| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Systematics |
2Jepson Herbarium and Department of Integrative Biology, University of California, Berkeley, California 94720-2465 USA; and 3Section of Evolution and Ecology, University of California, Davis, California 95616 USA
Received for publication March 8, 2001. Accepted for publication July 5, 2001.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: Compositae • external transcribed spacer • internal transcribed spacer • Lactuceae • molecular phylogeny • Munzothamnus; • Pleiacanthus • Stephanomeria
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Historically, the genus has long been known for the complex morphological intergradation that seems to connect the species. For example, Ferris (1960, p. 574), referring to the annuals, noted that "Different strains are sometimes locally recognizable but intergrading forms are constantly to be found even in the same region." Cronquist (1994, p. 448), referring to the very widespread perennial species S. tenuifolia and S. pauciflora, noted that the former species "consists of two striking but poorly defined varieties that show a degree of geographic segregation. They seem very different in their typical expression, but variation is continuous from one to the other," and when considering the two species together said, "Stephanomeria pauciflora is remarkably like S. tenuifolia ... in aspect. The only consistently dependable difference between the two lies in the pappus, ...."
The taxonomic distinctions of the annual species were poorly understood for years. The individual species did not become evident until their morphology and geographical distributions were correlated with their chromosome numbers and reproductive compatibilities (Gottlieb, 1971, 1972
). Stephanomeria exigua and S. virgata were shown to differ for a large number of traits, including their karyotypes, that were recombined in several other annual species, both at the diploid and tetraploid levels. The tetraploid S. elata was shown by cytogenetic analysis to have an allotetraploid origin from natural hybrids between the two diploids and to form a polyploid complex with them (Gottlieb, 1969
). The diploid S. diegensis also proved to be derived from hybrids between S. exigua and S. virgata (Gallez and Gottlieb, 1982
). Overall, the morphological intermediacy of S. elata and S. diegensis and the frequent presence of hybrid individuals wherever S. exigua and S. virgata grow together were the causes of the taxonomic complexities of the entire group of annuals.
Evolutionary studies of the genus also provided evidence for the direct origin of one diploid plant species from an extant one, the selfer S. malheurensis having arisen from the population of the outcrosser S. exigua subsp. coronaria at a single site in eastern Oregon where they grow side by side (Gottlieb, 1973, 1978
).
Relationships among the annual stephanomerias and circumscriptions for the species were determined from the results of an intensive series of experimental hybridizations carried out among scores of populations (Gottlieb, 1971
). Similar studies have not been conducted on the perennial species. Before the present study, the relationships among them were not understood, nor was it even clear whether all of the perennial species commonly assigned to the genus properly belong to it. For example, Stephanomeria blairii, recognized in The Jepson manual: higher plants of California (Hickman, 1993
), had been placed in Malacothrix and had also been considered as a monotypic genus Munzothamnus P. H. Raven. Stephanomeria spinosa, also recognized in The Jepson manual, was transferred into the genus from Lygodesmia D. Don on the basis of having a chromosome number and pollen sculpturing similar to those of Stephanomeria.
No taxonomic assessment of all the species of Stephanomeria at one time has ever been carried out. Because many of its species have been important in the study of plant evolutionary biology, and because a modern treatment is required for the Flora of North America North of Mexico project (L. D. Gottlieb, unpublished), it seemed appropriate to examine them now from the perspective of molecular systematics. We present here an analysis of DNA sequences from the internal transcribed spacer (ITS), the external transcribed spacer (ETS), and the 5.8S regions of 18S–26S nuclear rDNA from all the species of Stephanomeria as well as from four related genera (see Appendix 1).
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
, all 7 genera assigned to Microseridinae sensu Stebbins (1953)
, both names formalized as subtribes by Solbrig (1963)
, and representatives of other subtribes of Lactuceae. The taxa in Microseridinae have base chromosome numbers of x = 5, 6, and 9; those in Stephanomeriinae have x = 6, 7, 8, and 9. The taxa examined in this paper include all those with a base chromosome number of x = 8 and are assigned to Stephanomeria, Rafinesquia Nutt., Munzothamnus, and Pleiacanthus Rydb. as well as a taxon with n = 7 assigned to Prenanthella Rydb. The broader analysis showed that all these taxa belong to a single clade with 100% bootstrap support; this clade is the subject of the present paper. The analysis of all the Stephanomeriinae and other western North American Lactuceae will be presented in a separate publication. Four taxa were selected as outgroups, two from Microseridinae (Nothocalais troximoides and Uropappus lindleyi) and two from Stephanomeriinae sensu Stebbins (Atrichoseris platyphylla and Malacothrix saxatilis var. tenuifolia). The sequences from all diploid species generally recognized in Stephanomeria were examined; the tetraploid perennial S. parryi and the tetraploid annual S. elata were not included. Sources of all plant materials and GenBank accession numbers for DNA sequences have been archived at the American Journal of Botany Supplementary Data web site (http://ajbsupp.botany.org/). Voucher specimens are deposited at DAV, JEPS, and UC.
DNA isolation
Total DNAs were extracted from 1–2 g fresh leaves pulverized in liquid nitrogen by a cetyltrimethylammonium bromide (CTAB) protocol modified from Richard, Reichardt, and Rogers (1994)
or from dried herbarium specimens with the DNeasy Plant Mini Kit (Qiagen, Valencia, California, USA).
Polymerase chain reaction (PCR) amplification of the ITS region
The ITS region includes the ITS-1, 5.8S, and ITS-2 nuclear rDNA regions (Fig. 1). It was amplified as described by Baldwin and Wessa (2000)
with primers ITS-I (5'-GTCCACTGAACCTTATCATTTAG-3'; Urbatsch, Baldwin, and Donoghue, 2000
) and ITS4 (5'-TCCTCCGCTTATTGATATGC-3'; White et al., 1990
). The PCR products were visualized by agarose gel electrophoresis and cleaned with Ultra free-MC tubes (Millipore, Burlington, Massachusetts, USA).
|
) and 26S-IGS (5'-GGATTGTTCACCCACCAATAGGGAACGTGAGCTG-3'; Baldwin and Markos, 1998
) in the Expand High Fidelity PCR System enzyme mix (Roche, Indianapolis, Indiana, USA) and sequenced the 3' ETS region of the resulting fragment with primer 18S-E (5'-GCAGGATCAACCAGGTAGCA-3'; Baldwin and Markos, 1998
) as described by Baldwin and Markos (1998)
. A region of complete nucleotide identity was found in the eight species and a primer was designed within it: L-ETS (5'-GCATCGTTCGGTGCATTCT-3'). The L-ETS primer was then used with primer 18S-ETS (5'-ACTTACACATGCATGGCTTAATCT-3'; Baldwin and Markos, 1998
) to amplify the 3' ETS region.
Sequencing protocols
For sequencing the ITS region, we used primers ITS4 (also used for amplification of the ITS region; see above) and ITS5 (5'-GGAAGGAGAAGTCGTAACAAGG-3'; White et al., 1990
). We used primers L-ETS and 18S-ETS for sequencing of 3' ETS amplification products generated using the same primers (see above). Cycle-sequencing reactions were conducted using the Thermo Sequenase Dye Terminator Cycle Sequencing Kit (US79765, Amersham Pharmacia Biotech, Cleveland, Ohio, USA) with the addition of 4% DMSO (dimethyl sulfoxide) and with half-volume reactions. Reaction products were purified using Centri-seps columns (CS-901, Princeton Separations, Adelphia, New Jersey, USA). The DNA sequences were resolved on 4.8% polyacrylamide gels (Page-Plus acrylamide E562, Amresco, Solon, Ohio, USA) in an ABI 377 automated sequencer (Perkin-Elmer/Applied Biosystems). The sequences were analyzed with ABI Sequence Analysis and ABI Sequence Navigator software (Perkin-Elmer/Applied Biosystems). Nucleotide sequences of both DNA strands were obtained and compared to ensure accuracy. For Stephanomeria guadalupensis and S. thurberi, the fragments were cloned with the TOPO TA cloning kit (Invitrogen, Carlsbad, California, USA) because high-quality sequences could not be obtained directly from the pooled PCR products. The clones were reamplified (prior to sequencing) directly from plated, transformed colonies with M13 primers. Cells were added to the reaction mix and lysed at 94°C for 12 min in the thermocycler prior to 30 cycles of PCR (94°C for 1 min, 58°C for 1 min, and 72°C for 2 min), followed by a final extension period (72°C for 7 min). Among all sequences examined in the present study, 70 sites (0.13%) showed two bases; i.e., such sites either were truly polymorphic within the individual or were artifacts.
Phylogenetic analyses
Sequences were aligned with Clustal X (Thompson et al., 1997
) and adjusted manually. The boundaries between the spacers and adjacent rRNA genes (18S, 5.8S, and 26S) were determined by comparisons with earlier published angiosperm sequences (Baldwin, 1992
; Baldwin and Markos, 1998
). Gaps were treated as missing data in phylogenetic analyses. The aligned sequence matrix has been archived at the American Journal of Botany Supplementary Data web site (http://ajbsupp.botany.org/) and as hardcopy in JEPS archives.
Prior to phylogenetic analyses, the partition homogeneity test (Farris et al., 1995
), as implemented in PAUP* 4.0 (Swofford, 1998
), was conducted to test for incongruence of phylogenetic signal between the ETS and ITS regions. The test included 100 homogeneity replicates, each involving 20 heuristic searches with random stepwise-addition. Parsimony analyses were conducted using PAUP* 4.0 (Swofford, 1998
). Analyses were conducted of the entire aligned sequence matrix, with all characters and character-state transformations given equal weight. Maximum parsimony analysis was performed using 500 heuristic searches with random stepwise-addition. Reliability of clades was evaluated by bootstrap analyses, with 20 heuristic searches and random stepwise-addition for each of the 500 bootstrap replicates.
Taxon divergence
Divergence between every pair of taxa was calculated as the actual number of site differences between them excluding insertions/deletions (indels). When multiple sequences of a taxon were available, as was the case for 13 of the 24 taxa (19 in Stephanomeria plus 5 in related genera), all possible pairwise comparisons were made and a single divergence value was calculated by averaging.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
). The 5.8S rDNA was invariant in length at 164 bp, similar to the value reported previously (Hershkovitz and Lewis, 1996
). The 3' ETS region varied from 508 bp (in Stephanomeria tenuifolia) to 515 bp (in Munzothamnus blairii). A total of 33 indels were inferred from the aligned ITS and ETS regions. The ITS region had 19 indels and the ETS had 14. The longest indels were two 3-bp deletions (in Stephanomeria tenuifolia, STEN2—see Appendix 1 for taxon abbreviations), at sites 441–443 and 446–448. The only other indel longer than a single base was a 3 bp deletion in ITS-2 of Prenanthella exigua, sites 1124–1126.
Sequence diversity
For the aligned ITS and ETS sequences, 386 of 1162 sites (33.2%) were variable. The ETS sequence showed variation at 207 of 518 sites (40.0%) and, of these, 129 (24.9%) were potentially informative. For the ITS-1 and ITS-2 sequences, 173 of 480 sites (36.0%) were variable; of these, 115 (24.0%) were potentially informative. In the 5.8S rDNA, only six of 164 sites (3.7%) were variable, and four of them were potentially informative.
Phylogenetic results
The partition homogeneity test showed that the ITS and ETS data sets are not significantly heterogeneous (P = 0.07); therefore, they were combined for all subsequent analyses. Maximum parsimony analysis of the ETS + ITS matrix of 42 sequences and four outgroup taxa (two from Microseridinae and two from Stephanomeriinae sensu Stebbins [1953]
), resulted in 16 minimum length trees, each with a length of 658 steps, a consistency index (CI) of 0.74 (excluding uninformative characters), and a retention index (RI) of 0.83. The strict consensus of the 16 trees is shown in Fig. 2.
|
Stephanomeria blairii) and Pleiacanthus spinosus ( S. spinosa) have been included within Stephanomeria in all recent treatments of the genus. The inclusion of Prenanthella in the clade is unexpected. Rafinesquia, with two species, has been taxonomically stable and sequence evidence is consistent with this point of view. An ETS deletion of 1 bp (at site 456 in the ETS/ITS data matrix) reinforces evidence from nucleotide substitutions for monophyly of Rafinesquia.
|
The following three groups within the latter clade are convincing: (1) four perennials, mostly distributed outside the California Floristic Province—the recently described species S. fluminea, S. runcinata, the very widespread species S. tenuifolia, and the southwestern species S. thurberi (also supported by a 1-bp ITS-2 deletion at site 953 in the ETS/ITS data matrix); (2) two subspecies of S. exigua (subsp. deanei and subsp. exigua) and S. diegensis, the latter previously thought to have originated from hybridization between S. exigua and S. virgata (Gallez and Gottlieb, 1982
); and (3) S. malheurensis and its recent progenitor S. exigua subsp. coronaria (Gottlieb, 1973, 1978
). Relationships among the other annual taxa and the perennial S. pauciflora remain unresolved.
Pairwise divergence in Stephanomeria
The calculated pairwise divergence values between all pairs of taxa provide estimates of the relative differences among the annual taxa, among the perennial taxa, and between these two groups (Table 1). The annual taxa showed an average pairwise divergence of 11.0 sites, varying from 2.5 to 19.8. All the perennials had an average pairwise divergence of 30.7, varying from 1.0 to 49.3. Because the phylogenetic analysis showed that S. lactucina and S. cichoriacea belong to a different clade than the seven other perennial species, values for them were also calculated separately; they differed from their congeners at 44.7 sites, varying from 33.7 to 51.7. Five of the perennial species are in the same clade as the annuals. The average divergence among these perennials was 14.2, varying from 1.0 to 20.5, quite comparable to that among the annuals. Between the annuals and all perennials, the average difference was 25.2 sites.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
S. blairii) or Pleiacanthus spinosus ( S. spinosa) and that without these two taxa, Stephanomeria is a well supported (100% bootstrap; Fig. 3), monophyletic group of ten perennial and six annual species (counting S. elata and S. parryi, the two tetraploids not sampled here).
Munzothamnus blairii appears to belong in a monotypic genus. The taxon, endemic to San Clemente Island, Los Angeles County, California, has had various taxonomic homes. It was initially described as Stephanomeria blairii by Munz and Johnston (1924)
and later transferred by them to Malacothrix (Munz, 1935
). Raven (1963)
considered it "clearly a relictual and highly isolated genus" primarily because of its leaf shape and vegetative architecture and erected Munzothamnus for it. Stebbins, Jenkins, and Walters (1953)
emphasized the similar number and appearance of the chromosomes of M. blairii to those of Stephanomeria, as well as certain similarities between their pappi and concluded the taxon belonged in Stephanomeria. Tomb (1974)
agreed and added that pollen size and sculpturing were similar to that of Stephanomeria.
Carlquist (1974)
nicely described the mode of branch growth in Munzothamnus blairii. At the end of the first year, basal lateral buds on the main stem produce side shoots. At the end of the second year, those shoots produce terminal clusters of heads. At the end of the third year, the same side shoots produce new lateral shoots from distal nodes below the heads. Thus, the plants produce shoots from lateral buds both at the base of the stem and from nodes on side shoots below the terminal clusters. This architectural pattern is not found in Stephanomeria.
Pleiacanthus spinosus also now appears to belong in a monotypic genus. The taxon is widespread in open, sandy or gravelly washes and on slopes in desert shrub and pinyon–juniper communities in the deserts of Arizona and California and north to Oregon and Montana. It was first collected by Nuttall, who placed it in a subgenus of Lygodesmia D. Don that he erected for the purpose. Rydberg (1917)
elevated it to genus status, but his proposal was not taken up by others. The taxon remained in Lygodesmia until Tomb (1970)
transferred it to Stephanomeria. Tomb made the transfer because P. spinosus has the same chromosome number as Stephanomeria as well as similar echinate pollen grains and thus differs from Lygodesmia, which has n = 9 and echinolophate pollen. Our larger study of all genera usually associated with the Stephanomeriinae (to be reported separately) showed that Lygodesmia belongs neither to that subtribe or to Microseridinae or Malacothricinae. Pleiacanthus spinosus has several interesting unique traits: dense, long tufts of woolly hairs in the ground-level axils of bud scales of the stems, sharp-tipped branches and stems, and nonplumose pappus bristles of two lengths.
The sister relationship between Pleiacanthus spinosus and Prenanthella exigua revealed in this study (Fig. 3) harks back to the point of view of Gray (1874)
, who considered both taxa as congeners within Lygodesmia. Rydberg (1906, 1917)
removed Prenanthella exigua from Lygodesmia, but his ideas were ignored. Tomb (1972)
reinstated Prenanthella because its chromosome number (n = 7) differs from that of Lygodesmia and because its echinate pollen also distinguishes it from the latter genus. Pleiacanthus and Prenanthella differ in many respects including chromosome number and habit; both have nonplumose pappus bristles of two lengths, highly reduced heads with only 3–5 florets, and bractlike cauline leaves. The chromosome number of Prenanthella appears to have been derived independently from the same number found elsewhere among the western North American Lactuceae (i.e., Anisocoma Torr. & A. Gray, Calycoseris A. Gray, and Malacothrix).
Within Stephanomeria, the sister relationship between S. lactucina and S. cichoriacea was not foreseen (Fig. 3). The two species are found in very different habitats (montane vs. coastal) and differ in a large number of morphological traits. The number of site differences between them in this study was larger (31.7) than nearly all values found between other pairs of perennial taxa within the genus and suggests their relationship may indeed be ancient. The matter needs further study.
The two taxa from Baja California, S. monocephala and S. guadalupensis, are another example of extreme morphological and ecological divergence in Stephanomeria. Stephanomeria monocephala is known only from five sheltered sites in rocky, high montane habitats of the Sierra de San Pedro Mártir (Moran, 1969
). Unlike any other stephanomeria, it is caespitose and grows as a dense cushion with solitary heads borne on short peduncles. Stephanomeria guadalupensis is endemic to Isla Guadalupe, where it persists on nearly vertical cliffs. The species grows as clumps, 3–5 dm high (up to 15 dm high at flowering), and has the largest leaves of any stephanomeria (up to 30 x 9 cm).
The four perennial species from mostly outside the California Floristic Province form a well-supported subclade within the larger clade that includes S. pauciflora and all the annual species (Fig. 3). Stephanomeria tenuifolia is the most widespread species of the genus, distributed over 200 x 106 ha in interior, western North America, and is found in crevices below cliffs derived from volcanic, granitic, and other rocks in montane regions. The species shows remarkable variability in the form and dimensions of its stems and branches, varying from plants with numerous, slender, almost threadlike stems to stems that are sparingly branched and flexuose. Its architectural variability has perplexed many taxonomists and the species has collected a number of names, all now considered synonyms (L. D. Gottlieb, unpublished manuscript). In the present study, we sampled three populations: Park County, Montana, USA; Kittitas County, Washington, USA; and Plumas County, California, USA. The number of pairwise differences among them (10 or 12 sites) is about the same as that between an average pair of annual species. In reproductive features, S. tenuifolia closely resembles S. runcinata and S. fluminea from the same subclade (for example, the three species are distinctive in the genus in having heads with few florets and white pappus bristles that are plumose along their entire lengths). But the two close relatives of S. tenuifolia are ecologically specialized. Stephanomeria runcinata is frequently found on eroded siltstones, clay flats, and alkali soils and has a wide distribution in the Rocky Mountains and east and north to Saskatchewan, Nebraska, and North Dakota. Stephanomeria fluminea grows only on raised cobble benches in a few creek beds in northwestern Wyoming; morphologically, the species is quite unusual in the genus in having long cauline leaves (up to 6 cm) that persist and remain green at flowering (Gottlieb, 1999
). Stephanomeria runcinata and S. fluminea differ at only a single site, suggesting the latter species evolved quite recently. Indeed, its habitat was covered by the massive Pinedale glacier as recently as 18 000–20 000 yr ago. Stephanomeria thurberi is found principally in Arizona, New Mexico, and adjacent Mexico, growing in sandy sites in juniper–mesquite grasslands and also in yellow pine forests. The relationship of S. pauciflora, a species of the warm deserts, to the four species with more northern distributions is not resolved here. On the basis of pairwise site differences, S. pauciflora is about equally different from all of them (Table 1). All of the annual stephanomerias are found in the same clade as these five perennials. The short branch lengths of the annual lineages, which are similar to those of the four perennials (the ones in the group including S. tenuifolia), suggest all these species diverged much more recently than did the perennial species outside the clade.
Although the present sequence data do not resolve relationships among the annual taxa, the reproductive compatibilities of the annuals, which are expected to be closely correlated with these relationships, are very well known as a result of previous intensive biosystematic studies (Gottlieb, 1971
). Those studies sorted out the annual taxa by combining morphological analyses of numerous populations from throughout their distributions with studies of their breeding systems, cytological features, crossability, hybrid fertility, and chromosome homology as revealed by cytogenetic analysis of experimentally produced F1 hybrids.
Stephanomeria virgata contains two subspecies that share a number of morphological features, have a similar karyotype, and are highly compatible reproductively. The F1 hybrid progenies were 78–90% fertile, based on pollen stainability. Stephanomeria exigua is more complex and includes geographically separable subspecies of varying degrees of morphological similarity. Their F1 hybrid progenies had mean pollen stainabilities ranging from 16 to 81%, with an overall mean of 49%. Pollen stainability of F1 hybrids between the two species was significantly reduced, ranging from 8 to 21%, with an overall mean of 14% (based on 20 different progenies). Reductions in pollen stainability were closely correlated with differences in chromosome structural homology.
Stephanomeria virgata and S. exigua differ in numerous morphological traits and in their karyotype. The traits of S. diegensis are an amalgam of their differences, which suggested an hypothesis of hybridity to explain its origin. Stephanomeria diegensis is abundant and usually encountered in pioneer habits such as old clearings, chaparral openings, and sandy roadside embankments in southern California, USA. Experimental hybrids between S. diegensis and both S. virgata and S. exigua, involving both subspecies of the former and three of the latter, showed only 1–2% viable pollen, probably resulting from additional chromosomal repatterning (Gottlieb, 1971
). Electrophoretic differences in isozymes encoded by 20 loci were examined in the three species (Gallez and Gottlieb, 1982
). Stephanomeria virgata and S. exigua showed a relatively high genetic identity (I = 0.81) and had different alleles at 20% of the genes examined. The gene pool of S. diegensis proved to be a composite of the genes of the two species, with only one, very rare, unique allele. Thus, the electrophoretic results were concordant with the evidence from morphology, karyotype, and other attributes, together suggesting that S. diegensis arose following the stabilization of hybrid segregants from a natural cross between the two divergent diploid relatives.
The present data, including ten sequences from the five subspecies of S. exigua, four sequences from the two subspecies of S. virgata, and two fully identical sequences from S. diegensis, bear on this issue. The pairwise differences between each of the sequences and that of S. diegensis (Table 1) make evident that S. exigua subsp. deanei is most similar to S. diegensis, differing at only two or three sites. The next most similar sequence to that of S. diegensis is that of S. exigua subsp. exigua, which differs at 8 sites; the other three subspecies of S. exigua differ at 9–18 sites. The sequence of S. diegensis also differs from those of the two subspecies of S. virgata at 9 and 14 sites, respectively. Consequently, on this evidence, S. exigua subsp. deanei is the most likely candidate to have contributed the sequence of S. diegensis and to be a parent. The bootstrap tree (Fig. 3) strongly supports this relationship of S. diegensis to either S. exigua subsp. deanei or subsp. exigua.
If S. diegensis had a hybrid origin as described, there are three possible expectations for any gene locus: S. diegensis might have a sequence like one of the parents or, if the locus was at a different chromosomal position in the two parents, the sequences of both parents might be found, or the sequence might show some recombination of the sequences of the two parents. The sequence found in S. diegensis is unlikely to have resulted from recombination between sequences derived from S. exigua subsp. deanei and S. virgata, although at the two sites that S. diegensis differs from S. exigua subsp. deanei, it does resemble S. virgata. The two sites are 30 bases apart within the ETS region and are flanked by sites fewer than 200 bases apart at which S. diegensis differs from both subspecies of S. virgata. Thus, the required recombination would involve an improbable double crossover in a very small region. It is much more likely that the difference between S. diegensis and S. exigua subsp. deanei at the two sites in question arose by base substitutions, especially considering that S. diegensis resembles other subspecies of S. exigua at these positions. In conclusion, the ITS/ETS sequence data suggest the source of one parent of S. diegensis, but offer no information about a second parent.
The sequence data provide significant information that corroborates a previous hypothesis regarding the parentage of S. malheurensis (Gottlieb, 1973, 1977, 1979
; Brauner and Gottlieb, 1989
). This species is known only from a single locality in Harney County, Oregon, USA, where it grows on an island of soil derived from volcanic tuff. Stephanomeria exigua subsp. coronaria also grows at this site, but the locality is at the northern limit of its distribution. The two species have different breeding systems; S. malheurensis is predominantly self-pollinating and S. exigua subsp. coronaria is obligately outcrossing. The two species are very similar in morphological attributes. Electrophoretic analyses of their isozymes suggested that the genome of S. malheurensis was a subset of its sympatric congener. The two species are reproductively isolated by barriers that reduce their crossability and reduce the fertility of any hybrids between them. The early evidence suggested that S. malheurensis arose directly from the sympatric population of S. exigua subsp. coronaria following a mutation that modified its outcrossing breeding system and led to a rapid and abrupt series of events in a descendant lineage, reducing its variability as homozygosity built up. Such a mutation in the sympatric population of the proposed parent was eventually discovered (Brauner and Gottlieb, 1987
). The bootstrap tree (Fig. 3) shows the two taxa as components of a single subclade, with 96% bootstrap support. Four sequences of S. exigua subsp. coronaria were determined; they differ from the sequence of S. malheurensis at one to three sites, depending on the comparison, with an average pairwise difference of 1.8, a much smaller difference than between S. malheurensis and any other taxon (Table 1).
In summary, the sequence analysis of the ITS/ETS rDNA provides a valuable picture of the relationships of Stephanomeria to other genera as well as of many of its constituent species. Additional analysis of genes encoding proteins will hopefully further clarify the relationships among the annual members and reveal how they relate to the perennials.
|
|
FOOTNOTES |
|---|
4 Author for reprint requests (bbaldwin{at}uclink4.berkeley.edu
).
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Baldwin B. G. S. Markos 1998 Phylogenetic utility of the external transcribed spacers (ETS) of 18S–26S rDNA: congruence of ETS and ITS trees of Calycadenia (Compositae). Molecular Phylogenetics and Evolution 10: 449-463[CrossRef][Web of Science][Medline]
Baldwin B. G. M. J. Sanderson J. M. Porter M. F. Wojciechowski C. S. Campbell J. Donoghue 1995 The ITS region of nuclear ribosomal DNA: a valuable source of evidence on angiosperm phylogeny. Annals of the Missouri Botanical Garden 82: 247-277[CrossRef][Web of Science]
Baldwin B. G. B. L. Wessa 2000 Origin and relationships of the tarweed–silversword lineage (Compositae–Madiinae). American Journal of Botany 87: 1890-1908
Brauner S. L. D. Gottlieb 1987 A self-compatible plant of Stephanomeria exigua subsp. coronaria (Asteraceae) and its relevance to the origin of its self-pollinating derivative Stephanomeria malheurensis. Systematic Botany 12: 299-304[CrossRef][Web of Science]
Brauner S. L. D. Gottlieb 1989 Response to selection for time of bolting in Stephanomeria exigua subsp. coronaria and implications for the origin of S. malheurensis (Asteraceae). Systematic Botany 14: 516-524[CrossRef][Web of Science]
Carlquist S. 1974 Island biology. Columbia University Press, New York, New York, USA
Cronquist A. 1994 Intermountain flora: vascular plants of the Intermountain West, U.S.A., vol. 5. Asterales. New York Botanical Garden, Bronx, New York, USA
Farris J. S. M. Källersjø A. G. Kluge C. Bult 1995 Constructing a significance test for incongruence. Systematic Biology 44: 570-572[CrossRef]
Ferris R. S. 1960 Illustrated flora of the Pacific states, vol. IV, Bignoniaceae to Compositae. Stanford University Press, Stanford, California, USA
Gallez G. P. L. D. Gottlieb 1982 Genetic evidence for the hybrid origin of the diploid plant Stephanomeria diegensis. Evolution 36: 1158-1167[CrossRef][Web of Science]
Gottlieb L. D. 1969 The role of hybridization in the evolution of the annual species of Stephanomeria (Compositae). Ph.D. dissertation. University of Michigan, Ann Arbor, Michigan, USA
Gottlieb L. D. 1971 Evolutionary relationships in the outcrossing diploid species of Stephanomeria (Compositae). Evolution 25: 312-329[CrossRef][Web of Science]
Gottlieb L. D. 1972 A proposal for classification of the annual species of Stephanomeria (Compositae). Madroño 21: 463-481
Gottlieb L. D. 1973 Genetic differentiation, sympatric speciation and the origin of a diploid species of Stephanomeria. American Journal of Botany 60: 545-553[CrossRef][Web of Science]
Gottlieb L. D. 1977 Phenotypic similarity of Stephanomeria exigua subsp. coronaria and its recent derivative "Malheurensis.". American Journal of Botany 64: 873-880[CrossRef][Web of Science]
Gottlieb L. D. 1978 Stephanomeria malheurensis (Compositae), a new species from Oregon. Madroño 25: 44-46
Gottlieb L. D. 1979 The origin of phenotype in a recently evolved species. In O. T. Solbrig, S. Jain, G. B. Johnson, and P. H. Raven [eds.], Topics in plant population biology, 264–286. Columbia University Press, New York, New York, USA
Gottlieb L. D. 1999 A new species of Stephanomeria (Asteraceae) from northwestern Wyoming. Madroño 46: 58-60
Gray A. 1874 Notes on Compositae and characters of certain genera and species, etc. Proceedings of the American Academy of Arts and Sciences 9: 216-217
Hershkovitz M. A. L. A. Lewis 1996 Deep-level diagnostic value of the rDNA-ITS region. Molecular Biology and Evolution 13: 1276-1295[Abstract]
Hickman J. [ed.]. 1993 The Jepson manual: higher plants of California. University of California Press, Berkeley, California, USA
Moran R. 1969 Twelve new dicots from Baja California, Mexico. Transactions of the San Diego Society of Natural History 15: 291
Munz P. A. 1935 A manual of southern California botany. Claremont Colleges, Claremont, California, USA
Munz P. A. I. M. Johnston 1924 Miscellaneous notes on plants of southern California, III. Bulletin of the Torrey Botanical Club 51: 301-302
Raven P. H. 1963 A flora of San Clemente Island, California. Aliso 5: 345
Richard E. M. Reichardt S. Rogers 1994 Preparation of genomic DNA from plant tissue. In F. M. Ausubel [ed.], Current protocols in molecular biology, vol. 1, 2.3.1.–2.3.7. John Wiley, New York, New York, USA
Rydberg P. A. 1906 Studies on the Rocky Mountain flora, XVI. Bulletin of the Torrey Botanical Club 33: 161
Rydberg P. A. 1917 Flora of the Rocky Mountains and adjacent plains. Steinman & Foltz, Lancaster, Pennsylvania, USA
Solbrig O. T. 1963 Subfamilial nomenclature of Compositae. Taxon 12: 229-235[CrossRef]
Stebbins G. L. 1953 A new classification of the tribe Cichorieae, family Compositae. Madroño 12: 65-81
Stebbins G. L. J. Jenkins M. Walters 1953 Chromosomes and phylogeny in the Compositae, tribe Cichorieae. University of California Publications in Botany 26: 401-430
Swofford D. L. 1998 PAUP*: phylogenetic analysis using parsimony (* and other methods), version 4. Sinauer, Sunderland, Massachusetts, USA
Thompson J. D. T. J. Gibson F. Plewniak F. Jeanmougin D. G. Higgins 1997 The Clustal-X Windows interface: flexible strategies for multiple sequence alignment aided by quality analysis tools. Nucleic Acids Research 24: 4876-4882
Tomb A. S. 1970 Novelties in Lygodesmia and Stephanomeria (Compositae: Cichorieae). Sida 3: 530-532
Tomb A. S. 1972 Re-establishment of the genus Prenanthella Rydb. (Compositae: Cichorieae). Madroño 24: 223-228
Tomb A. S. 1974 Chromosome numbers and generic relationships in subtribe Stephanomeriinae (Compositae: Cichorieae). Brittonia 26: 203-216[CrossRef][Web of Science]
Urbatsch L. E. B. G. Baldwin M. J. Donoghue 2000 Phylogeny of the coneflowers and relatives (Heliantheae: Asteraceae) based on nuclear rDNA internal transcribed spacer (ITS) sequences and chloroplast DNA restriction site data. Systematic Botany 25: 539-565[CrossRef][Web of Science]
White T. J. T. Bruns S. Lee J. Taylor 1990 Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. In M. Innis, D. Gelfand, J. Sninsky, and T. White [eds.], PCR protocols: a guide to methods and applications, 315–322. Academic Press, San Diego, California, USA
This article has been cited by other articles:
|
|
 |
|
  V. S Ford, J. Lee, B. G Baldwin, and L. D Gottlieb Species divergence and relationships in Stephanomeria (Compositae): PgiC phylogeny compared to prior biosystematic studies Am. J. Botany, March 1, 2006; 93(3): 480 - 490. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Andreasen and B. G. Baldwin Reexamination of relationships, habital evolution, and phylogeography of checker mallows (Sidalcea; Malvaceae) based on molecular phylogenetic data Am. J. Botany, March 1, 2003; 90(3): 436 - 444. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
