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Systematics |
Plant and Microbial Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
Received for publication March 20, 2003. Accepted for publication August 5, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Asteraceae • cpDNA • hawkweed • Hieracium pilosella • hybrid • introduced • New Zealand • reticulate • variation • weed
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). But the biology of invasive species also provides valuable, observable events that parallel and potentially illuminate prehistoric range changes. A large part of research in the field of molecular ecology has focused on determining population genetic effects of geologically recent environmental perturbations, in particular Pleistocene glaciation (Hewitt, 1996
; Taberlet et al., 1998
; Abbott et al., 2000
). This research contributes to our understanding of how factors such as dispersal ability, reproductive behavior, and genetic diversity are implicated in the colonization of new habitat. However, interpretation of phylogeographic data is to a greater or lesser extent post hoc, because measurements are made long after the colonization event. Invasive or weedy species offer a finer degree of resolution as it is through the auspices of human introduction that potentially invasive taxa are presented with new habitats. Thus, we expect to be able to observe many of the initial ecological/evolutionary responses of novel biotic and environmental interaction. In particular, we have the opportunity to study in detail the role of genetic diversity (Neuffer and Hurka, 1999
), polyploidy, and hybridization in invasion (e.g., Stebbins, 1985
; Raybould et al., 1991
; Abbott, 1992
; O'Hanlon et al., 1999
; Ellstrand and Schierenbeck, 2000
), features that must apply during natural range changes.
The genus Hieracium (Asteraceae) is among the most species rich of plants with more described species than any other genus in Europe (Sell and West, 1976
). The systematics of the hawkweed genus is extremely complex and contentious, with much disagreement among systematists even with respect to numbers of species (Gadella, 1991
). The instability of the taxonomy is probably a reflection of the polyphyletic origin of many Hieracium taxa (Krahulcova et al., 2000
), though there is little direct evidence for this (but see Shi et al., 1996
). Many Hieracium species comprise a complex of ploidal levels, and in at least some instances, putative species may be composites of several independent hybridization events (Krahulcova et al., 2000
). Interestingly, reproductive systems vary among ploidal levels even within species; for example, in H. pilosella even ploidal numbers (e.g., 2, 4) are generally obligate sexuals while those with odd ploidal numbers (e.g., 3, 5) are usually facultative apomicts (Gadella, 1987
, 1991
). In related taxa such as H. praealtum, this is not the case; tetraploid and pentaploid plants are known, but all are apomictic.
The Hieracia of New Zealand comprises a minute, and thus manageable, fraction of the full diversity of the genus. The 10 species reported from the field are all accidental introductions from Europe first reported in New Zealand over 100 years ago (Travers, 1884
). These plants are generally thought to have arrived as contaminants of imported grass seed. The most likely source was the United Kingdom because this was the origin of most human and agricultural colonists. In addition, British colonialists founded acclimatization societies for the purpose of introducing plants and animals to New Zealand. Subgenus Pilosella is represented in South Island New Zealand by five species (H. pilosella, H. praealtum, H. caespitosum, H. aurantiacum, H. x stoloniflorum; Webb et al., 1988
). Of these, H. pilosella has, perhaps with the aid of human habitat modification, become a successful and troublesome weed among high country grasslands that predominate in the South Island (Treskonova, 1991
; Jenkins and Jong, 1997
). Hieracium is much scarcer in North Island and, so far, of little ecological significance. Hieracium pilosella forms small, prostrate leaf rosettes and, like other members of the subgenus, spreads vegetatively via stolons. In New Zealand, H. pilosella often forms dense mats that exclude other plants, but it is not a significant weed in Europe, where it occurs as a component of mixed grassland floras.
In Europe, five cytopes of Hieracium pilosella have been identified in the field, ranging from diploid to heptaploid (Gadella, 1991
). The range is dominated by sexual tetraploids and apomictic pentaploids, with higher ploidies occurring mostly in the north (United Kingdom, Scandinavia) and montane areas of central Europe. Morphologically too, H. pilosella is extremely variable. For instance, Gadella (1987)
reports that Zhan (1923)
recognized 624 variants of the species.
In New Zealand, initial cytological studies of H. pilosella indicated that populations were predominantly, if not entirely, pentaploid (Makepeace, 1981
; Jenkins, 1991
). However, subsequent studies have found aneuploids (Chapman and Lambie, 2000
) and hybrids (Morgan-Richards et al., in press
), which may reflect the presence of obligate sexuals. Tetraploid sexual plants have been confirmed in a number of populations, and these may have evolved in situ (Chapman et al., 2003
) through residual sex of facultatively apomictic pentaploids (Houliston and Chapman, 2001
). There is also considerable morphological variation (Chapman and Brown, 2001
) and evidence of high genetic diversity (Chapman et al., 2000
). Considering the recent, probable small size and apomictic reproduction of the initial introduction, H. pilosella in New Zealand is surprisingly variable. The degree to which this variability reflects initial introduced diversity vs. postcolonization changes remains unresolved (Chapman and Brown, 2001
) and is the subject of ongoing research.
We undertook a study of chloroplast DNA (cpDNA) sequence variation to further our understanding of the extent and origin (pre- or postintroduction) of genetic diversity of H. pilosella introduced to New Zealand. Our rationale was that any variants encountered were extremely unlikely to have evolved since colonization and could not be the result of recombination (unlike nuclear markers). We surveyed all other species of Hieracium subgenus Pilosella in New Zealand to seek evidence of hybridization, introgression, or reticulate evolution since colonization. To facilitate this and to shed light on the possible European source(s) of New Zealand lineages, we examined samples representing the same and closely related species sourced directly from European populations, in the first cpDNA survey of the group.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). However, our approach to sequencing DNA from plants maintained in the glasshouse was selective; we took a random subsample of pentaploid plants but examined all surviving tetraploids. Other sites were sampled much less intensively in the field, but we collected at many locations. Given that H. pilosella can be highly clonal, this strategy minimized the risk of resampling clones within sites and maximizes the chances of finding different lineages. At all sites, individuals (ramets) were collected at well-spaced intervals (>30 cm and usually >100 cm). In New Zealand, H. pilosella is most abundant in the high country (600–1000 m asl; Treskonova, 1991
; Svavarsdóttir et al., 1999
) where it has invaded native grasslands; these were the source of most of our material. But we also located and collected specimens from low altitude sites where populations generally consist of small and possibly ephemeral mats. We also sourced DNA from herbarium specimens in the University of Canterbury collection. European specimens were obtained as leaf samples, dried and stored with silica gel, and in some cases additional samples were provided as DNA sequences by collaborators. Our sampling in Europe reflects the availability of collectors and the distribution of taxa (e.g., H. praealtum in the United Kingdom is a localized garden escape and thus not in our sample). The provenance of our specimen of H. auricula is not known because it has been in culture for some time; however, we retained it in the study because it was the sole representative of a haplotype that forms an internal node in our tree.
Taxonomy
The systematics and nomenclature of the Hieracia is far from stable and several competing systems are in current use in Europe. For plants collected in New Zealand, we have followed Webb et al. (1988)
and as far as possible accommodated material from Europe within this system. To do this, we have relied on local expertise, and we acknowledge that without comparison of morphological and cytogenetic data some question as to placement of individuals may exist. However, we believe the determination of H. pilosella specimens is reliable and can be accommodated within all current taxonomic systems.
Ploidy
The ploidy of New Zealand specimens was determined using a combination of flow-cytometry and direct counts of chromosomes (Morgan-Richards et al., in press
). For European material for which DNA was extracted from silica-gel-dried specimens or from herbarium specimens or was obtained as raw DNA, we were not able to determine ploidy.
DNA extraction
Each sample was ground using a hot (60°C) ceramic pestle and mortar with preheated extraction buffer comprising 1000 µL 2% hexadecyltrimethylammonium bromide (CTAB) buffer in 100 mmol/L Tris-HCl pH 8.0, 1.4 mol/L NaCl, 20 mmol/L EDTA, and 2 µL ß-mercaptoethanol. Homogenates were incubated at 60°C for 30–60 min. An equal volume of 24 : 1 chloroform : isoamyl alcohol was added, and the mixture vigorously shaken and centrifuged at 16 060 g for 5 min. Supernatants were pipetted into fresh tubes and combined with a two-thirds volume of cold 100% isopropanol. DNA was pelleted after 15–60 min by spinning at 16 060 g for 10 min and then washed with 500 µL 70% ethanol by spinning briefly. The 70% ethanol was discarded and the pellet dried and dissolved in 30 µL of water.
DNA amplification and sequencing
We targeted noncoding regions of cpDNA as described by Taberlet et al. (1991)
. Polymerase chain reaction (PCR) was used to amplify the trnL-trnF chloroplast gene fragment (Taberlet et al., 1991
). Primers c and f (Taberlet et al., 1991
) were used to amplify the trnL intron and trnL/trnF intergenic spacer. The polymerase chain reactions were carried out in 25-µL volumes containing 2.5 mmol/L MgCl2, 200 µmol/L dNTPs, 1x PCR buffer, 0.625 units Taq (Roche Diagnostics, Mannheim, Germany) and 1.5 µL of diluted (1 : 20–1 : 100) DNA template. Thermal cycling conditions were: 2 min at 94°C, 35 cycles of 15 s at 94°C, 30 s at 48°C, and 90 s at 72°C, followed by 3 min at 72°C. The PCR products were purified using High Pure spin columns (Roche Diagnostics, Mannheim, Germany). Cycle sequencing utilized Dyenamic chemistry (Amersham Biosciences, Freibury, Germany) and the Taberlet et al. (1991)
primers c, e, and f following the manufacturer's protocols. Cycle sequencing products were electrophoresed on an ABI automated sequencer (PE Biosystems, Foster City, California, USA). Confirmation of base calls by comparison with chromatograms and alignment of sequences was achieved by eye using SeqEd version 3.01. We constructed a minimum spanning tree of the haplotypes identified manually.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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12 species were surveyed from 72 locations in Europe (Table 1, Appendix 1) in 36 regions (Fig. 2). In total, 285 plants were sequenced.
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Only two other haplotypes (D, G) were present in more than one taxon (Table 1, Appendix 1). Most samples of haplotype D came from H. aurantiacum (NZ and Europe), but also from H. glaciale (Europe) and H. praealtum (Europe and NZ). The two instances of haplotype D in New Zealand H. praealtum were in rare pentaploid individuals (Hope Saddle, Fig. 1); other pentaploid and all tetraploid H. praealtum in New Zealand had haplotype A. Haplotype G was found in H. x stoloniflorum (NZ), H. aurantiacum, H. florentinum, and H. pilosella from Europe.
New Zealand Hieracium pilosella
In New Zealand, haplotype A was the most frequent (70%) in both our total sample and within H. pilosella alone (Table 1, Table 3). A was present in representatives of all three ploidal levels of H. pilosella encountered (Table 4). Conversely, the other haplotypes found in New Zealand H. pilosella (B, C, and M) were restricted to pentaploids, tetraploids, and hexaploids, respectively (excluding F1 hybrids, see later). C was restricted to three adjacent locations (Drac Flat, Lyndon, and Rakaia), and M was found at just one location (Twizel) (Fig. 1, Table 4). All the tetraploid individuals in our New Zealand sample came from three intensively surveyed locations (Drac Flat, Lyndon, and Rakaia), but because we sequenced all tetraploids that we found, but not all the pentaploids, the ratio in our sample is skewed. However, the dominance of one haplotype (C) at Drac Flat is informative. Across the majority of locations, only pentaploids were found and these were either haplotype A or B (Fig. 1). At seven of 20 locations where more than one individual was examined, both A and B were found (Table 3).
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) from three sites (Drac Flat, Lyndon, Cowans Hill). Half had haplotype A, one had B (Lyndon), and eight had C (Lyndon 1, Drac Flat 7) (Table 3, Table 4).
European taxa
Haplotype A was also encountered with high frequency in Europe, being present in 53% of all plants and 50% of H. pilosella examined (Table 1). Of 12 taxa sampled, eight had Haplotype A and four of these had at least one other haplotype (Appendix 1). The actual extent of haplotype sharing is almost certainly much higher than this because our sample of most taxa was very small. In Europe, rare haplotypes were found in a number of taxa: I in two H. caespitosum (of eight), J in a single H. lactucella, and H in a single H. auricula.
Phylogeny
The relationship among these haplotypes was inferred using a fully resolved parsimony spanning tree. Many of the haplotypes we encountered appear as internal nodes in our tree, consistent with shallow or recent splitting (Fig. 3a). Interestingly, four of the five internal nodes consisted of haplotypes found in H. pilosella.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Both are consistent with recent speciation. Our data do not allow us to study the phylogenetics nor the systematics of species, but our data do demonstrate the complex recent history of the group and help explain taxonomic difficulties. Hieracium pilosella introduced to New Zealand brought with it much of the European chloroplast variation. It has not increased cpDNA diversity via hybridization since introduction, but hybridization may nevertheless have a significant impact on invasiveness in New Zealand.
Reticulate evolution
We found six haplotypes within H. pilosella and extensive sharing among ploidal levels and sharing among species. Despite the predominance of apomixis the sharing of haplotypes can be explained by reticulate evolution. In New Zealand Hieracium pilosella, a low level of residual sex has been demonstrated in pentaploid apomicts (Houliston and Chapman, 2001
) and a breakdown in microspecies boundaries has been observed (Chapman and Brown, 2001
). Furthermore, sex between pentaploid apomicts not only results in pentaploids of mixed parentage but can also yield tetraploids, some of which are obligate sexuals (Chapman and Bicknell, 2000
; Chapman et al., 2003
). These and the sexual progeny of interspecies hybrids (Morgan-Richards et al., in press
; see Invasion later) may further contribute to reticulation. Nuclear markers have revealed genetic variation in apomictic microspecies of Hieracium (hawkweeds, Shi et al., 1996
; Chapman et al., 2000
) and Taraxacum (dandelions). In contrast, Wittzel (1999)
found no cpDNA variation within apomictic species of Taraxacum, the expected result for true clonal lineages (Richards, 1996
). The presence of multiple haplotypes (or other genetic variation) within apparently apomictic species could result from the failure to correctly identify microspecies (i.e., different lineages) as Wittzel (1999)
suggested for the Taraxacum data of King (1993)
. However, failure to correctly identify microspecies could be attributed to morphological instability, which would be likely if microspecies/species hybridized. It has become increasingly apparent that many (and perhaps all) apomicts are not in fact obligate apomicts but have some potential for sexual reproduction (Nogler, 1984
; Bayer et al., 1990
; Briggs and Walters, 1997
). Gene flow in predominantly selfing polyploids could result in exchange of chloroplast and other markers among lineages (microspecies, e.g., Doyle et al., 1999
) and thus inconsistent morphology.
Origins of New Zealand Hieracium
Hieracium pilosella and presumably the other species of the genus found today in New Zealand arrived sometime in the mid-1800s (Travers, 1884
). Introduction probably resulted from seed accidentally harvested with European grass seed used for development of New Zealand pastureland. It is assumed that the United Kingdom was the source of this seed but this is not supported by our data. We found more cpDNA haplotypes in New Zealand H. pilosella than in the United Kingdom. The two haplotypes in the United Kingdom (A, B) were ubiquitous throughout Europe. In particular, we did not find haplotype C in the United Kingdom, despite sampling from several populations. However, haplotype C was present in our smaller sample from eastern/northern Europe (Czech Republic, Finland). Also in our central European sample were H. pilosella and H. aurantiacum with haplotype G (Austria). Although G was not found in either of these species sampled in New Zealand, it was present in samples of the hybrid taxon derived from these parentals (H. x stoloniflorum). We note, too, that New Zealand H. x stoloniflorum, which are evidently derived from introduced hybrids rather than arising here, are hexaploid (Jenkins and Jong, 1997
; Morgan-Richards et al., in press
), and this ploidy is reported from Bavaria (Germany) and the Czech Republic but not the United Kingdom (Krahulcova et al., 2000
). It is significant too that all of the subgenus Pilosella taxa in New Zealand, except H. pilosella, exist only as garden escapes in the United Kingdom (Stace, 1997
). The combined evidence unexpectedly indicates that some, if not all, extant New Zealand Hieracium taxa came from central Europe rather than the United Kingdom.
Invasion
Why some introduced taxa succeed in founding persistent populations is not well understood. Acclimatization societies in New Zealand actively introduced foreign organisms and thus supplemented the large number that arrived accidentally (Travers, 1884
). A study of bird introductions to New Zealand revealed that colonization success might have as much to do with management as ecological characteristics of individual species (Veltman et al., 1996
). In the case of Hieracium pilosella, it cannot be known which, and in what relative frequencies, lineages (genotypes) were initially introduced. Initial studies of H. pilosella ploidy in New Zealand indicated most plants were pentaploids and tetraploids were absent (Makepeace, 1981
; Jenkins and Jong, 1997
). Sexual tetraploids have since been found (Chapman et al., 2003
), but so has evidence that apomict pentaploids are able to generate other ploidies through residual sex (Chapman and Bicknell, 2000
; Houliston and Chapman, 2001
). It has been inferred that tetraploid sexuals may have evolved in New Zealand (Chapman et al., 2003
).
The present study indicates that at least one sexual tetraploid lineage (4x C) was introduced directly from Europe. To have evolved in New Zealand would require a population of haplotype C pentaploids and given that the level of residual sex in apomicts is low (up to 2.3%; Houliston and Chapman, 2001
), a rather large population would be predicted. No haplotype C pentaploids have been found in New Zealand. All H. pilosella with haplotype C come from a small area in central South Island, and most individuals come from <25 m2 of one site (Drac Flat). It is not surprising therefore that earlier surveys did not encounter these tetraploids. This scenario might also apply to haplotype A tetraploids, and it is interesting to note that all sexual tetraploids found so far come from sites in the vicinity of the earliest record of Hieracium in New Zealand (Travers, 1884
). From our cpDNA and ploidy data, we can conservatively estimate that at least four different lineages (A, B, C, M) were introduced to New Zealand. There would have been a minimum of six lineages if shifts in ploidal level observed in plants with haplotype A occurred prior to colonization.
Although tetraploid H. pilosella are rather abundant in Europe and may have been relatively abundant in the introduced seed, it is likely that many features of pentaploids (e.g., facultative apomixis, more and longer stolons, more seeds; Gadella, 1987
) gave them advantages as colonizers. Pentaploids have the potential to colonize from single seeds, whereas isolated tetraploids might soon fail as they are self-incompatible (Krahulcova et al., 2000
). A rapid spread of pentaploids and localization of tetraploid sexuals can easily be visualized.
However, sexual tetraploids (whether introduced or derived) may be having a greater impact than previously realized. In New Zealand, H. pilosella unlike other related species (H. caespitosum and H. praealtum) appears to have rapidly increased in abundance since the 1960s (Connor, 1991
; Treskonova, 1991
; Scott, 1993a
, b
; Rose et al., 1995
; Svavarsdóttir et al., 1999
). Evidence from flow cytometry and crossing experiments indicate that this may be the result of interspecies hybridization (Morgan-Richards et al., in press
). We have observed tetraploid interspecies hybrids (probably H. pilosella x H. praealtum) at five different sites, and sequences indicate that three different cpDNA lineages have been involved in the formation of H. pilosella hybrids (Table 4). Many of the hybrids are sexual and able to back cross to parentals, and morphological and flow cytometric data indicate that many putative pentaploid H. pilosella individuals throughout New Zealand are probably pentaploid hybrids (Morgan-Richards et al., in press
). It is possible that interspecies hybridization has resulted in especially invasive lineages (Doyle et al., 1999
; Ellstrand and Schierenbeck, 2000
).
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FOOTNOTES |
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2 Current address: Allan Wilson Centre for Molecular Ecology and Evolution, Massey University, Private Bag 11-222, Palmerston North, New Zealand (s.trewick{at}massey.ac.nz
)
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