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- Materials and methods
1. Introduced plants have the potential to rapidly evolve traits of ecological importance that may add to their innate potential to become invasive. During invasions, selection may favour genotypes that are already pre-adapted to conditions in the new habitat and, over time, alter the characteristics of subsequent generations.
2. Spotted knapweed (Centaurea stoebe) occurs in two predominantly spatially separated cytotypes in its native range (Europe–Western Asia), but currently only the tetraploid form has been confirmed in the introduced range (North America), where it is invasive. We used several common garden experiments to examine, across multiple populations, whether tetraploids and diploids from the native range differ in life cycle, leaf traits and reproductive capacity and if such differences would explain the predominance of tetraploids and their advance into new habitats in the introduced range. We also compared the same traits in tetraploids from the native and introduced range to determine whether any rapid adaptive changes had occurred since introduction that may have enhanced invasive potential of the species in North America.
3. We found tetraploids had lower specific leaf area, less lamina dissection and fewer, narrower leaves than diploids. Diploids exhibited a monocarpic life cycle and produced few if any accessory rosettes. Diploids produced significantly more seeds per capitulum and had more capitula per plant than tetraploids. In contrast, the vast majority of European tetraploids continued to flower in both seasons by regenerating from multiple secondary rosettes, demonstrating a predominantly polycarpic life cycle.
4. During early growth tetraploids from North America achieved greater biomass than both tetraploids and diploids from the native range but this did not manifest as larger above-ground biomass at maturity. In North American tetraploids there was also evidence of a shift towards a more strictly polycarpic life cycle, less leaf dissection, greater carbon investment per leaf, and greater seed production per capitulum.
5. Synthesis. Our results suggest that the characteristics of tetraploid C. stoebe pre-adapted them (compared to diploid conspecifics) for spread and persistence of the species into habitats in North America characterized by a more continental climate. After the species’ introduction, small but potentially important shifts in tetraploid biology have occurred that may have contributed significantly to successful invasion.
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- Materials and methods
The invasiveness of a plant species may be derived from a combination of pre-adapted traits and rapid adaptive changes in the species following introduction (Müller-Schärer & Steinger 2004). Interspecific studies examining traits of invasive plants have identified that traits associated with reproductive capacity and leaf traits associated with rapid carbon capture, such as high specific leaf area (SLA), leaf area ratio, photosynthetic rate, and net assimilation rate, may pre-adapt species to become invasive (Pyšek & Richardson 2007). Once introduced to a new habitat and released from co-evolved natural enemies (Liu & Stiling 2006) these species may be able to exploit resources to a greater extent than co-occurring native species (Blumenthal 2006; Blumenthal et al. 2009). Recent studies have highlighted the importance of co-determinants such as residence time, range size and propagule pressure that interact with plant traits in the invasion process and show that the relative importance of traits is context-dependent and differs between species (Pyšek, Křivánek & Jarošík 2009b; Pyšek et al. 2009a). This partly explains the difficulty in generalizing across species about the importance of plant traits in the process leading from a species’ introduction to invasion.
The most commonly tested hypothesis related to evolutionary change is whether plants in the introduced range, once released from their co-evolved natural enemies, have been able to reallocate resources previously used on defence, to achieve increased growth, the so-called Evolution of Increased Competitive Ability (EICA) hypothesis (Blossey & Nötzold 1995). Whilst increased growth and/or reduced defence has commonly been detected in invasive conspecifics (Siemann & Rogers 2003a,b; Jakobs, Weber & Edwards 2004; Wolfe, Elzinga & Biere 2004; Agrawal et al. 2005; Blumenthal & Hufbauer 2007; Zou, Rogers & Siemann 2008a; Zou et al. 2008b; Abhilasha & Joshi 2009), such reallocation of resources may be minor and not always manifest as increased above-ground vegetative biomass (Maron et al. 2004a; Genton et al. 2005; Barney, Whitlow & DiTommaso 2009). Changes in traits other than size, e.g. numbers or size of propagules and SLA, may be equally important for promoting invasiveness (Bossdorf et al. 2005). Higher SLA and associated increased rates of carbon capture could be allocated to a more extensive root system or more propagules per plant rather than in greater leaf biomass or shoot size. Whilst SLA has been found to be generally higher in invasive plants relative to co-occurring native species (Leishman et al. 2007), a shift in this trait within a species could be the result of selection for adaptation to a new environmental niche during invasion.
Recent studies indicate that interspecific variation in ploidy level in species is associated with invasiveness (Pyšek et al. 2009a) and that within species, polyploids rather than diploids are the dominant invasive forms (Lafuma et al. 2003; Pandit, Tan & Bisht 2006; Schlaepfer et al. 2008; Treier et al. 2009). Hence, the potential for becoming invasive not only depends on the variation in ploidy of a species but also on the ploidy level of a cytotype. Although the reason for this is currently unclear, it has been proposed that a broader tolerance of environmental conditions in polyploids, their greater potential to rapidly evolve via an ability to carry greater genetic diversity and the potential for duplicated genes to evolve new functions in the longer term, may explain their success (Soltis & Soltis 2000). It is also possible that, in a similar interaction to that proposed for resource availability and release from natural enemies (Blumenthal 2006), some polyploids may possess traits that give them advantages over diploids when herbivores/pathogens are absent, such that they are able to increase their spread and maintain occupancy of colonized sites (Müller-Schärer, Schaffner & Steinger 2004).
In the invasive plant Centaurea stoebe L. tetraploids and diploids occur primarily in discrete populations across the native range in Europe whilst tetraploids dominate the non-native range in North America entirely (Treier et al. 2009). Such a pronounced shift in cytotyope frequency could be due to founder events or be derived from a larger degree of environmental niche overlap between the native and introduced ranges of tetraploids than in diploids (Treier et al. 2009). In the latter instance, assuming both cytotypes were introduced to North America, the combination of traits possessed by tetraploids may have made them inherently more invasive than diploids. Moreover, tetraploid C. stoebe have been shown to be polycarpic which, in the absence of specialist herbivores, could give them greater lifetime fecundity than monocarpic diploids thus facilitating their rapid spread (Müller 1989; Broz et al. 2009).
In conjunction with changes in life cycle associated with ploidy, a significant shift in the climatic niche has occurred between C. stoebe’s native and introduced ranges (Broennimann et al. 2007). Tetraploids in North America now occupy areas with drier and more continental conditions than do tetraploids in Europe, whereas the niche of native diploids and tetraploids is largely overlapping (Treier et al. 2009). In addition, North American tetraploid genotypes have reduced expression of genes related to constitutive defence, consistent with reduced defence costs in the absence of herbivores resulting in rapid adaptive change (Broz et al. 2009). Such a reduction of defence costs may have contributed to the observed increased vegetative growth and competitiveness of invasive tetraploid genotypes relative to native genotypes in this species (Ridenour et al. 2008; He et al. 2009). The aforementioned findings indicate that C. stoebe may have undergone rapid evolutionary change in North America.
In this study, we sought to compare C. stoebe leaf traits, growth, life cycle and reproductive capacity of: (i) diploids with tetraploids from the native range in Europe and (ii) tetraploids from the native range (Europe) with tetraploids from the introduced range (North America). The first comparison is aimed at assessing what traits, possibly associated with previously identified life cycle differences between diploid and teraploid cytotypes, may potentially explain the dominance of the tetraploid cytotype in North America and the spatial separation of cytotypes in Europe. The second comparison is aimed at determining whether rapid adaptive changes, e.g. in reproduction, leaf traits or growth, have occurred in tetraploids from the introduced range relative to tetraploids from the native range. We chose to address this using a series of common garden experiments. In the absence of detailed population genetics information, our approach used a large number of populations from each cytotype and geographic region to average effects across multiple genotypes and to avoid confounding population level differences.