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2Laboratory of Systematic and Evolutionary Botany, Chinese Academy of Sciences, Beijing 100093, China; and 3Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824
Received for publication May 26, 1998. Accepted for publication July 29, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: concerted evolution • chromosome • gene conversion • in situ hybridization • nuclear ribosomal DNA • Paeonia • phylogeny • physical mapping
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Raven, 1975
). Peonies (Paeonia, Paeoniaceae), with chromosomes that are large in size and few in number, have been a major focus of cytological studies (e.g., Stebbins, 1938
; Tzanoudakis, 1983
; Hong, Zhang, and Zhu, 1988
). The genus Paeonia comprises three sections and 
35 species (Stern, 1946
): section Moutan with six diploid (2n = 10) shrubby species, section Oneapia with two diploid herbaceous species, and section Paeonia with both diploid and tetraploid (2n = 20) herbaceous species.
Cytological studies of most species of Paeonia have revealed a quite uniform karyotype for the entire genus (Stebbins, 1938
; Hong, Zhang, and Zhu, 1988
; Pei, 1993
). Of five pairs of mitotic chromosomes in the diploid species, the three largest pairs are similar in size and have median to submedian centromeres, the fourth pair has a submedian centromere, and the smallest pair has a subterminal centromere. The only consistent karyotypic difference among peony species is the higher arm ratio (more unequal length between short and long arms) of the largest pair of chromosomes (chromosome 1) in section Moutan compared with those of the other two sections (Hong, Zhang, and Zhu, 1988
). Consequently, karyotypes of peonies yield little phylogenetic information within the genus.
Physical maps of genes by fluorescent in situ hybridization (FISH) represent a potentially new source of chromosomal characters that may be phylogenetically informative. In plants, physical mapping has focused on highly repetitive DNA or multigene families because technical difficulties remain for mapping low-copy genes (Jiang and Gill, 1994a
). The most frequently mapped gene is the 18S–5.8S–26S ribosomal RNA gene (rDNA). Evolutionary implications of variation in number and location of rDNA loci have been explored in some plant groups (e.g., Maluszynska and Heslop-Harrison, 1993
; Jiang and Gill, 1994b
; Thomas et al., 1997
), but the question concerning potential phylogenetic utility of such variation in general remains open.
Another appealing reason for investigating the number and location of rDNA loci in peonies is to gain a better understanding of concerted evolution of rDNA. Concerted evolution, via gene conversion or unequal crossing over, plays an essential role in maintenance of sequence homogeneity of a multigene family, such as rDNA (Arnheim, 1983
). To understand mechanisms of concerted evolution, a series of theoretical studies was done to investigate the impact of the number of loci, the number of gene copies at each locus, and the chromosomal location of the loci on the tempo of concerted evolution (Arnheim, 1983
; Ohta and Dover, 1983
; Slatkin, 1986
). However, little empirical evidence has been gathered to test the hypotheses derived from the theoretical studies.
A remarkable example of interlocus (interchromosomal) concerted evolution of rDNA in plants was obtained by studying sequences of internal transcribed spacers (ITS) of rDNA in cotton (Wendel, Schnabel, and Seelanan, 1995
). Each allotetraploid species of cotton has fixed ITS sequences of one of the two diploid parental species through concerted evolution. Further, the fact that the rDNA loci were mapped on several chromosomes in cotton (Crane et al., 1993
; Hanson et al., 1996
) suggested that concerted evolution of rDNA had occurred between nonhomologous chromosomes.
Allotetraploid species have been documented in Paeonia section Paeonia based on cytogenetic and molecular data (Stebbins, 1948
; Tzanoudakis, 1983
; Sang, Crawford, and Stuessy, 1995
, 1997
). Polymorphism of ITS sequences was not observed in some of the allotetraploid species, implying that the parental sequence polymorphism has been homogenized by concerted evolution in these species (Sang, Crawford, and Stuessy, 1995
, 1997
; Sang and Zhang, in press
). Clarification of the number and location of rDNA loci in Paeonia, therefore, should provide additional insights into mechanisms of concerted evolution of rDNA in plants.
In this paper, we mapped the 18S–5.8S–26S rDNA on the chromosomes of five diploid peony species, P. rockii and P. delavayi representing two subsections of section Moutan, and P. emodi, P. tenuifolia, and P. veitchii of section Paeonia. Our primary goals were to: (1) determine the number and location of the rDNA loci in these peony species; (2) infer evolutionary changes in number and location of rDNA loci in Paeonia; and (3) gain a better understanding of concerted evolution of rDNA in Paeonia.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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FISH
Slides were incubated in 100 µg/mL DNase-free RNase A at 37oC for 1 h followed by three washes in 2X SSC for 5 min, then incubated in 1 µg/mL proteinase K at 37oC for 10 min followed by two washes in PBS buffer with 50 mmol/L MgCl2 and three washes in 2X SSC for 5 min. Slides were postfixed in 1% formaldehyde for 10 min, dehydrated in ethanol (70, 90, and 100%), and air-dried. Freshly denatured hybridization solution (20–40 µL), containing 50% (v/v) formamide, 10% (w/v) dextran sulfate, 2X SSC, 1.5 ng/µL labeled probe, 250 ng/µL salmon sperm DNA (Sigma), 0.125% SDS, and 50 mmol/L sodium phosphate (pH 7.0), was applied to each slide. The slides were then placed in a moist chamber and incubated first at 90°C for 10 min, then at 37°C for 16 h.
The slides were washed twice at 42°C in a solution of 20% formamide, 0.1X SSC, and 0.2% SDS for 5 min, once in 1X SSC with 0.2% SDS for 3 min, and twice in 2X SSC with 0.1% SDS for 3 min, followed by three washes at room temperature in 2X SSC for 3 min, and a brief wash in 100 mmol/L Tris-HCl (pH 7.5), 150 mmol/L NaCl, and 0.1% Triton X-100. The slides were then incubated in 100 mmol/L Tris-HCl (pH 7.5), 150 mmol/L NaCl, 0.1% Triton X-100, and 5% BSA at 37°C for 30 min. The immunological reaction was carried out by incubating the slides at 37°C in 10 µg/mL sheep anti-DIG antibody conjugated with fluorescein (Boehringer Mannheim) for 45 min. The slides were washed three times in 100 mmol/L Tris-HCl (pH 7.5), 150 mmol/L NaCl, and 0.1% Triton X-100 for 5 min; stained in 2 µg/mL propidium iodide (PI) solution for 20 min; dehydrated in ethanol (70, 90, and 100%), each 5 min; air-dried and mounted in SlowFadeTM (Molecular Probes, Eugene, Oregon).
Observation and calculation
Slides were observed with a Zeiss 210 Laser Scanning Confocal Microscope using the 488 line of a dual line argon ion laser. A LP 590 barrier filter was used to collect the red (PI) image, and a BP 520–560 filter was used to collect the green (fluorescein) image. The red image was collected first in a confocal mode. The fluorescein image was then collected in a nonconfocal mode so as to maximize the signal, and overlaid onto the PI image. The overlay image was stored on the computer disk. The images were brought together to make the plate (Fig. 1) using the program Adobe Photoshop. During this process, no modification was made to the individual images.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In all five species, rDNA loci are clearly located on the short arms of the two pairs of small chromosomes, chromosomes 4 and 5, with submedian and subterminal centromeres, respectively (Fig. 1). In P. delavayi, the chromosomes that do not have rDNA sites constitute the large pair of chromosomes with submedian centromeres (Fig. 1a). In P. rockii, two of the three large pairs of chromosomes, of which one has median and one has submedian centromeres, do not have rDNA sites (Fig. 1b). In P. tenuifolia, two large pairs of chromosomes do not have rDNA sites. Whenever any of the three pairs of large chromosomes showed unequal arm length, rDNA sites were always located on the short arms.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Ricroch, Peffley, and Baker, 1992
; Fukui, Ohmido, and Khush, 1994
; Castilho and Heslop-Harrison, 1995
; Linares et al., 1996
; Martel, Ricroch, and Sarr, 1996
; Xu and Earle, 1996
). It is, therefore, notable that P. emodi and P. veitchii have an rDNA locus on every pair of chromosomes.
The number of rDNA loci varies within each of the three sections of Paeonia. In section Moutan, P. rockii and P. delavayi have six and eight rDNA sites, respectively. In section Paeonia, P. tenufolia has six rDNA sites while P. emodi and P. veitchii have ten rDNA sites. In section Oneapia, P. californica and P. brownii have six and eight rDNA sites, respectively (Zhang and Sang, 1998
). This suggests that the number of rDNA loci has changed readily within the genus.
The location of rDNA loci can be determined based on the karyotype of a species. All of the species have rDNA loci on the two pairs of short chromosomes, 4 and 5. Although it is difficult to rank with certainty the three pairs of large chromosomes by size, one pair, which does not bear an rDNA locus in the species with six or eight rDNA loci, seems to be the largest in size, i.e., chromosome 1. Especially in P. rockii and P. delavayi of section Moutan, this pair of chromosomes has submedian centromeres (Fig. 1a, b) and is the one designated as chromosome 1 by Stebbins (1938)
and Hong, Zhang, and Zhu (1988)
. Therefore, we can conclude that chromosome 1 in P. rockii, P. delavayi, P. tenuifolia, and P. californica does not have an rDNA locus.
Previous cytological studies indicated that chromosomes 2 and 3 do not differ significantly in length or arm ratio and thus cannot be distinguished morphologically (Hong, Zhang, and Zhu, 1988
; Pei, 1993
). The location of rDNA loci offers a new marker for identifying homologous chromosomes (Jiang and Gill, 1994b
). We hypothesize that the two large chromosomes bearing rDNA sites in P. rockii, P. tenuifolia, and P. californica are homologs and designate them as chromosome 3. This hypothesis can be further tested by comparing maps of additional molecular markers.
Phylogenetic implications of variation in number and location of rDNA loci can be examined by mapping the number of rDNA loci on the phylogenetic tree of these species revealed previously by multiple molecular markers (Sang, Donoghue, and Zhang, 1997
; Fig. 2). The most parsimonious interpretation of evolution of the number and location of rDNA loci is that the state of three loci, one each on chromosomes 3, 4, and 5, is a plesiomorphy of the genus, i.e., existing in the most recent common ancestor of Paeonia. The increase in number of rDNA loci occurred later in each of the three sections. The increase from three to four loci represents a parallel gain of an rDNA locus on chromosome 2 in section Moutan and section Oneapia (Fig. 2). The number of rDNA loci, therefore, is subject to homoplasy when used as a phylogenetic character in Paeonia. Therefore, caution should be exercised in phylogenetic interpretation of variation in physical maps of rDNA loci.
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). Such translocation could be facilitated by the telomeric location of rDNA loci because translocation occurring at the middle of chromosomes is more likely to be deleterious (Hanson et al., 1996
). Variability in number of rDNA loci within each section of Paeonia, therefore, may be attributed to their telomeric location. The lack of an rDNA site on one homolog of the chromosome 5, 5' (Fig. 1f), in one individual of P. veitchii may have resulted from a spontaneous deletion of a fragment from the short arm of the chromosome.
Regarding concerted evolution of rDNA, the previous studies have shown that at least three allotetraploid species, P. arietina, P. officinalis, and P. parnassica, had completely homogenized ITS sequences (Sang, Crawford, and Stuessy, 1995
, 1997
). Comparison of ITS and cpDNA phylogenies (Sang, Crawford, and Stuessy, 1997
) and phylogenetic analyses of Adh gene sequences (Sang and Zhang, in press
) suggested that these three species originated from hybridization between a close relative of P. tenuifolia and a basal lineage of section Paeonia. Although the number of rDNA loci has not been determined in any of the allotetraploid species, they are likely to have at least six rDNA loci given that each diploid genome contains at least three loci. It is, therefore, striking that concerted evolution could have homogenized ITS sequences among such a large number of rDNA loci dispersed on several pairs of nonhomologous chromosomes of an allotetraploid. The telomeric location of rDNA loci may have facilitated the process of sequence homogenization if unequal crossing-over was the primary mechanism of concerted evolution in peonies.
The previous finding of uneven occurrence of sequence homogenization in the ITS regions of certain putative hybrid species of Paeonia suggested that gene conversion may have played an important role in concerted evolution of rDNA in peonies (Sang, Crawford, and Stuessy, 1995
). An interesting question, then, is to what extent the number of rDNA loci and the number of nonhomologous chromosomes bearing these loci affect rates of gene conversion. Theoretical studies indicated that dispersion of a gene family onto several chromosomes has relatively small effect on the rate of gene conversion unless mechanisms exist that prevent gene conversion among nonhomologous chromosomes (Ohta and Dover, 1983
). The conversion rate within each chromosome and the total number of gene copies in the family are critical factors accounting for the rates of gene conversion. The conversion rate between nonhomologous chromosomes becomes critical only when the number of nonhomologous chromosomes is of the same magnitude or larger than the number of genes on an individual chromosome (Ohta and Dover, 1983
). For rDNA in peonies, the number of gene copies is obviously much larger than the number of nonhomologous chromosomes bearing rDNA loci. Therefore, the results of this study support the theoretical prediction that dispersion of rDNA loci on several nonhomologous chromosomes would not necessarily lead to slower rates of gene conversion.
In allotetraploids of Brassica, however, the parental polymorphism of rDNA was maintained in the allotetraploid species despite their relatively ancient origins (Waters and Schaal, 1995
; O'Kane, Schaal, and Al-Shehbaz, 1996
). Physical mapping of rDNA loci in diploid Brassica species detected two to five rDNA loci at both telomeric and interstitial locations of chromosomes (Maluszynska and Heslop-Harrison, 1993
). In both peony and cotton, where homogenization of ITS sequence have been detected in allotetraploids, all of the rDNA loci were mapped at telomeric or subtelomeric locations of chromosomes (Hanson et al., 1996
). Therefore, we conclude that chromosomal location of rDNA loci has a more substantial impact than the number of loci on the tempo of concerted evolution through either unequal crossing-over or gene conversion.
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FOOTNOTES |
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4 Author for correspondence (Tel.: 517-355-4689; Fax: 517-353-1926; e-mail: sang{at}pilot.msu.edu
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Castilho, A., and J. S. Heslop-harrison. 1995 Physical mapping of 5S and18S–25S rDNA and repetitive DNA sequences in Aegilops umbellulata. Genome 38: 91–96.
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Fukui, K., N. Ohmido, and G. S. Khush. 1994 Variability in rDNA loci in the genus Oryza detected through fluorescence in situ hybridization. Theoretical Applied Genetics 87: 893–899.
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