The Journal of Heredity 2002:93(5)
© 2002 The American Genetic Association 93:339-345
Evolutionary Trends of Different Repetitive DNA Sequences During Speciation in the Genus Secale
From the Department of Cell Biology and Genetics, University of Alcala, 28871 Alcala de Henares (Madrid), Spain.
Address correspondence to Angeles Cuadrado at the address above, or angeles.cuadrado{at}uah.es.
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The presence and distribution of two simple sequence repeats (SSRs), three highly repetitive sequences from rye, and the 5S rDNA have been investigated in 3 rye cultivars and 10 wild-related species of the genus Secale. The following conclusions can be drawn in addition to detailed knowledge of the sequence content of chromatin in each accession studied: (1) Every species is unique in either or both the complement and chromosomal distribution of the six repeated sequences analyzed. (2) These sequences reveal multiple landmarks along all the rye chromosomes arms. (3) High polymorphism as well as heterozygosity between homologues in the distribution of the (AAG)5 and (AAC)5 was revealed in the outbreeding species of the Secale strictum complex. (4) It is possible to deduce trends in the complexity of repetitive DNA during the evolution of the genus. A possible evolutionary pathway that accounts for the present-day Secale species is presented.
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The genus Secale includes cultivated rye and between 4 and 11 wild species, depending on the criteria used for species definition. The commonly recognized species of the genus Secale are three annuals, S. cereale L., S. vavilovii Grouch, and S. sylvestre Host, and the perennial S. strictum (syn. S. montanum). All the taxa in the genus are diploid with 14 chromosomes, and all can be intercrossed to yield partially fertile hybrids. Within the genus Secale, one or two translocations involving different chromosome arms have been confirmed by meiotic analysis. It is thought, therefore, that structural changes in chromosomes have played an important role in speciation (Heemert and Sybenga 1972; Riley 1955; Stuz 1972).
S. cereale, an annual outbreeder, includes cultivated rye plus a number of weedy rye types: S. cereale afghanicum Vav., S. cereale dighoricum Vav., S. cereale segetale Zhunk., and S. cereale ancestrale Zhunk. All these forms cross readily with cultivated rye, the hybrids are vigorous, their meiosis is completely normal, and the pollen fertility is as high as that of the parental taxonomic unit (Khush 1962). Because cultivated rye S. cereale does not appear in ancient archaeological materials, its origin is considered to be quite recent (bronze age). Weedy types of S. cereale may be somewhat older than cultivated rye.
S. strictum has been considered to be a complex group that includes distinct geographical isolates: the outbreeders S. strictum strictum, S. strictum kuprijanovii Grossh, and S. strictum anatolicum Boiss, and the inbreeder S. strictum africanum Stapf. Some authors have postulated that these subspecies could be considered as isolated populations belonging to the single species S. strictum. However, S. strictum ssp. africanum, which is self-fertile and endemic to South Africa, differs from the rest of S. strictum subspecies in breeding habit and chromosome arrangement (Khush 1962). It has a relatively recent origin. It migrated to South Africa in the Pleistocene, where it has evolved separately from the rest of the genus. It has not been determined whether S. strictum ssp. africanum is an independent species.
Finally, the annual and inbreeder species S. vavilovii, which is closely related to the group of S. cereale and S. silvestre, is the most different species of the group, being separated early from the rest of the species in the genus.
The phylogenetic relationships of all these species have been formerly interpreted from different types of data—morphological, ecological, cytogenetic, and hybrid fertility assays. Following the pioneer work of Vavilov (1926) on the origin of cultivated rye, there was general agreement that the weedy forms of S. cereale are the direct progenitors of cultivated rye. However, at one time or another almost every species of Secale has been suggested as the immediate ancestor of S. cereale. Previous approaches to explain the phylogeny of the genus were made, checking the presence, structure, and chromosomal locations of a series of defined repeated sequences making up much of the heterochromatic regions of rye chromosomes (Bedbrook et al. 1980; Cuadrado and Jouve 1997; Jones and Flavel 1982). However, it was difficult to deduce evolutionary trends (due to the complexity of these types of sequences) and to identify the chromosomes involved in structural interchanges using the hybridization sites revealed only by these probes. The inability to make a common bifurcating lineage of species and sequences means, of course, that these highly repeated sequences alone cannot be used to shed more light on the phylogeny of Secale. The synthetic simple sequence repeats (SSRs) have proven to be useful as cytological markers in identifying chromosomes and for understanding genome organization in plants (Cuadrado and Schwarzacher 1998; Cuadrado et al. 2000; Fuchs et al. 1998; Schmidt and Heslop-Harrison 1996). However, these types of sequences have not been used extensively in phylogenetic studies in rye.
The present work was designed to review the phylogenetic relationships of the 11 taxonomic units of the genus Secale by studying the presence and distribution of different DNA repetitive sequences. In this article our goal was to construct a physical map with multiple landmarks over all rye chromosomes. We tried to relate the amount and the pattern of distribution of both SSR markers and the previously described pSc119.2, pSc34, pS74, and pTa794 probes with genome remodeling during the speciation of the genus Secale. These will form the basis not only for the identification of chromosome rearrangements between the rye species but also describe the evolutionary trends of different repetitive sequences.
|
Materials and Methods |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Plant Materials
The plant materials used in this investigation consisted of various accessions belonging to 11 taxonomic units of the genus Secale (Table 1). S. cereale cv. Imperial is used as the tester. The addition lines of Imperial rye chromosomes into Triticum aestivum cv. Chinese Spring were proposed as a standard series of rye chromosomes in the workshop report on rye chromosome nomenclature and homology relationships (Sybenga 1983).
|
DNA Probes and Labeling
The probes were two synthetic oligonucleotides—(AAC)5 and (AAG)5—three different repetitive clones—pSc119.2, pSc74, and pSc34—containing highly repeated DNA sequences of 120, 350–480, and 610 bp, respectively, derived from S. cereale (Bedbrook et al. 1980; McIntyre et al. 1990), and pTa794 containing the 5S ribosomal sequence (Gerlach and Dyer 1980).
The synthetic oligonucleotides were labeled with digoxigenin-11-dUTP (Roche) or biotin-16-dUTP (Roche) according to a standard random primer protocol (Cuadrado et al. 2000). The plasmid clones were amplified and labeled with digoxigenin-11-dUTP (Roche) or rhodamine-4-dUTP (Amersham) using the polymerase chain reaction (PCR) with universal forward and reverse sequencing primers, 30 s annealing at 55°C and a 90 s extension at 72°C.
Cytologic Preparation and Fluorescence In Situ Hybridization (FISH)
Root-tip chromosome preparations, chromosome pretreatments, and in situ hybridization with hybridization stringency of 85% were performed following the experimental procedures described by Cuadrado and Jouve (1997). Reprobing was performed as described by Cuadrado and Jouve (1994). To avoid false negatives, in each FISH experiment the same hybridization mixture, with combined probes, was used in slides made from different species.
Sites of hybridization of biotin- and digoxigenin-labeled probes were detected with strectavidin-Cy3 (Sigma) and anti-digoxigenin FITC (Roche), respectively. No detection procedure was needed after direct rhodamine labeling. Epifluorescence signals were recorded by Fuji 400 film. Karyotypes were established by using at least 10 high-quality metaphase chromosome spreads from slides made from at least three seeds of each genotype. For Figure 1, negatives were digitized and printed using Adobe Photoshop 5.0 after contrast and brightness optimization of the entire image.
|
|
Results |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Metaphase chromosomes from all Secale species were identified individually by morphology and by combined simultaneous and/or reprobing preparations with the cloned probes pSc119.2 and pTa794 (Figure 1d,h,i,s). This allowed the identification of most rye chromosome arms in the species of the genus Secale (Cuadrado and Jouve 1997; Cuadrado et al. 1995). The external morphology of the seven chromosomes is well maintained over all the genus: one satellited (1R), three metacentrics (2R, 3R, and 7R), and three submetacentrics (4R, 5R, and 6R), with the exception of the S. strictum complex. In this group, the higher variation was noticed for chromosome 2R that is near median and small (Figure 1j,l,n,p).
Using the in situ hybridization pattern with the (AAC)5 probe alone, it is possible to distinguish all seven rye chromosomes in all the taxa analyzed (Figure 1a,b,d,f,i,j,l,n,p). The possibility of individual rye chromosome identification by using a single probe allowed us to establish a physical map with this probe. The (AAC)5 patterns of cv. Imperial were chosen to build up an (AAC)n karyogram of cultivated rye. It has two types of sites: "common" (present in Imperial) and "additional" (not found in Imperial, but present in the other varieties) (Figure 2).
|
The physical map of all the repeated DNA sequences in the cultivated rye is practically identical to those of all the weedy ryes; even less variation has been found between all these forms than between different cultivated rye varieties. A consensus map for the strictum complex that includes all the detected sites in the three subspecies strictum, anatolicum, and kuprijanovii has been considered (Figure 3). The similarities and high polymorphism found in the pattern of distribution of the hybridization sites for the probes in the allogamous subspecies of the strictum complex make it impossible to deduce evolutionary trends within this group.
|
The Presence and Distribution of the (AAC)n
The (AAC)5 probe hybridized to all the chromosome arms in the 11 taxonomic units investigated, from the most primitive species to cultivated rye. The cultivars Riodeva, Petkus, and Imperial showed similar but not identical labeling patterns. The most easily noticeable differences were the absence of hybridization on the subtelomeric site at the long arm of the satellited chromosome and at the interstitial site of the long arm of chromosome 2R in Petkus (Figure 1a). Differences were also found in Riodeva, lacking the interstitial hybridization signal on the long arm of chromosome 5R (Figure 1b). Moreover, the long arms of chromosomes 3R and 7R in Riodeva and Petkus have presented interstitial sites not observed in Imperial (Figure 1a,b). The physical map of the (AAC)5 sequence of all the weedy ryes—ancestrale, dighoricum, segetale, and afghanicum—is practically identical to that of cultivated rye. Even less variation was observed in the physical distribution of this sequence between the weedy ryes than between the three S. cereale cultivars analyzed (Figure 1a,b,d,e). Accordingly we will use the same standard karyogram to identify the chromosomes in the weedy forms of S. cereale. The (AAC)5 pattern found in S. vavilovii resembles that of S. cereale, with the only exception being the distal hybridization site found in the long arm of chromosome 6R of S. cereale that was not visible in S. vavilovii (Figure 1f).
S. silvestre showed the most different pattern of distribution of (AAC)n of all the taxa investigated. This sequence appeared in many additional sites. The most easily noticeable differences were observed in chromosomes 1R and 5R, which deviated from those in S. cereale by the presence of many interstitial in situ signals on the long and short arms (Figure 1i, middle).
The (AAC)n in situ pattern showed considerable variation between the subspecies of S. strictum. This was particularly true in S. africanum, which was the unique taxonomic unit of the strictum complex analyzed that showed a signal at the distal position in 1RL. Moreover, the chromosome arm 4RL does not have the distal signal that characterizes the chromosome 4R in the other members of this complex group of subspecies (Figure 1p). Even more variation was detected between homologues for the distribution of (AAC)n between and within individuals of the subspecies strictum, anatolicum and kuprijanovii (Figure 1j,l,n).
The Presence and Distribution of (AAG)n
Three major intercalary sites of (AAG)5 on the arms 2RL, 3RS, and 6RL are present in S. cereale and its related weedy forms (Figure 1c). Besides these signals, S. vavilovii shows an intercalary site on arm 5RL that is polymorphic for presence/absence (Figure 1g).
Up to 11 intercalary sites have been detected on chromosomes 1R, 3R, 4R, 5R, and 6R in the all S. strictum group. However, not more than eight signals per metaphase were observed. High polymorphism as well as heterozygosity between homologues was revealed for the distribution of (AAG)5 in different chromosomes in most of these species (Figure 1k,m,o,q). Besides the signals on 3RS and 6RL observed in S. cereale and S. vavilovii, it has been observed on one site on 3RL in strictum (Figure 1k); one or two sites on 4RL in anatolicum (Figure 1m), strictum (Figure 1k), and africanum (Figure 1q); one or two sites on 5RL in kuprijanovii (Figure 1o) and anatolicum (Figure 1m); one site on 5RS in africanum (Figure 1q) and strictum (Figure 1k); and one in the satellite of chromosome 1R in strictum (Figure 1k), kuprijanovii (Figure 1o), and africanum (Figure 1q). A rich pattern of 14 intercalary bands distributed over the seven chromosomes was observed in S. silvestre (Figure 1i).
The Presence and Distribution of the Highly Repetitive DNA Families
The localization of repetitive rye DNA sequences of 120, 350–480, and 610 bp have been studied in combination with the above-mentioned SSR probes to analyze the evolving trends of these noncoding sequences in the genus. Clear differences in both the amount and distribution of the highly repeat sequences have been found between the taxa investigated (Figure 3). However, despite the existing polymorphism for their presence/absence, the following general conclusions can be drawn.
Probe pSc119.2 localized for the 120 bp repetitive sequences and hybridized to practically all of the telomeric regions and in some major interstitial sites in single loci of 1RS, 1RL, 4RL, 5RL, 6R, 7RS, and 7RL in all species (Figure 1d,i [top]). Additional sites also are observed in particular taxonomic units: in 2RS in anatolicum, 4RS in S. vavilovii, and 6RS (polymorphic for presence/absence) in some subspecies of S. strictum and S. cereale (Figure 1d). Among the interstitial single sites, that of chromosome 6RL sometimes appears as a doublet in S. vavilovii, S. strictum, and S. cereale (Figure 1d). Moreover, a second interstitial site on chromosome arm 4RL has been found in some cultivars of rye (Figure 1i top; see also Cuadrado et al. 1995 [Figures 1 and 2b,d,e] and Cuadrado and Jouve 1997 [Figures 2 and 3c,d,m]).
The probe pSc74 that localizes for the repetitive family of 480 bp did not show in situ hybridization in S. silvestre or S. strictum africanum. It is restricted to subtelomeric positions in both arms on the metacentric chromosomes (1R, 2R, 3R, and 7R) and on the short arm of the heterobrachial chromosome pairs (4R, 5R, and 6R) in the rest of the species analyzed. Interstitial sites on 4RL, 5RL, and 6RL were observed in the S. strictum and S. cereale subspecies (Figure 3). Differences in this family were higher between the strictum complex and between rye cultivars. In these cases they went from samples with only two subtelomeric sites on the short arm of chromosomes 4R and 5R (i.e., strictum) to samples exhibiting all possible sites (i.e., anatolicum). The polymorphism for the presence/absence of the repetitive family of 480 bp in a particular site was usually frequent between and within plants of the same accessions.
The repetitive sequence of 610 bp is restricted exclusively to a subtelomeric position in some taxa (Figure 3). This repeated DNA family was absent in S. vavilovii and almost totally absent in S. silvestre and S. africanum, where only one site was observed on chromosome arms 4RS and 5RS, respectively (Figure 1r). Moreover, one to two polymorphic sites on 4RS or 5RS have been observed in strictum (data not shown). This family was polymorphic for the presence/absence in different telomeres of the other subspecies of S. strictum and in S. cereale, which showed from samples with signals in all the telomeres (with the exception of 4RL, 5RL, and 6RL) to samples with only a few signals.
The Presence and Distribution of Ribosomal 5S Genes
The genomes of all the taxa of the genus presented two 5S rDNA loci in common: one in a distal position with respect to the NOR constriction in 1RS and the other localized in a subterminal site of chromosome arm 5RS (Figure 1h,s). Moreover, S. vavilovii, S. cereale ancestrale, and some cultivars of S. cereale show a third polymorphic locus on a distal position of the arm 3RS (Figure 1h).
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The presence and distribution of highly repetitive DNA sequences of rye were previously investigated in different species of the genus Secale by Jones and Flavell (1982) and Cuadrado and Jouve (1997). The analysis has been extended to 11 taxa of the genus Secale, including the three main repetitive families of 120, 350–480, and 610 bp, the 5S rDNA and two SSRs. The present results on the localization of the highly repeated sequences in S. strictum anatolicum fits well with that of S. montanum previously reported (Cuadrado and Jouve 1997), probably due to a misclassification of the accession.
The morphology of the seven chromosomes of rye and the pattern of distribution of the repetitive sequences detected with pTa794 and pSc119.2 have diagnostic value in the genus Secale (Cuadrado et al. 1995) with minor exceptions for the chromosomes 2R and 3R. These can be distinguished using the third loci for 5S rDNA that is present in some cultivars of S. cereale, ancestrale, and vavilovii (Figure 1h). However, 3R is easily distinguished from 2R based on the different location of (AAG)5 in all the subspecies of S. cereale, the subspecies of the complex strictum, and in S. vavilovii (Figure 1c,g). In the same manner, the SSR sequences have diagnostic value. Thus (AAG)5 only permits chromosome identification in S. silvestre, and (AAC)5 allows the unequivocal identification of all rye chromosomes in all the taxa investigated (Figure 2). The use of a unique SSR system for chromosomal identification has many advantages over Giemsa C-banding and facilitates the localization of a particular probe using different fluorochromes in simultaneous or successive fluorescence hybridization experiments (Wiegant et al. 1993). It also offers the possibility of screening large populations of plants for the presence of particular chromosomes in breeding programs.
Evolution of the DNA Sequences and Phylogenetic Relationships in Genus Secale
The evolutionary trends of the different repetitive sequences accompanying the speciation of the genus can be inferred by comparing the pattern of distribution of each individual family in all rye species. Figure 3 presents a schematic phylogenetic tree based in the evolutionary trends of six different repetitive DNA sequences during the speciation of the genus Secale. Conclusions from other cytogenetic, morphological, and ecological assays have been considered and it has been assumed that the amplification of the same complex in different species is very low unless the sequence is predisposed to being amplified, and the probability of insertion and amplification of new element copies is much more common than the removal by unequal recombination of current copies.
From the sequences investigated, the families of 480 bp and 610 bp and the 5S rDNA locus of 3RS are Secale specific. However, the repetitive family of 120 bp and the SSR (AAG)5 and (AAC)5 have been mapped in other genera of the tribe Triticeae (Bustos et al. 1996; Castilho and Heslop-Harrison 1995; Cuadrado et al. 2000; Mukai et al. 1993; Pederson et al. 1996). Moreover, the 1RS and 5RS loci of 5S rDNA observed in all the taxonomic units of the genus Secale have been observed in the homologous chromosomes of other species of the tribe Triticeae (Castilho and Heslop-Harrison 1995; Mukai et al. 1990). These results show that these families were present in the common ancestral genome of rye, wheat, and other genera of the tribe.
The pattern of distribution of the family of 120 bp is maintained in all Secale species from primitive forms to the cultivated ryes (comparison in Figure 3, row b). The maximal variation in its distribution in the group is noticed in the interstitial sites of chromosome 6R in the most evolved species (see comparatively Figure 1c,i). This result suggests that the 120 bp family supported events of interstialization after the separation of S. silvestre and S. strictum africanum from the remaining species.
The 610 bp family is absent in S. vavilovii and almost totally absent in S. silvestre and strictum and africanum (Figure 1r). These results suggest that this sequence was formerly present and further amplified from one or two sites, which by reasonable assumption seems to correspond to the telomere of 4RS or 5RS. The amplification probably occurred after the ancestor of all rye species diverged from the basic Triticeae genome and before speciation within the genus. Thereafter the 610 bp passed to telomeric sites of other chromosomes, as is observed in the subspecies of S. strictum and in S. cereale. Differences in this family have been observed in the open-pollinated forms, and is particularly observable in the subspecies of the complex strictum and rye cultivars. This polymorphism could be due to deletion and/or amplification from the preexisting sequences. In the same manner, the absence of this family in S. vavilovii can be explained by deletion. The presence of the 610 bp family in S. silvestre and S. strictum and its absence in other Triticeae species fits well with the hypothesis of S. strictum being the ancestor of the early emerging species S. silvestre than with the existence of a common ancestor for both species (Khush and Stebbins 1961). The similar pattern of the 120 bp family in all Secale species and its different distribution in other species of the tribe Triticeae also contributes to support the hypothesis of a primitive S. strictum as the ancestor of S. silvestre.
The absence of the 480 bp family in S. silvestre and S. strictum africanum suggests that these sequences may have been amplified during the evolution of the genus Secale subsequent to the divergence of both taxa from the common ancestor. The rest of species show the 480 bp family that seems to have appeared first in the telomeres and before in the intercalary regions of the long arms of the heterobrachial chromosomes. Finally, the repeated sequence seems to have suffered a duplication appearing as a doublet on 5RL and 6RL in the most evolved species of the genus, S. cereale and its related weedy forms.
It is noteworthy that S. silvestre differs from the other Secale species by the amount and distribution of both SSR sequences (AAG)5 and (AAC)5. The differences observed in the presence of DNA sequences between species of the same genus can be explained by two hypothesis: (1) as a consequence of independent events of amplification in previously diverged species, or (2) as consequence of events of differential deletion in evolving species, subsequent to its amplification in a common ancestor. According with the increasing trend of other repetitive sequences (Bennetzen and Freeling 1997), it is reasonable to assume that S. silvestre was separated early from the rest of species and thereafter the sequences were distributed throughout the arms of all chromosomes by an intraspecific recombination mechanism in S. silvestre. The notable differences in the pattern of distribution of (AAC)5 and (AAG)5 between S. silvestre and the rest of species of the genus support the early separation of S. silvestre from S. strictum (Figure 3).
S. strictum africanum, geographically separated from the rest of the genus, differs from the other members of the strictum complex in breeding habits, chromosome arrangements, and the absence of the 480 bp family. A reduction and/or elimination of a family can result from unequal crossing over or by intrastrand recombination. Losing a telomeric block of repeat sequences is easy to explain by deletion. However, the loss of a site at an intercalary position is more difficult to explain. These reasons favor early separation of S. strictum africanum from the other members of the strictum complex. The isolation of S. africanum may have occurred before the amplification of the 480 bp family and its accumulation at the telomeres and transfer to interstitial sites. The events of interstialization of the 120 bp family support the fact that S. strictum africanum was separated early.
The conservative pattern of distribution that presents (AAC)5 in the remaining taxa could be explained by moderate polymorphism between and within species and/or the rearrangements that occurred during the evolution of the genus Secale. The distribution of (AAG)5 exhibits major differences. All the chromosomes are labeled at single sites in the subspecies of the complex strictum, with the only exception being 2R and 7R. However, S. cereale and S. vavilovii showed a nearly identical pattern, having common sites in three arms. Because the deletion of the same family at the same site in different taxa is highly improbable, the similar pattern of (AAG)n in S. cereale and S. vavilovii suggests a close relationship between species and suggests their common origin and separation from S. strictum followed by a divergent evolving process. This assumption is supported by the fact that the same translocation that distinguished S. strictum strictum (= montanum) from S. vavilovii has been found in S. cereale (Singh and Röbbelen 1977). This hypothesis is also supported by the specific finding of the polymorphic locus 5S rDNA in arm 3RS in both species. This result suggests that this system suffers an event of amplification after the separation of the common ancestor of S. cereale and S. vavilovii from the other species.
In summary, our results support that S. silvestre, an annual inbreeding species, was torn off from S. strictum in the Miocene and constitutes an early separated branch originating after the amplification of the 610 bp family. S. silvestre differs from the other species of the genus by the amount and distribution of (AAG)n and (AAC)n. The results suggest that both SSRs were distributed throughout the arms of all chromosomes after the divergence of S. silvestre.
The second stage in the evolution of the genus is relatively young, in the Pleistocene, after the geographical separation of the perennial inbreeding species S. africanum. Because deletion may be more difficult or may occur less frequently, it seems reasonable to assume that africanum constitutes an isolated branch that was separated from the rest before the amplification of the 480 bp family, the interstialization of the 120 bp family, and the bifurcation of the rest of the species. The similar pattern of distribution of the clusters of (AAC)n, (AAG)n, and the presence of the 5S rDNA locus in 3RS in S. cereale and S. vavilovii support the close relationship and common origin of both species. After an indefinite period of time they became disjoined and evolved separately. The main events concerning the repetitive families have been the transference of the 480 bp family from the telomeres toward interstitial sites in S. cereale, and the deletion of the 610 bp family in S. vavilovii. The homologous physical maps for the distribution of all the repetitive families studied in cultivated and weedy forms of S. cereale sustain the hypothesis of considering all these forms as geographical races of this unique species. In the same context, it should be assumed that the subspecies of the complex group strictum are really the result of independent evolution of isolated populations belonging to the single species S. strictum, with the exception of the most primitive subspecies, africanum, which must be considered as an independent taxonomic unit.
Chromosomal Rearrangements
Within the genus Secale up to three evolutionary translocations involving different chromosome arms were confirmed by meiotic analysis (Khush 1962; Khush and Stebbins 1961; Heemert and Sybenga 1972; Singh and Röbbelen 1977; Stutz 1972). Naranjo and Fernández-Rueda (1991), using C-bands, telocentrics, and translocations as diagnostic cytologic markers, demonstrated that only 1RS, 1RL, 2RL, 3RS, and 5RS showed a normal homologous relationship to wheat. The remaining arms of S. cereale appeared to be involved in chromosome rearrangements that occurred during the evolution of the genus Secale. They demonstrated the existence of a multiple translocation involving 4RL, 5RL, 6RS, and 7RS in S. cereale relative to wheat. The restriction fragment length polymorphism (RFLP)-based genetic map also suggests this multiple translocation (Devos et al. 1993). Cuadrado and Jouve (1997), using highly repeat families of rye in FISH experiments were unable to identify the chromosomes involved in these structural interchanges. However, they suggested that the arms 2RL, 4RS, and 5RL of the species S. strictum, S. vavilovii, and S. silvestre, respectively, could be involved in some translocation.
Comparison of the physical maps reported in the present study allow the identification of additional putative regions involved in translocations differentiating the different taxa. Thus dissimilarity in the pattern of distribution of (AAC)5 in 6RL between S. vavilovii and S. cereale seems to suggest that the distal region of this arm has been involved in interchanges differentiating both species. Heemert and Sybenga (1972), using aneuploids and a standard tester set of reciprocal translocations in S. cereale, identified this chromosome as being involved in the translocations that structurally differentiate the genome of S. cereale from those of S. montanum and S. vavilovii. Differences in the pattern of distribution of (AAC)5 in the arm 4RL between africanum and the rest of the complex group strictum seem to suggest that the distal region of this arm could be involved in a translocation. This was previously suggested for the species S. africanum and S. montanum (Khush 1962).
Due to the high degree of polymorphism and the differences found between the Secale species, we cannot confirm that differences in the physical mapping with these repeats will be due to chromosomal rearrangements. Maybe the potential of such an approach will be in the identification of chromosomes involved in pairing figures in meiosis of the partially fertile hybrids, using simultaneous visualization of multiple DNA probes by FISH. This kind of analysis proved useful in studying the homologies between T. aestivum and S. cereale in wheat x rye hybrids (Cuadrado et al. 1997)
|
Acknowledgments |
|---|
The authors would like to thank the CICYT (Comisión Asesora de Ciencia y Tecnología) of Spain for financial support of this work (grant no. AGL2000-0762-C02-01), the University of Alcalá (grant no. E030/00), and Adrian Burton for linguistic assistance.
|
Footnotes |
|---|
Corresponding Editor: Stephen J. O'Brien
Received November 16, 2001
Accepted June 10, 2002
|
References |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
-
Bedbrook JR, Jones J, O'Dell M, Thompson RD, Flavell RB, 1980. A molecular description of telomeric heterochromatin in Secale species. Cell. 19:545-560.[CrossRef][Web of Science][Medline]
Bennetzen JL, Freeling M, 1997. The unified grass genome: synergy in synteny. Genome Res. 7:301-306.
Bustos A, Cuadrado A, Soler C, Jouve N, 1996. Physical mapping of repetitive DNA sequences and 5S and 18S–26S rDNA in wild species of the genus Hordeum. Chromosome Res. 4:491-499.[CrossRef][Web of Science][Medline]
Castilho A, Heslop-Harrison JS, 1995. Physical mapping of 5S and 18S–25S rDNA and repetitive DNA sequences in Aegilops umbellulata. Genome. 38:91-96.
Cuadrado A, Ceoloni C, Jouve N, 1995. Variation in highly repetitive DNA composition of heterochromatin in rye studied by fluorescence in situ. Genome. 38:1061-1069.
Cuadrado A, Jouve N, 1994. Mapping and organization of highly-repeated DNA sequences by means of simultaneous and sequential FISH and C-banding in 6x-triticale. Chromosome Res. 2:331-338.[CrossRef][Medline]
Cuadrado A, Jouve N, 1997. Distribution of highly repeated DNA sequences in species of the genus Secale. Genome. 40:309-317.[Medline]
Cuadrado A, Schwarzacher T, 1998. The chromosomal organization of simple sequence repeats in wheat and rye genomes. Chromosoma. 107:587-594.[CrossRef][Web of Science][Medline]
Cuadrado A, Schwarzacher T, Jouve N, 2000. Identification of different chromatin classes in wheat using in situ hybridization with simple sequence repeat oligonucleotides. Theor Appl Genet. 101:711-717.[CrossRef]
Devos KM, Atkinson MD, Chinoy CN, Francis HA, Harcourt RL, Koebner RMD, Liu CJ, Masojc P, Xie DX, Gale MD, 1993. Chromosomal rearrangements in the rye genome relative to wheat. Theor Appl Genet. 85:673-680.[Web of Science]
Fuchs J, Kohne M, Schubert I, 1998. Assignment of linkage groups to pea chromosomes after karyotyping and gene mapping by fluorescent in situ hybridization. Chromosoma. 107:272-276.[CrossRef][Web of Science][Medline]
Gerlach WL, Dye TA, 1980. Sequence organization of the repeating units in the nucleus of wheat which contain 5S rRNA genes. Nucleic Acids Res. 8:4851-4865.
Heemert CV, Sybenga J, 1972. Identification of the three chromosomes involved in the translocation which structurally differentiates the genome of Secale cereale L., from those of Secale montanum and Secale vavilovii Grossh. Genetica. 43:387-393.[CrossRef]
Jones JDG, Flavell RB, 1982. The structure, amount and chromosomal localization of defined repeated DNA sequences in species of the genus Secale. Chromosoma. 86:613-641.[CrossRef]
Khush GS, 1962. Cytogenetic and evolutionary studies in Secale, II Interrelationships in the wild species. Evolution. 16:484-496.[CrossRef]
Khush GS, Stebbins GL, 1961. Cytogenetic and evolutionary studies in Secale, I. Some new data on the ancestry of S. cereale. Am J Bot. 48:723-730.[CrossRef]
McIntyre CL, Pereira S, Moran LB, Apples R, 1990. New Secale cereale (rye) DNA derivatives for the detection of rye chromosome segments in wheat. Genome. 33:317-323.
Mukai Y, Endo TR, Gill BS, 1990. Physical mapping of the 5S rRNA multigene family in common wheat. J Hered. 81:290-295.
Mukai Y, Nakahara Y, Yamamoto M, 1993. Simultaneous discrimination of the three genomes in hexaploid wheat by multicolor fluorescence in situ hybridization using total genomic and highly repeated DNA probes. Genome. 36:489-494.
Naranjo T, Fernández-Rueda P, 1991. Homology of rye chromosome arms to wheat. Theor Appl Genet. 82:577-586.
Pedersen C, Rasmussen SK, Linde-Laursen I, 1996. Genome and chromosome identification in cultivated barley and related species of the Triticeae (Poaceae) by in situ hybridization with the GAA-satellite sequence. Genome. 39:93-104.[Medline]
Riley R, 1955. The cytogenetics of the differences between some Secale species. J Agric Sci. 46:377-383.
Schmidt T, Heslop-Harrison JS, 1996. The physical and genomic organization of microsatellites in sugar beet. Proc Natl Acad Sci USA. 93:8761-8765.
Singh RJ, Röbbelen G, 1977. Identification by Giemsa technique of the translocations separating cultivated rye from three wild species of Secale. Chromosoma. 59:217-229.[CrossRef]
Stutz HC, 1972. On the origin of cultivated rye. Am J Bot. 59:59-70.[CrossRef]
Sybenga J, 1983. Rye chromosome nomenclature and homology relationships. Z Pflanzenzücht. 90:297-304.
Vavilov NI, 1926. Studies on the origin of cultivated plants. Bull Appl Bot. 16:1-248.
Wiegant J, Wiesmeijer CC, Hoovers JM, Schuuring E, d'Azzo A, Vrolijk J, Tanke HJ, Raap AK, 1993. Multiple and sensitive fluorescence in situ hybridization with rhodamine, fluorescein and coumarim-labeled DNAs. Cytogenet Cell Genet. 63:73-76.[Web of Science][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  E. I. Mikhailova, D. Phillips, S. P. Sosnikhina, A. V. Lovtsyus, R. N. Jones, and G. Jenkins Molecular Assembly of Meiotic Proteins Asy1 and Zyp1 and Pairing Promiscuity in Rye (Secale cereale L.) and Its Synaptic Mutant sy10 Genetics, November 1, 2006; 174(3): 1247 - 1258. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||