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Systematics and Phytogeography |
2Landcare Research, P.O. Box 69, Lincoln, New Zealand 8152; 3Herbarium, Royal Botanic Gardens Kew, Richmond, Surrey TW9 3AB, UK
Received for publication August 2, 2001. Accepted for publication December 11, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: diversification • DNA • ITS • New Zealand • phylogeny • rbcL • Stylidiaceae
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Pole, 1994
). Dispersal across an oceanic barrier is an infrequent event that geographically isolates new founders from their mainland progenitors. Once established the new immigrants must overcome the deleterious effects of an initial genetic bottleneck. With the variability of their mainland relatives no longer accessible, random mutation and recombination are the only sources of new variation to rebuild a depauperate gene pool. According to Lloyd (1985)
, plants with contrasting traits need not be equally successful in colonizing islands. Many of the unique features of the New Zealand flora probably reflect the nonrandom success of different individuals in reaching, becoming established, and diversifying there. The trigger plant family (Stylidiaceae) is comprised of five genera and over 240 species that are native to Australia, Southeast Asia, New Zealand, and South America (Fig. 1). The subalpine and alpine zones in New Zealand are a center of diversity for Phyllachne and Forstera, two of the five genera that comprise the family. Seven of the nine species belonging to these genera are found there, with all but P. colensoi endemic. The monotypic genus Oreostylidium is also restricted to montane and subalpine zones of New Zealand. These three genera, however, comprise only 4% of the total number of species in the family; the majority (92%) are placed in the genus Stylidium, while the remainder belong to Levenhookia (Table 1). The southwest of Western Australia is a center of diversity for both Stylidium and Levenhookia.
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). In Stylidium (the trigger plants), the column is held under tension and "triggers" in response to an insect probing for nectar, subsequently depositing pollen on the insect before gradually resetting. In order to accommodate this movement the labellum is reflexed and reduced in size. Trigger plant flowers are morphologically diverse, with variation in the size and shape of flowers, presence and form of corolla appendages, inflorescence structure and corolla color combining to produce an array of flowers of contrasting appearance. In contrast to Stylidium, and to a lesser degree Levenhookia, Phyllachne, Forstera and Oreostylidium possess simple, white, actinomorphic flowers with immobile columns.
The debate surrounding the origins of the subalpine and alpine flora of New Zealand has centered on the relative importance of long-distance dispersal. While explaining the distribution of some elements of the flora in terms of dispersal, Wardle (1968, 1978)
proposed that New Zealand's subalpine and alpine flora arose largely through diversification during the late Tertiary. Wardle (1968)
suggested that Phyllachne, Forstera, Oreostylidium, and Donatia (the hypothesized sister group to Stylidiaceae) are examples of seemingly ancient genera that existed in Antarctica and dispersed to New Zealand via Tasmania and the subantarctic islands. The ancestors of these groups were suggested to have survived in peneplained uplands with soils too leached to support continuous forest, before evolving into the subalpine and alpine habitats first created in the late Pliocene. In contrast, Raven (1973)
argued that much of the New Zealand flora arrived via Australia. The collision of the Australian plate with the Asian plate resulted in the uplift of mountains in Malaysia, Australia, and New Zealand during the late Pliocene and Pleistocene. According to Raven, the uplift of the mountain ranges created suitable habitats for the migration of subalpine and alpine plants between Asia and Australia. In view of their obscure phyletic relationships, Raven also considered Phyllachne, Forstera, and Donatia to be examples of genera of great antiquity. Recent molecular studies on the New Zealand subalpine and alpine flora confirm that intensified speciation occurred during times of marked climatic and geologic perturbation during the Pliocene (Winkworth et al., 1999
). Our aim was to reconstruct phylogenetic relationships among the New Zealand lineages of Stylidiaceae, to estimate the timing of diversification, and to explore ecological and adaptive morphological features that could account for the differences in species richness.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). However, on the basis of a number of morphological, anatomical, and embryological features (including the absence of the floral column) the Donatiaceae are now considered a separate family (Rapson, 1953
; Philipson and Philipson, 1973
; Gustafsson and Bremer, 1995
) and sister to Stylidiaceae (Gustafsson and Bremer, 1995
; Laurent, Bremer, and Bremer, 1999
). Voucher information and GenBank accession numbers are listed in the Appendix stored at (http://ajbsupp.botany.org/v89/wagstaff.pdf). The complete data sets are available upon request from the first author and were deposited in TreeBASE (http://www.herbaria.harvard.edu/treebase).
Molecular data
Our molecular sampling strategy capitalized on the unique characteristics of rbcL and internal transcribed spacer (ITS) sequences. The gene rbcL is evolving at a relatively slow rate allowing sequence comparisons among distantly related outgroups, and a large number of published sequences are available for comparison (see Chase et al., 1993
; Källersjö et al., 1998
; and references therein). Finally, Albert et al. (1994)
and Bremer and Gustafsson (1997)
suggest that rbcL approaches clock-like behavior in its evolution, so that the amount of sequence divergence may be used to estimate the timing of evolution. By comparison, the ITS region is evolving more rapidly than rbcL and provides more informative characters to resolve relationships at lower taxonomic levels (Baldwin et al., 1995
).
Total DNA was extracted from either fresh leaves or leaf fragments dried with silica gel using a modification of the hot CTAB (hexadecyltrimethyl-ammonium bromide) method of Doyle and Doyle (1987)
. The gene rbcL and the ITS region (the 3' end of the 18S rDNA gene; ITS-1; the 5.8S rDNA gene; ITS-2; and the 5' end of the 28S rDNA gene) were amplified by polymerase chain reaction (PCR). Primer sequences and our amplification and sequencing techniques follow Olmstead et al. (1992)
for rbcL and Wagstaff and Garnock-Jones (1998)
for the ITS region. Excess primers and unincorporated nucleotides were removed from the PCR products by spin column centrifugation (QIAquick PCR purification kit; QIAGEN, Clifton Hill, Victoria, Australia). The purified DNA samples were then labeled with Big Dye terminators (PE Applied Biosystems, Perkin-Elmer, Sydney, New South Wales, Australia). After this, unincorporated dye terminators were removed by alcohol precipitation in the presence of 3.0 mol/L sodium acetate pH 5.2. The sequencing was performed at the Waikato University DNA Sequencing Facility. Both the forward and reverse DNA strands were sequenced to minimize errors and to confirm our results.
The sequence alignment for the ITS region was facilitated by ClustalX (Thompson et al., 1997
), and gaps were inserted to ensure positional homology. A gap penalty setting of 75 and a gap extension penalty of 6.6 were initially used to identify and position large gaps in the sequence data. Then low-scoring segments were realigned using a gap penalty setting of 15 and a gap extension penalty of 6.6 with the removing new gaps option turned on. These settings opened and positioned small gaps.
Data analysis
The phylogenetic analyses were accomplished using PAUP* version 4.0d65 (Swofford, 1998
), and MacClade version 3.04 (Maddison and Maddison, 1992
) was used to explore character evolution. The analyses were conducted using the PAUP* settings random addition with ten replicates, tree-bisection-reconnection (TBR) branch swapping, MULPARS in effect, and steepest descent. The characters were all unordered and weighted equally, and gaps were treated as missing data. The rbcL and ITS data sets were analyzed separately to explore possible conflict. In the absence of conflict, the data sets were then combined. The topological constraint option in PAUP* and the branch moving option in MacClade were used to assess the degree of congruence among competing hypotheses of relationships.
Support for the inferred clades was estimated by jackknife (Farris et al., 1996
) and bootstrap (Felsenstein, 1985
) analyses. The searches were performed with 1000 replications excluding uninformative sites; the starting trees were obtained by random addition with one replication for each replication, TBR branch swapping, and MULPARS in effect.
The relationship between sequence divergence and time for the gene rbcL has been discussed by Albert et al. (1994)
and Bremer and Gustafsson (1997)
and was calculated using the equation: substitution rate = patristic distance (Dp)/number of nucleotides/inferred time since cladogenesis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The ITS sequences were 782 nucleotides long with gaps inserted to assure positional homology. Missing data and gaps accounted for 16.4% of the matrix (0.5 and 15.8%, respectively). The ITS-1 spacer varied in length from 164 nucleotides in Stylidium calcaratum to 251 in Donatia, whereas the ITS-2 spacer varied from 209 in Levenhookia leptantha to 281 in S. emarginatum. Of the 782 total sites in the ITS matrix, 458 sites were constant, 85 variable sites were parsimony-uninformative, and 239 sites were parsimony-informative characters.
A heuristic search of the ITS sequences similarly recovered only one maximum parsimony tree in a single island of 670 steps (consistency index = 0.685; retention index = 0.809; rescaled consistency index = 0.554; excluding uninformative characters) (Figs. 2, 4). The slightly lower consistency, retention, and rescaled consistency indices indicate more homoplasy in the ITS sequence data. The ITS tree also branches symmetrically with two notable clades being resolved, the first clade consisting of Levenhookia, Oreostylidium, and Stylidium (bootstrap 97%; jackknife 100%) and the second clade consisting of Forstera and Phyllachne (bootstrap 83%; jackknife 91%). As in the rbcL tree S. calcaratum emerges at the base of the first clade, whereas Levenhookia is now nested within the first clade emerging as sister to S. emarginatum (bootstrap 51%; jackknife 71%). Oreostylidium again is nested well within the first clade emerging as sister to S. graminifolium (bootstrap 99%; jackknife 100%). The topology of the second clade is also similar to that obtained by rbcL with F. bellidifolia again diverging at the base of the second clade (bootstrap 100%; jackknife 100%) and with neither Phyllachne nor Forstera being monophyletic. However, the New Zealand species form two uniform groups that are well supported.
Absolute genetic distance values indicated that the gene rbcL evolves at a slower rate than the ITS region; the mean (±1 SD) number of nucleotide substitutions in rbcL that distinguish Donatia novae-zelandiae from the Stylidiaceae terminals was 57.9 ± 14.26, whereas the mean number of substitutions in ITS was 163.3 ± 18.9, which is nearly a threefold difference in the substitution rate. The rbcL data also exhibited substantially more rate variation across lineages than the ITS data (Figs. 3, 4); the standard deviation around the mean was proportionally higher in the rbcL data. The rate difference between the Forstera/Phyllachne and the Stylidium/Oreostylidium clades was particularly striking; however, the sequences within each of these clades appeared to be evolving in a relatively clock-like manner.
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Because the results from the rbcL and ITS sequences were largely congruent, the two data sets were combined. The combined analysis also recovered one maximum parsimony tree of 943 steps (consistency index = 0.688; retention index = 0.812; rescaled consistency index = 0.559; excluding uninformative characters) (Fig. 5) that shared aspects of the rbcL and ITS trees, but the relationships were more highly resolved and better supported. The tree again branched symmetrically with the same two clades resolved; the first clade comprised Levenhookia, Oreostylidium, and Stylidium (bootstrap 86%; jackknife 95%) and the second was composed of Forstera and Phyllachne (bootstrap 98%; jackknife 100%). As in the rbcL tree, L. leptantha diverged at the base of the first clade (bootstrap 69%; jackknife 84%). The two accessions of O. subulatum form a well-supported clade (bootstrap 99%; jackknife 100%) nested well within the first clade with S. graminifolium well supported as their sister (bootstrap 100%; jackknife 100%). The branching order of the second clade was identical to that obtained from the ITS sequences. Forstera bellidifolia diverged at the base of the second clade (bootstrap 100%; jackknife 100%). As in the previous analyses, neither Forstera nor Phyllachne is monophyletic, but two smaller clades are monophyletic. The first of these consists of P. rubra, P. colensoi, and P. clavigera (bootstrap 86%; jackknife 95%) and the second of F. sedifolia, F. tenella, and F. bidwillii (bootstrap 58%; jackknife 74%).
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) with the following parameters estimated from the combined parsimony tree shown in Fig. 5: (1) the assumed nucleotide frequencies were A = 0.25459, C = 0.23492, G = 0.26019, and T = 0.25030; (2) the estimated proportion of invariable sites = 0.478127 (observed proportion of constant sites = 0.761447); and the estimated value of the gamma shape parameter = 0.541289. The maximum likelihood analysis of the combined data yielded a single tree (–log = 7789.31019) (Fig. 6). The branching order was identical to the combined parsimony analysis, but the branch lengths differed. The maximum likelihood result may provide a more realistic estimate of divergence because corrections are made for multiple substitutions along long branches. These observations also indicate that our phylogenetic inferences are relatively robust to the different assumptions of these two approaches to phylogeny reconstruction.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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the evolution of floral morphology, breeding systems, ecological preferences, growth forms, and dispersal mechanisms are all closely interrelated to enhance reproductive success in island plants, and we discuss some of these interrelationships below.
Our results suggest that the New Zealand Stylidiaceae fall into two distinct lineages that differ substantially from one another in species richness (Figs. 2–6), a result consistent with that obtained by Laurent, Bremer, and Bremer (1999)
. One lineage is composed of seven species placed in Forstera and Phyllachne, while the other New Zealand group consists solely of Oreostylidium subulatum, which is nested within a clade consisting of Levenhookia and Stylidium. The occurrence of curved, monothecous anthers in both Phyllachne and Forstera, as opposed to the dithecous anthers found in Levenhookia, Stylidium, and Oreostylidium, lends additional support to this result. This feature was used by Mildbraed (1908)
to separate the two groups into the tribes Phyllachneae and Stylidieae.
Origin and dispersal
Colonization of New Zealand by members of the Stylidiaceae involved at least two instances of long-distance dispersal. The distribution of Donatia, the outgroup in our study, is very similar to that of Phyllachne. Donatia fascicularis is found in southern South America to latitude 40°S, and D. novae-zelandiae is found in both New Zealand and Tasmania. The Tasmanian species F. bellidifolia diverges at the base of the Forstera/Phyllachne clade and the South American species P. uliginosa is sister to the New Zealand species, hence the origin of the New Zealand clade is equivocal; with a South American or Tasmanian origin being equally parsimonious. Conspecific populations of P. colensoi in New Zealand and Tasmania probably reflect more recent dispersal from New Zealand to Tasmania. The New Zealand ancestor of O. subulatum arrived by long-distance dispersal from Australia. Oreostylidium subulatum is nested in the largely Australian genus Stylidium; S. graminifolium, its sister in our analysis, is widely distributed in eastern Australia and Tasmania.
The present distribution of Stylidiaceae suggests that Antarctica may have played an important role as a corridor for the migration of Stylidiaceae between Australia, New Zealand, and South America during the Tertiary (Fig. 1). Isolation of Antarctica appears to have been initiated south of Tasmania at the boundary between the Eocene and Oligocene some 35 million years ago, separating Antarctica from the Australian areas (Coleman, 1980
; Johnson and Veevers, 1984
; Veevers, Powell, and Roots, 1991
). New Zealand at this time had already been isolated for more than 45 million years, but it is possible that the ancestors of New Zealand Stylidiaceae reached New Zealand across now submerged islands and/or now uninhabitable areas along the Antarctic coast long after the isolation of New Zealand. Up to the Oligocene–Miocene boundary 23 million years ago, the Antarctic Peninsula and southern South America were linked by a land bridge, the Scotia Arc, which finally broke open to form the Drake Passage and establish the circum-Antarctic current (Barker and Burrell, 1977
; Coleman, 1980
; Dalziel, 1983
). There is abundant fossil evidence that suggests the Transantarctic Mountains were clothed in beech forests until as recently as the Pliocene (Webb and Harwood, 1993
).
Divergence estimates based upon the gene rbcL (Fig. 3) suggest that dispersal to New Zealand occurred at two distinct times during the late Tertiary. The mean (±1 SD) number of nucleotide substitutions that distinguish Phyllachne uliginosa from the Forstera/Phyllachne terminals is 8.14 ± 0.89, whereas the accessions of Oreostylidium subulatum are distinguished from Stylidium graminifolium by 4.00 ± 1.41 substitutions. Based upon the substitution rate of 0.74 substitutions per million years calculated for the gene rbcL by Bremer and Gustafsson (1997)
, the Forstera/Phyllachne lineage in New Zealand shared a common ancestor with the South American P. uliginosa about 6 million years ago and O. subulatum shared a common ancestor with the Australian S. graminifolium about 3 million years ago.
These divergence times are consistent with the substitution rate estimated from the fossil record (Fig. 6). According to Macphail (1997)
Forstera-type pollen (Tricolpites stylidioides) first appears in southeastern Australia possibly as early as the Oligocene some 39 million years ago, whereas Stylidiaceae pollen is known only from the Quaternary in New Zealand (Mildenhall, 1980
). The mean distance from the terminals to the base of the Forstera/Phyllachne clade is 0.074 ± 0.005 substitions per site, and dividing this by 39 million years yields a substitution rate of approximately 0.0019 nucleotide substitutions per site per million years (Fig. 6). The mean number of substitutions per site from the New Zealand Forstera/Phyllachne terminals to the base of the New Zealand clade is 0.012 ± 0.002, which also equates the first appearance in New Zealand to about 6 million years ago (0.012 nucleotide substitutions per site/0.0019 nucleotide substitutions per site per million years). The mean number of substitutions per site from the Oreostylidium terminals to the base of the Oreostylidium clade is 0.0035, which equates to about 2 million years (0.0035/0.0019), a first appearance in New Zealand that is somewhat more recent than the estimate obtained from rbcL sequences alone, but is more consistent with the Quaternary fossil record. The difference may reflect a more realistic estimate of branch lengths obtained by maximum likelihood analysis.
The estimated divergences times (Figs. 3, 6) suggest diversification of Stylidiaceae was likely influenced by major geological and climatical changes and fluctuations in sea level that occurred during the late Tertiary and Quaternary. Uplift of the mountains in Australia and New Zealand to their present elevation and subsequent episodes of glaciation created a diversity of habitats largely during the Pliocene and Pleistocene (Raven, 1973
). Although montane and cool-temperate environments existed earlier, subalpine and alpine environments probably did not. The first major lowering of temperatures in New Zealand took place in the Late Pliocene about 2.4 million years ago, and there appear to have been at least three glacial cycles subsequently. During the last, or Otiran, cycle, there was heavy glaciation in the South Island along and west of the Main Divide. Stewart Island and the subantarctic islands were glaciated, and all plants seem to have been eliminated from some of the smaller islands. In Australia, the major effect of the pluvial cycles during the Pleistocene was to promote moist fertile corridors across now arid regions facilitating contact between floras of the eastern and western temperate zones of Australia (Raven, 1973
).
Patterns of diversification
The New Zealand Stylidiaceae can be readily distinguished by their growth habit, floral morphology, and ecological requirements, and these factors may have influenced their ability to diversify in New Zealand (Fig. 7). Species of Phyllachne have short stem innovations that are densely branched, forming compact cushions (Fig. 7A), a feature that is also characteristic of Donatia. In contrast, the stems in Forstera are elongate and simply branched to form more delicate perennial herbs (Fig. 7D). Oreostylidium is distinct from the other New Zealand species and shares a basal-rosette habit (Fig. 7H) with its sister S. graminifolium (Fig. 7E).
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). According to Godley (1960)
the ecological conditions and taxonomic affinities of cushion bogs in New Zealand and southern Chile are remarkably similar where D. fascicularis and P. uliginosa are characteristic components of the Magellanic moorland region of southern Chile. In New Zealand D. novae-zelandiae occurs in mainly subalpine habitats, but descends to low altitudes in Westland and beside Foveaux Strait (Wardle, 1991
). Along with P. colensoi it is often dominant in upland bogs. Phyllachne colensoi is widely distributed in high mountain habitats including bogs throughout mainland New Zealand, with the exception of Mt. Taranaki. In mainland New Zealand the range of P. colensoi overlaps with P. clavigera, which is found from latitude 42° S and in the subantarctic islands and with P. rubra, which has a more restricted distribution on the Central Otago plateau.
Conspecific populations of Phyllachne colensoi in New Zealand and Tasmania probably reflect recent dispersal from New Zealand. This species is widespread in the alpine zone of New Zealand, but in Tasmania it is recorded from only 12 locations and is generally restricted to areas above 1250 m on well-drained sites or rocky slopes. The restricted distribution of P. colensoi in Tasmania may reflect its recent establishment and the lower habitat diversity in the subalpine and alpine zones (Gibson and Kirkpatrick, 1985
).
The spreading, sparsely branched growth habit of Forstera (Fig. 7D) probably evolved as this species expanded into less stressful subalpine and montane herbfields. The more robust species F. sedifolia and F. mackayii are found in high montane to subalpine regions, whereas the somewhat more slender species F. bidwillii and F. tenella are found in the subalpine to low montane areas.
Although Oreostylidium subulatum shares a similar growth habit to Stylidium graminifolium (Fig. 7E, H), it has not diversified in New Zealand. It is restricted to damp grasslands and herbfields in montane and lower subalpine areas (Allan, 1961
), whereas S. graminifolium is widespread in eastern mainland Australia and Tasmania, occurring on a diversity of soil types throughout coastal and montane regions (Raulings and Ladiges, 2001)
. Lloyd (1985)
postulated that many Australian plant groups are absent or poorly represented in New Zealand because they possess characters that are poorly adapted to New Zealand habitats.
Like many of the flowering plants in mountainous regions of New Zealand, all members of the New Zealand Stylidiaceae (including Donatia novae-zelandiae, Donatiaceae) possess small, unspecialized, white flowers (Fig. 7B, C, G). There is a general lack of specialist insect pollinators in New Zealand and hence a disproportionate reliance on unspecialized pollinators that promiscuously visit a wide range of plants and forage in an imprecise manner. A generalized floral structure may confer a selective advantage by permitting more species of insects and particularly smaller, less specialized insects to serve as pollinators (Lloyd, 1985
). The flowers of Phyllachne are sessile and solitary at the apices of the stems, scarcely emerging from the surface of the foliage (Fig. 7B). However, the total floral display can be quite striking given the densely branched nature of the stems. A broad spectrum of insect visitors has been documented for P. colensoi in both New Zealand (Primack, 1983
) and Tasmania (Corbett, 1995
). In Forstera, the flowers form a less conspicuous floral display, borne singularly or in pairs on elongate scapes (Fig. 7C). Forstera mackyii and F. bidwillii have colored pollinator guides at the throat of the flower. There is only one pollination record for Forstera (Primack, 1983
).
In contrast to the other New Zealand Stylidiaceae, Oreostylidium subulatum has undergone a dramatic evolutionary transformation in its floral morphology (Fig. 7G). It possesses actinomorphic, white, solitary flowers with an insensitive column (Fig. 7G, H), whereas its sister, Stylidium graminifolium (along with most of the other species in the genus Stylidium), possesses zygomorphic, often colorful flowers, typically borne on multiflowered inflorescences and with sensitive columns (Fig. 7E, F). Laurent, Bremer, and Bremer (1999)
suggested that the small, white flowers of O. subulatum developed by paedomorphosis (accelerated sexual development in a morphologically immature plant). The flowers of O. subulatum also lack the sensitive column that enables precise pollen placement, so presumably they are visited by a range of insects. This is in contrast to most species of Stylidium, in which pollinator constancy has been observed (Erickson, 1958
).
It is plausible that Oreostylidium subulatum underwent this transformation prior to dispersal to New Zealand, but we propose that this evolutionary change occurred relatively rapidly and after it became established in New Zealand. The initial O. subulatum founder population was presumably small and may have grown from a single seed. The lack of specialist pollinators may have also constrained its establishment and diversification in New Zealand.
A well-documented system of balanced lethal mutations has been shown to minimize the products of self-pollination in southwest species of Stylidium (Coates and James, 1979
; James, 1979
; Burbidge and James, 1991
). Willis and Ash (1990)
later demonstrated that a self-incompatibility system also operates in the eastern Australian species S. productum and S. graminifolium. It is unknown whether an equivalent system operates in the closely allied species Oreostylidium subulatum. This is a particularly interesting area for further research given that Godley (1979)
has illustrated that self-incompatibility in the native New Zealand flora is a relatively infrequent phenomenon.
Although Stylidiaceae flowers appear designed to promote cross-pollination, high levels of inbreeding can be generated by geitonogamous self-pollination, which is facilitated by many-flowered inflorescences. The elongate scapes in Stylidium graminifolium (Fig. 7E) and allied trigger plants support between 10 and 110 flowers, for example (Raulings and Ladiges, 2001)
. In contrast, the flowers of Oreostylidium subulatum are borne individually on short scapes (Fig. 7H). The inconspicuous flowers combined with the loss of column movement may indicate a switch toward obligate autogamy, as reported in some species of Parahebe (Garnock-Jones, 1976
; Wagstaff and Garnock-Jones, 2000
). Carlquist (1978)
suggested the reduction in inflorescence size, loss of column sensitivity, and recurved column in the northern Australian species S. reductum were also indicative of a switch to self-pollination. In an Oreostylidium population examined at Maungatua the stigmas developed in the bud suggesting that autogamy is extremely likely, at least in this population. Oreostylidium is recorded as white-flowered (Allan, 1961
), although the flowers on the Maungatua population had a pinkish-red abaxial flush and the column was similarly colored at the base. Garnock-Jones (1976)
noted that the corollas of taxa adapted to autogamy are usually uniformly white. The evolution of autogamy would have facilitated the establishment of an initial Oreostylidium founder population, but would limit the amount of genetic variation within and among populations, thus restricting its ability to adapt to environmental changes in the longer term.
In summary the sequence data support two distinct clades of New Zealand Stylidiaceae that differ in species richness. The origin of the Forstera/Phyllachne clade is equivocal; however, our results clearly demonstrate an Australian origin for the New Zealand endemic Oreostylidium subulatum. Forstera and Phyllachne are conspicuous members of the subalpine and alpine flora of New Zealand, while O. subulatum has a more restricted distribution. Differences in the time of establishment, the availability of suitable pollinators, and their breeding systems partly account for contrasting evolutionary success in the New Zealand Stylidiaceae. Finally our results have important implications for the classification of New Zealand Stylidiaceae. Based upon our study, we would propose two taxonomic changes, first to include Oreostylidium in the genus Stylidium, and second, to include Phyllachne in the genus Forstera, Forstera being the older name. There are precedents for both of these changes. Valid combinations have been published for Stylidium subulatum and Forstera clavigera. These changes would render Stylidium and Forstera monophyletic.
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FOOTNOTES |
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4 Author for reprint requests (wagstaffs{at}landcare.cri.nz
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Allan H. H. 1961 Flora of New Zealand. R. E. Owen Government Printer, Wellington, New Zealand
Baldwin B. G. M. J. Sanderson J. M. Porter M. F. Wojciechowski C. S. Campbell M. 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][ISI]
Barker P. F. J. Burrell 1977 The opening of Drake Passage. Marine Geology 25: 15-34
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Bean A. R. 2000 A revision of Stylidium subg. Andersonia (R.Br. ex G.Don.) Mildbr. (Stylidiaceae). Austrobaileya 5: 589-649
Bremer K. M. H. G. Gustafsson 1997 East Gondwana ancestry of the sunflower alliance of families. Proceedings of the National Academy of Sciences, USA 94: 9188-9190
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Carlquist S. 1969 Studies in Stylidiaceae: new taxa, field observations, evolutionary tendencies. Aliso 7: 13-64
Carlquist S. 1974 Island biology. Columbia University Press, New York, New York, USA
Carlquist S. 1978 New species of Stylidium, with comments on evolutionary patterns in tropical Stylidiaceae. Aliso 9: 308-322
Chase M. W. et al 1993 Phylogenetics of seed plants: an analysis of nucleotide sequences from the plastid gene rbcL. Annals of the Missouri Botanical Garden 80: 528-580[CrossRef][ISI]
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Coleman P. J. 1980 Plate tectonics background to biogeographic development in the southwest Pacific over the last 100 million years. Palaeogeography, Palaeoclimatology, Palaeoecology 31: 105-121
Corbett C. 1995 Pollination ecology in a Tasmanian alpine environment. Honors thesis, Department of Geography and Environmental Science, University of Tasmania, Hobart, Australia
Curtis W. M. 1963 The student's Flora of Tasmania. L. G. Shea, Government Printer, Tasmania, Australia
Dalziel I. W. D. 1983 The evolution of the Scotia Arc: a review. In R. L. Oliver, P. R. James, and J. B. Jago [eds.], Antarctic earth science, 283–288. Cambridge University Press, London, UK
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