Molecular characterization of Sec2 loci in wheat--Secale africanum derivatives demonstrates genomic divergence of Secale species.

The unique 75 K γ-secalins encoded by Sec2 loci in Secale species is composed of almost half rye storage proteins. The chromosomal location of Sec2 loci in wild Secale species, Secale africanum, was carried out by the wheat--S. africanum derivatives, which were identified by genomic in situ hybridization and multi-color fluorescence in situ hybridization. The Sec2 gene-specific PCR analysis indicated that the S. cereale Sec2 was located on chromosome 2R, while the S. africanum Sec2 was localized on chromosome 6Rafr of S. africanum. A total of 38 Sec2 gene sequences were isolated from S. africanum, S. cereale and S. sylvestre by PCR-based cloning. Phylogenetic analysis showed that S. africanum Sec2 diverged from S. cereale Sec2 approximately 2-3 million years ago. The illegitimate recombination of chromosome 2R-6R involving the Sec2 loci region may accelerate sequence variation during evolutionary process from wild to cultivated Secale species.

1. Introduction

The genus Secale includes cultivated rye and 4 to 11 wild species depending on the criteria used for species definition [1,2]. Morphological, biochemical and molecular cytogenetic evidence supported that the wild perennial rye species are considered to be the progenitor of cultivated rye (Secale cereale L.) [2,3]. Within the genus Secale, several translocations and structural changes in chromosomes have been confirmed by genetic and genomic analysis, and they were thought to have played an important role in speciation [4,5,6,7]. Cultivated rye serves as a valuable source of potentially useful genes for wheat improvement [6]. The wild species of rye are also valuable sources for wheat breeding [2]. As an endangered wild species of Secale, Secale africanum Stapf. was reported to possess less heterochromatin than S. cereale [8,9,10,11,12,13]. To introduce novel disease resistance genes from S. africanum into common wheat, an amphiploid from the Triticum turgidum—S. africanum intergeneric hybrid was crossed with cultivated wheat [14,15], and a large number of wheat—S. africanum introgression lines were developed [10]. Recently, S. africanum chromosomes 1Rafr, 2Rafr and 6Rafr were transferred into a cultivated wheat background [12,13,16]. These chromosomes are novel sources to compare the individual chromosome evolution between cultivated and wild Secale species.

Secalin proteins are a prolamin (alcohol-soluble) group of seed storage proteins present in rye grain. These proteins can be classified into four major groups: (I) the high-molecular-weight (HMW) secalins; (II) the Mr 40 K γ-secalins; (III) the Mr 75 K γ-secalins; and (IV) the ω-secalins [17]. The 75 K γ-secalins of rye are the most abundant group of secalins, which are present in endosperm with aggregates stabilized by intra-/inter-molecular disulfide bonds [18,19]. The 75 K γ-secalins are encoded by locus Sec2, which represents a unique family of 75 K γ-secalins that does not have analogues in other cereals [20]. The Sec2 locus is found on chromosome 2R in cultivated rye (S. cereale) or 6R in wild perennial rye (S. montanum) [21,22,23,24]. The analysis of the wheat—Secale translocation lines indicated that Sec2 increased polymeric glutenin, which could potentially provide superior dough properties and increase protein levels in the endosperm [23]. However, there is little sequence information for paralogous genes within the Sec2 in different rye species [25,26].

In the present study, we isolated the Sec2 loci from newly developed wheat—S. africanum chromosome substitution lines and compared the Sec2 loci between wild and cultivated Secale, with the aim to reveal the sequences evolution and chromosome rearrangement in Secale genomes.

2. Results

2.1. Identification of Wheat—S. africanum Amphiploid and Substitution

With the combination of fluorescence in situ hybridization (FISH) probed by repetitive sequences pSc119.2 and genomic in situ hybridization (GISH) using S. cereale cv. JZHM genomic DNA as the probe, we are able to determine that the S. africanum chromatin introgressed in the wheat (Triticum aestivum) background. All S. africanum chromosomes were identified in T. durum—S. africanum amphiploid YF-1 (Figure 1A,B). The S. africanum chromosome 6Rafr displayed a strong FISH signal along the telomeric region on about 1/3 of the short arms, where two bands were located in the terminal and sub-terminal region of long arms. Some faint FISH signals also appeared close to the centromeric region. FISH with probes of Oligo-pTa535 and Oligo-pSc119.2 revealed that the T. aestivum—S. africanum-derived lines HH-41 and Mn512 were chromosome 6Rafr (6D) substitution lines (Figure 1C,D). The chromosome 2Rafr was shown by the pSc119.2 signal at both ends in YF-1. FISH probed with Oligo-pAs1, Oligo-pSc119.2 and Oligo-(GAA)7 and sequential GISH also confirmed that the T. aestivum—S. africanum-derived line LF34 was a chromosome 2Rafr (2D) substitution line (Figure 1E,F). We found that the banding patterns of S. africanum chromosomes 2Rafr was the absence of any telomeric region as compared with the YF-1. The results showed that S. africanum chromosomes 6Rafr and 2Rafr displayed a clear FISH banding pattern, and thus, they can easily be distinguished in the wheat background.

Figure 1

FISH and genomic in situ hybridization of T. durum—S. africanum amphiploid (A,B); T. aestivum—S. africanum chromosome 6Rafr (6D) substitution lines HH-41 (C); Mn512 (D); and 2Rafr (2D) substitution line LF34 (E,F). The bar is 10 μm.

2.2. Localization of Sec2 in the S. africanum Chromosome

In order to localize the Sec2 (Gli-R2) locus encoding 75 K γ-secalin proteins to the specific Secale chromosome, the reported Sec2-specific PCR primer pairs were used [24]. The genomic DNA isolated from wheat lines CS, CY12, MY26, S. cereale cv. JZHM (JH), S. cereale cv. QLHM (QH), CS—S. cereale cv. Imperial addition lines (CSDA1R to CSDA7R), T. durum—S. africanum amphiploid YF-1 and wheat—S. africanum 6Rafr (6D) substitution line HH41 were amplified by the Sec2 primer set (Figure 2). The Sec2 gene on S. cereale chromosome 2R is clearly visible, since it can be amplified in CSDA2R. However, there was no amplification of Sec2 in wheat—S. africanum 2Rafr derivative line LF34, while in wheat—S. africanum 6Rafr substitution line HH41, about a 1200–1500 bp product was amplified with Sec2-specific primers. The PCR results suggested that the Sec2 loci is located on S. africanum chromosome 6Rafr.

Figure 2

PCR patterns of Sec2 for wheat lines (CS, CY12 and MY26), S. africanum, S. cereale cv. JZHM, wheat—S. africanum amphiploid (YF-1), 2Rafr (2D) substitution (LF34), 6Rafr (6D) substitution (HH41) and CS—S. cereale cv. Imperial addition lines (CSDA1R to CSDA7R).

2.3. The Sequence Homology of Sec2 Genes

Genomic DNA was isolated from YF-1 and HH41 representing S. africanum, CSDA2R representing S. cereale, together with S. sylvestre. Genomic DNA was used as templates, and PCR amplification for the complete sequences of Sec2 genes was conducted using the Sec2 complete sequence primer [26]. The PCR products were cloned in vector pT7 Blue R-Vector and sequenced. A total of 38 unique clones, including nine from YF-1, 11 from HH41, nine from CSDA2R and nine from S. sylvestre, were sequenced. All sequences were deposited into GenBank under the Accession Numbers JX877738 to JX877776 (Table 1). The nucleotide sequences comparison of the entire sequence showed a high degree of homology with other Sec2 sequences. Sequence prediction indicated that 27 of 38 sequences included complete open reading frames (ORF) of genes. The rest of the 11 sequences structurally similar to the full-ORF genes were pseudogenes, because they contained a typical in-frame premature stop codon resulting from the transition of T by C at the first base of the glutamine codon (CAA, CAG), either at repetitive central domains or C-terminal domains. As shown in Table 1, the S. africanum Sec2 gene sequences contained 55% (11 of 20) pseudogenes, while no pseudogenes in S. sylvestre Sec2 genes were found.

Table 1

The Sec2 sequences isolated from wheat—S. africanum derivatives, S. cereale and S. sylvestre.

Materials	Full-ORF	Pseudogenes
CSDA2R	JX877767 (1434 bp, 54.19 kD), JX877768 (1416 bp, 53.58 kD), JX877769 (1416 bp, 53.42 kD), JX877770 (1416 bp, 53.58 kD), JX877771 (1395 bp, 52.64 kD), JX877772 (1215 bp, 45.73 kD), JX877773 (1416 bp, 53.58 kD), JX877774 (1434 bp, 54.22 kD), JX877775 (1395 bp, 52.64 kD), JX877776 (1395 bp, 52.64 kD)
S. sylvestre	JX877738 (1503 bp, 56.73 kD), JX877739 (1503 bp, 56.71 kD), JX877740 (1503 bp, 56.71 kD), JX877741 (1503 bp, 56.71 kD), JX877742 (1203 bp, 45.03 kD), JX877743 (1503 bp, 56.70 kD), JX877744 (1503 bp, 56.58 kD), JX877745 (1503 bp, 56.71 kD), JX877746 (1503 bp, 56.73 kD)
YF-1	JX877747 (1428 bp, 53.98 kD), JX877748 (1428 bp, 54.03 kD), JX877749 (1260 bp, 47.38 kD), JX877750 (1260 bp, 47.38 kD)	JX877751(1371 bp), JX877752(1356 bp), JX877753 (1356 bp), JX877754 (1356 bp), JX877755 (1356 bp)
HH41	JX877756 (1428 bp, 54.39 kD), JX877757 (1464 bp, 55.23 kD), JX877758 (1428 bp, 53.90 kD), JX877759 (1452 bp, 54.64 kD), JX877760 (1311 bp, 49.04 kD)	JX877761 (1320 bp), JX877762( 1332 bp), JX877763 (1332 bp), JX877764 (1332 bp), JX877765 (1320 bp), JX877766 (1332 bp)

2.4. The Amino Acid Sequences of Sec2 Genes

The general structure of Sec2 protein consists of four main structural domains, including a conservative signal peptide with 19 amino acids, a steady short N-terminal region with 12 amino acids containing a cysteine, a repetitive domain and a conserved C-terminal domain with 143 amino acids, including seven or eight cysteines [24]. On the basis of the deduced amino acid sequence of the Secale Sec2 genes, the 26 complete ORFs represented a presumptive mature protein with 241–305 residues and a calculated molecular weight of 47.3–56.7 kD (Table 1). The alignment of the amino acid sequences of Sec2 genes indicated that all Sec2 genes contained eight conserved cysteine residues at the end of the C-terminal region (Figure 3). The S. africanum-derived Sec2 sequences displayed a cysteine at the C-terminal region, while the S. sylvestre Sec2 sequences showed an extra cysteine at repetitive regions, but a lack of the cysteine at the C-terminal region. The extra cysteine was possibly due to a point mutation (C to G), which makes TGC (encoded C) from TCC (encoded S).

Figure 3

Alignment of deduced amino acid sequence N- and C-terminal regions of S africanum Sec2 and S. sylvestre Sec2. The cysteine residues are boxed. S. sylvestre Sec2 is marked by a left line.

2.5. Phylogenetic Tree and the Evolutionary Analyses

Sequence comparison was performed among the Secale Sec2 genes to understand the relatedness and the divergent time by construction of phylogenetic trees. In addition to the nucleotide sequence of the other reported eight Secale Sec2 alleles, four γ-gliadin and ω-gliadin sequences from wheat were included as the outgroup. As shown in Figure 4, the phylogenetic tree indicated that the sequences from wheat were clearly separated from those from Secale. The gliadin genes from S. africanum Sec2 sequences and S. sylvestre Sec2 sequences were clustered in one group, while the genes from S. cereale were clustered into different groups. The number of polymorphic sites (S) and nucleotide diversity (π) among Sec2 sequences of S. sylvestre, S. africanum and S. cereale were calculated. Total S. cereale Sec2 with S = 230 and π = 0.0553 was shown to be significantly higher than those of S. africanum Sec2 with S = 79 and π = 0.0399, while the S. sylvestre Sec2 sequences exhibited the lowest variation with S = 32 and π = 0.0016. This fact suggests that the Sec2 genes in S. cereale exhibited higher diversity than those in S. africanum and S. sylvestre. Based on the evolutionary rate of 6.5 × 10−9 of cereal prolamin genes in wheat and its relatives [27,28], we found that the divergence of S. sylvestre Sec2 gene sequences spans around four million years (Mya), while S. cereale and S. africanum Sec2 genes separated approximately 2–3 Mya.

Figure 4

Phylogenetic tree generated from the Sec2 gene family from Secale and the control gliadin of wheat by MEGA4 using the neighbor-joining (NJ) method. Numbers above the branches show bootstrap frequencies based on 1000 replicates.

3. Discussion

Studies indicated that cultivated rye evolved from wild perennial rye through the appearance of annual weedy forms [1,9]. The wild rye, S. africanum, is considered to be an ancient species of S. cereale. Devos et al. [6] compared the molecular marker homologous groups between wheat and rye and found that the chromosomal rearrangement occurred between wheat and rye chromosomes. Recently, Martis et al. [7] used high-throughput genome sequences to compare the Secale with barley, Brachypodium and rice genomes. They revealed that the modern rye genomes exhibited significant chromosomal rearrangements compared to their ancestral genomes. However, the studies had difficulty revealing the genomic variation and recombination during the evolution or domestication between wild and cultivated Secale species. In order to interpret the divergence and evolution among Secale species by comparing the corresponding chromosomes or chromosome regions, we produced and identified several wheat—S. africanum derivative lines, including the addition, substitution or translocation lines [12,13,14]. These lines will allow the comparison of different Secale genomes based on cytogenetic, biochemical and molecular evidence.

More precise evidence has been accumulated over the past decade by the establishment of the chromosomal locations of an array of biochemical and molecular markers, which was used to interpret the origins of the genetic recombination in rye chromosomes [6,7,13]. A group of endosperm storage proteins were reported to be located on chromosome 2R in S. cereale and then located on chromosome 6Rmon of S. montanum [17]. The loci of Gli-2 (Sec2) are located on 6AS, 6BS and 6DS in wheat, 2RS in S. cereale and 6Rmon in S. montanum. This indicated a translocation between part of the short arm of a group 2 chromosome and part of the short arm of a group 6 chromosome (2S/6S translocation) in the genome of S. cereale, relative to those of wheat and S. montanum [22]. In the present study, we found that the Sec2-specific markers were assigned to S. africanum chromosome 6Rafr, not 2Rafr, which supports the theory that translocations have taken place in the ancestor of cultivated rye [6,7].

The Sec2 (Gli-R2) locus encoding 75 K γ-secalin proteins is located relatively close to the telomeric C-band on the short arm of S. cereale chromosome 2R [24]. A genomic DNA clone coding for a rye secalin gene (gSec2A) was isolated from a wheat-rye translocation line carrying the 2RS.2BL chromosome [25]. Recently, Wang et al. [29] identified 59 Sec2 sequences from a cultivated rye and derivative lines after crossing with bread wheat. Chen et al. [26] isolated four Sec2 gene sequences from each of S. cereale, S. vavilovii, S. sylvestre and S. strictum using a PCR-based strategy, implying that the Sec2 genes are conserved in genus Secale. Our studies reported new Sec2 gene sequences from S. sylvestre and S. africanum in wheat—S. africanum amphiploid and substitution lines and found the novel variations at the nucleotide and amino acid level in the Sec2 gene loci. It will be interesting for further studies to be done on the genetics and interaction between wheat and Secale gliadin proteins for wheat and triticale breeding for quality improvement.

Since the unique family of the 75 K γ-secalins Sec2 loci does not have analogues in other cereals [20], the estimation of seed protein evolutionary divergence time can be performed after species become reproductively isolated. Therefore, molecular dating can estimate the divergence of a particular genetic locus, but not the divergence of species [30]. The estimation of S. africanum Sec2 diverged from S. cereale Sec2 approximately 2–3 million years ago (Mya). It can also be interpreted that S. africanum is one of the perennial Secale, with restricted geographic distribution in South Africa, today on the verge of extinction [8,31]. Rye and wheat diverged 7 Mya, and both lineages and the barley lineage diverged from a common Triticeae ancestor around 11 Mya [32]. The complicated chromosomal rearrangements of the rye genome might have occurred during evolution less than 2–3 Mya. Higher diversity of S. cereale Sec2 gene sequences in 2R than S. africanum Sec2 in 6Rafr was possibly a result of the location of the 2RS terminal region in a hotspot region in the cereal genome. It will be able to address the issue of a correspondence between the chromosomal gaps and rearrangements for the adaptation of the wild Secale species.

Recently, the occurrence of chromosome structure variation induced by the S. cereale chromosome in wheat-rye derivatives was observed [33]. In the self-progeny of the 6R (6D) substitution line, several kinds of altered chromosomes were also discovered [34]. It is likely that the wheat—S. cereale chromosome 6R monosomic addition line could induce the alterations of wheat chromosomes and the abnormal behavior of wheat chromosomes during mitosis. However, in the present study, no wheat chromosome structural changes were observed in the wheat—S. africanum 6Rafr (6D) substitution lines. We assumed that the S. cereale chromosome 6R might have lost some factors that affect the stability of the wheat background, while the ancestry of S. africanum contained chromosome 6Rafr.

4. Experimental Section

4.1. Plant Materials

Secale africanum, S. sylvestre and S. vavilovii were kindly provided by Perry Gustafson of the University of Missouri, Columbia, MO, USA. The T. durum—S. africanum amphiploid (YF-1), S. cereale cv. Jinzhouheimai (JZHM) and wheat cultivars Mianyang11 (MY11), Chuanyu12 (CY12) and Mianyang 26 (MY26) are maintained in our laboratory. Chinese spring—S. cereale Imperial rye addition lines (CSDA1R to CSDA7R) were obtained from Bernd Friebe, Wheat Genetic and Genomic Resources Center, Kansas State University, Manhattan, KS, USA. The S. africanum chromosome 2Rafr (2D) substitution line LF24 and chromosome 6Rafr (6D) substitution line HH41 were selected from the F7 generation of the cross between wheat cultivars Mianyang11 (MY11) and YF-1.

4.2. Genomic in Situ Hybridization and Fluorescence in Situ Hybridization

Seedlings were grown in petri dishes, and the root tips of about 2 cm were collected and pretreated in water at 0 °C for 24 h and fixed in ethanol-acetic acid (3:1) for 1 week. Root-tip squashes and chromosome preparation were done according to Yang et al. [10]. For genomic in situ hybridization (GISH) analysis, the sheared genomic DNA of S. cereale cv. JZHM was labeled with Alexa Fluor-488-5-dUTP (Vector Laboratories, Burlingame, CA, USA), and sheared genomic DNA of Chinese spring wheat was used as the blocking. The GISH protocols are from Yang et al. [10]. The multi-color fluorescence in situ hybridization (mcFISH) by the oligonucleotide probes representing the repetitive sequences was used for identifying the wheat chromosomes according to the recent study of Tang et al. [35]. Oligo-pSc119.2-1 and Oligo-(GAA)7 repeats were end-labelled with 6-carboxyfluorescein (6-FAM) for green signals, and the Oligo-pAs1 and Oligo-pTa535-1 were labeled with 6-carboxytetramethylrhodamine (Tamra) for red signals. Oligonucleotide probes were synthesized by Shanghai Invitrogen Biotechnology Co., Ltd. (Shanghai, China). The synthesized probes and the FISH hybridization process was according to Tang et al. [35]. The slides were mounted in 4',6'-diamidino-2-phenylindole (DAPI) dissolved in Vectrashield® antifade solution (Vector Laboratories, Burlingame, CA, USA). Microphotographs of GISH and FISH chromosomes were taken with an Olympus BX-51 microscope using a DP-70 CCD camera.

4.3. Primer Design, PCR Cloning and Sequencing

Total genomic DNA was isolated from young leaves as described by Li et al. [36]. The DNA concentration was determined using a Sizhumen DNA-protein photometer and also by comparison with a known lambda DNA standard on a 1% agarose gel. The Sec2-specific PCR primer pair synthesis and PCR protocol followed that of Chen et al. [26]. The target genes amplified by PCR were excised from 1.0% agarose gels and purified using a gel extraction kit (Qiagen, Valencia, CA, USA). The purified products were ligated into the pT7 Blue R-Vector using T4 ligase and then introduced into Escherichia coli DH5α by heat shock transformation. Nucleotide sequencing was performed on a polyacrylamide gel with the ABI prism 377 sequencer (Perkin Elmer) as an automated fluorescent sequencing system.

4.4. Phylogenetic Analyses

The controlled wheat gliadin used as comparison controls was obtained from the NCBI website [37] The Sec2 gene sequences from the wheat—S. africanum 6Rafr substitution line, CSDA2R and S. sylvestre sequences cloned here were analyzed by the ORF finder program at the NCBI network service [37]. Sequences were aligned using BioEdit software. The sequences HQ266670–HQ266677 of S. cereale Sec2 genes published by Wang et al. [29] were also included in our study. All DNA sequences were aligned using ClustalW Version 1.8 [38]. Multiple alignment parameters were scored up to 12 for the gap opening penalty and 0.1 for the gap extension penalty. Alignments were confirmed manually using sequential pairwise comparisons. MEGA4 was used for calculating pairwise sequence divergences and nucleotide compositions and for performing neighbor-joining (NJ) analyses [39]. The phylogenetic tree was linearized assuming equal evolutionary rates in all lineages and was drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. A consensus tree was generated using 1000 bootstrap replicates. DnaSP Version 5.0 [40] was used to compare the nucleotide diversity [41] of the Sec2 DNA polymorphism among Secale species.