positions of the B genome relative to S progenitors and G sisters.
AABB and AAGG genome origins have been attributed to the same single hybridization event (Wagenaar 1961; Tanaka et al. 1979; Gill and Chen 1987; Provan et al. 2004) or to separate alloploidization events (Mori et al. 1995; Brown-Guedira et al. 1996; Rodriguez, Perera et al. 2000; Huang et al. 2002). In support of the former view, T. dicoccoides and T. araraticum have almost identical morphology, but they have F1 hybrids showing 100% sterility (Tanaka et al. 1979) (with normal chromosome pairing, Rao and Smith 1968; Rawal and Harlan 1975; Tanaka et al. 1979). In addition, some lines of T. araraticum produce hybrids with a significant level of fertility when crossed to T. dicoccoides (Rao and Smith 1968; Rawal and Harlan 1975). Our data resolve this issue. The hybridization events leading to AABB and AAGG genomes occurred independently as evidenced 1) by their distinct positions in AFLP analyses, 2) by the finding that each has sequestered different samples of A. speltoides haplotype diversity, and 3) from the comparison of divergence within and among A, B, and G genome haplotypes (table 3). The B and G genomes are clearly distinct, incompatible with the view of a single-hybrid origin (Rodriguez, Maestra et al. 2002).
Wheat is no exception to the rule that specific polyploids arose recurrently during flowering plant evolution (Soltis 2005), accompanied by extensive and rapid genome restructuring (Leicht and Bennett 1997). Alloploidization often involves intergenomic recombination (McGrath et al. 1990; Jang and Gill 1994; Song et al. 1995; Soltis 2005) and rapid loss of DNA (Özkan et al. 2001), whereby subsequent diploidization restores disomic genetics (Levy and Feldman 2002). The genetic control of chromosome pairing provides insights on wheat alloploid evolution. Aegilops speltoides forms are known that suppress pairing among homologous chromosomes (Ph1 activity) (Aghaee-Sarbarzeh et al. 2000). If Ph1 genotypes participate in polyploidization events, interspecific hybrids acquire a bivalent type of chromosome pairing, the case of T. dicoccoides (Sears 1976). Other lines of A. speltoides do not show Ph1-like activity (Kimber and Feldman 1987), having loci that suppress Ph1, thus allowing homologous pairing in interspecific crosses (Sears 1976). The absence of Ph1 or Ph1-like activity favors tetravalent formation and, possibly, intergenomic translocations. Triticum araraticum has extensive DNA loss (Özkan et al. 2001) and 6 chromosomal rearrangements relative to T. dicoccoides (Rodriguez, Maestra et al. 2000; Rodriguez, Perera et al. 2000), 4 of which are intergenomic G–A translocations. A possibility is that in the AAGG genome synthesis, the Ph1 allele of A. speltoides was suppressed and later on restored via genetic segregation (today T. araraticum has an active Ph1 allele). Similar evolutionary mechanisms may underlie the cytogenetic distinctness of S, B, and G genomes, whose evolutionary relationships are nonetheless revealed by AFLP and haplotype data.
Previous studies suggested that T. araraticum inherited a A. speltoides cytoplasm (Mori et al. 1997; Wang et al. 2000; Provan et al. 2004) but were conflicting for T. dicoccoides (Wang et al. 2000; Provan et al. 2004). Our ndhF data assign the A. speltoides cytoplasm to both T. dicoccoides and T. araraticum. This cytoplasm is distinct from that of other Sitopsis species.
We identified no A. speltoides line that shares greater similarity to all polyploids sampled than any other A. speltoides line. Furthermore, the A. speltoides haplotypes most similar or identical to B and G genome haplotypes are dispersed across different individual lines. Further sampling within A. speltoides might uncover lines that carry the same combination of haplotypes as the B genome donor contained. However, because the species is an outbreeder, it is more likely that no modern A. speltoides lines have preserved the B donor genotype in its contiguous ancestral state.
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Supplementary Material |
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TOPAbstractIntroductionMethodsResultsDiscussionSupplementary MaterialAcknowledgementsReferences |
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Supplementary material mentioned in the text, comprising 10 supplementary tables and 2 supplementary figures are available online at http://en.tecnoparco.org/Default.aspx?tabid=118 and at Molecular Biology and Evolution online (http://www.mbe.oxfordjournals.org). Sequence data from this article are deposited in GenBank Data library under accession no. provided in Supplementary table S10.
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Acknowledgements |
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TOPAbstractIntroductionMethodsResultsDiscussionSupplementary MaterialAcknowledgementsReferences |
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We thank Charlotte Bulich and Isabelle Fuchs for excellent technical assistance, the Deutsche Forschungsgemeinschaft for financial support, and the Alexander von Humboldt Foundation for a stipend to H.Ö.
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Footnotes |
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Peter Lockhart, Associate Editor
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References |
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