Increased Phenotypic Plasticity to Climate May Have Boosted the Invasion Success of Polyploid Centaurea stoebe

Phenotypic plasticity may allow organisms to cope with altered environmental conditions as e.g. after the introduction into a new range. In particular polyploid organisms, containing more than two sets of chromosomes, may show high levels of plasticity, which could in turn increase their environmental tolerance and invasiveness. Here, we studied the role of phenotypic plasticity in the invasion of Centaurea stoebe (Asteraceae), which in the native range in Europe occurs as diploids and tetraploids, whereas in the introduced range in North America so far only tetraploids have been found. In a common garden experiment at two sites in the native range, we grew half-sibs of the three geo-cytotypes (native European diploids, European tetraploids and invasive North American tetraploids) from a representative sample of 27 populations. We measured the level and the adaptive significance of phenotypic plasticity in eco-physiological and life-history traits in response to the contrasting climatic conditions at the two study sites as well as three different soil conditions in pots, simulating the most crucial abiotic differences between the native and introduced range. European tetraploids showed increased levels of phenotypic plasticity as compared to diploids in response to the different climatic conditions in traits associated with rapid growth and fast phenological development. Moreover, we found evidence for adaptive plasticity in these traits, which suggests that increased plasticity may have contributed to the invasion success of tetraploid C. stoebe by providing an advantage under the novel climatic conditions. However, in invasive tetraploids phenotypic plasticity was similar to that of native tetraploids, indicating no evolution of increased plasticity during invasions. Our findings provide the first empirical support for increased phenotypic plasticity associated with polyploids, which may contribute to their success as invasive species in novel environments.     

Effects of polyploidy on secondary chemistry, physiology, and performance of native and invasive genotypes of Solidago gigantea (Asteraceae)

The role of polyploidy in facilitating invasiveness of introduced plants has not been well explored. Examination of traits of diploid and polyploid plants in both their native and introduced ranges can shed light on evolutionary processes occurring postintroduction in invasive plants. We determined the distribution and prevalence of cytotypes of Solidago gigantea in both its native range (USA) and introduced range (Europe), and measured a suite of biochemical, physiological, and reproductive characters for plants from both continents. Tetraploids were the most frequent cytotype encountered on both continents, while hexaploids were found only in the USA. Hexaploids were the most distinctive cytotype, with fewer differences observed between diploids and tetraploids. Comparison of diploids and tetraploids in the USA and Europe showed that traits changed in concert for both cytotypes. Both diploids and tetraploids in Europe had reduced concentrations of three classes of secondary chemical and invested relatively more into rhizomes than into flowers. The same changes occurring in both cytotypes in the introduced range show that altered phenotypes of European plants are not due to shifts in the proportions of cytotypes but instead occur within them. There was no evidence that polyploids evolve more quickly in the introduced range.     

Plant origin and ploidy influence gene expression and life cycle characteristics in an invasive weed

Background  

Ecological, evolutionary and physiological studies have thus far provided an incomplete picture of why some plants become invasive; therefore we used genomic resources to complement and advance this field. In order to gain insight into the invasive mechanism of Centaurea stoebe we compared plants of three geo-cytotypes, native Eurasian diploids, native Eurasian tetraploids and introduced North American tetraploids, grown in a common greenhouse environment. We monitored plant performance characteristics and life cycle habits and characterized the expression of genes related to constitutive defense and genome stability using quantitative PCR.     

DNA ploidy-level variation in native and invasive populations of Lythrum salicaria at a large geographical scale

Aim  This study aimed to document precisely the patterns of DNA ploidy variation in the native and secondary ranges of Lythrum salicaria distribution. The hypothesis that species invasiveness had been induced by a switch in ploidy level was addressed.
Location  Europe, Middle East, North America.
Methods  DNA ploidy levels of 1884 progenies of 578+ plants collected at 124 localities were determined by DAPI flow cytometry.
Results  Large cytotype variation (2 x , 3 x , 4 x and 6 x ) was found across the native area of distribution (64 populations covering 12 European and two Middle Eastern countries). DNA hexaploids were detected for the first time, and rare DNA triploids were reliably confirmed. DNA tetraploids largely prevailed across the native range studied, while DNA diploids and DNA hexaploids were recorded only in Israel and Turkey, respectively. DNA triploid progenies occurred in one population from Hungary (together with DNA tetraploids). Sympatric growth of DNA tetraploids and DNA hexaploids was repeatedly encountered in Turkey. In contrast, cytotype uniformity was a typical feature of the invasive North American plants. Sixty populations, covering 13 states of the USA and provinces of Canada, were characterized by the presence of only DNA tetraploids.
Main conclusions  Several L. salicaria cytotypes (2 x , 3 x , 4 x , 6 x ) occur in the native range of distribution, with much variation concentrated in the Middle Eastern countries, whereas only DNA tetraploids appeared to occur in North America. Our data show that the invasive spread of North American populations was not triggered by differences in ploidy level. Alternative explanations should be sought.     

Ecological, morphological and allozymic differentiation between diploid and tetraploid knapweeds (Centaurea jacea) from a contact zone in the Belgian Ardennes

In the northeastern part of Belgium, the Centaurea jacea complex shows extensive morphological variation and is represented by a diploid (2 n = 22) and a tetraploid (2 n = 44) cytotype. Polysomic inheritance of allozyme markers in the tetraploids, suggesting autopolyploidy, is here demonstrated for the first time. In order to test whether the two cytotypes occupy distinct habitats and possess different gene pools, patterns of allozymic and morphological variation were investigated in relation to ploidy level and site characteristics in 26 populations from the Belgian Ardennes. The two cytotypes showed a parapatric distribution, the diploids occurring at higher elevations (mostly above 500 m) than the tetraploids (mostly below 500 m). Three mixed populations were found near the contact zone of the two cytotypes. Within the mixed populations no triploid plant and no evidence for gene flow between cytotypes were found, despite widely overlapping flowering periods. The two cytotypes can be distinguished on the basis of morphological traits and enzymatic gene pools. The congruence of morphological and allozymic variation with chromosome numbers suggests a secondary contact between the two cytotypes with limited gene flow between them. The origin and persistence of the parapatric distribution are discussed.     

Does higher ploidy level increase the risk of invasion? A case study with two geo-cytotypes of Solidago gigantea Aiton (Asteraceae)

Aims Understanding the role of genetics in biological invasions has become an important aspect for modern plant ecology. Many studies suggest that increased ploidy level benefits the success of an invasive species, but the basis for this phenomenon is not fully understood. In its native, North American range, Solidago gigantea has three geo-cytotypes comprising di-, tetra- and hexaploid populations, while in Europe, where it is highly invasive, S. gigantea stands are composed primarily of tetraploid individuals. Our study investigates whether North American hexaploids can induce a greater risk of invasion, due to their higher performance in a non-native range, as compared to the existing tetraploids of that range.Methods We performed greenhouse and common garden experiments along with microsatellite analyses to test whether differences in chromosome number and origin of the species mean superior fitness in the introduced range.Important findings Genetic diversity was significantly higher in the native hexaploid populations (A R = 6.04; H e = 0.7794), rather than the non-native tetraploid populations (A R = 4.83; H e = 0.6869). Furthermore, differentiation between geo-cytotypes was moderate (ρ ST = 0.1838), which was also confirmed by their clear segregation in principal component analysis and structure analyses, proving their different genetic structure. In contrast to genetic diversity, the non-native tetraploid geo-cytotype performed better in the common garden experiment, implying that higher genetic diversity does not always mean better success. Our results suggest that native hexaploids do not present a greater risk, as assessed by their performance in the introduced range, when compared to the non-native tetraploids, as was suggested by previous studies. Nevertheless, their introduction is still undesirable due to their different genetic structure, which, through hybridization, could give a new drive to the invasion of S. gigantea .     

The population genetics of the fundamental cytotype-shift in invasive <Emphasis Type="Italic">Centaurea stoebe</Emphasis> s.l.: genetic diversity,genetic differentiation and small-scale genetic structure differ between cytotypes but not between ranges

Polyploids are overrepresented in invasive species. Yet, the role of genetic diversity and drift in colonization success of polyploids remains unclear. Here, we investigate genetic diversity, genetic differentiation and small-scale genetic structure in our model system, the three geo-cytotypes of Centaurea stoebe: monocarpic diploids and polycarpic (allo)tetraploids coexist in the native range (Eurasia), but only tetraploids are reported from the invasive range (North America). For each geo-cytotype, we investigated 18–20 populations varying in size and habitat type (natural vs. ruderal). Population genetic analyses were conducted at eight microsatellite loci. Compared to diploids, tetraploids revealed higher genetic diversity and lower genetic differentiation, whereas both were comparable in tetraploids between both ranges. Within spatial distances of a few meters, diploid individuals were more strongly related to one another than tetraploids. In addition, expected heterozygosity in diploids increased with population size and was higher in natural than in ruderal habitats. However, neither relationship was found for tetraploids. The higher genetic diversity of tetraploid C. stoebe may have enhanced its colonization abilities, if genetic diversity is correlated with fitness and adaptive capabilities. Furthermore, the inheritance of a duplicated chromosome set as well as longevity and frequent gene flow reduces drift in tetraploids. This counteracts genetic depletion during initial introductions and in subsequent phases of small or fluctuating population sizes in ruderal habitats. Our findings advocate the importance of studying colonization genetic processes to gain a more mechanistic understanding of the role of polyploidy in invasion dynamics.     

Absence of gene flow between diploids and hexaploids of Aster amellus at multiple spatial scales

The potential for gene exchange across ploidy levels has long been recognized, but only a few studies have explored the rate of gene flow among different cytotypes. In addition, most of the existing knowledge comes from contact zones between diploids and tetraploids. The purpose of this paper was to investigate relationships between diploid and hexaploid individuals within the Aster amellus aggregate. A. amellus is known to occur in diploid and hexaploid cytotypes in Europe, with a complex contact zone in central Europe. Patterns of genetic diversity were investigated using seven microsatellite loci at three different spatial scales: (1) in the single known mixed-ploidy population; (2) in populations at the contact zone and (3) in a wider range of populations across Europe. The results show clear separation of the cytotypes at all three spatial scales. In addition, analysis of molecular variance strongly supported a model predicting a single origin of the hexaploids, with no or very limited gene flow between the cytotypes. Some hexaploid individuals found in the mixed-ploidy population, however, fell into the diploid cluster. This could suggest recurrent polyploid formation or occasional cross-pollination between cytotypes; however, there are strong post-zygotic breeding barriers between the two cytotypes, making the latter less plausible. Overall, the results suggest that the cytotypes could represent two cryptic species. Nevertheless, their formal separation is difficult as they cannot be distinguished morphologically, occupy very similar habitat conditions and have largely overlapping distribution ranges. These results show that polyploid complexes must be treated with caution as they can hide biological diversity and can have different adaptation potentials, evolving independently.     

Effect of triploid fitness on the coexistence of diploids and tetraploids

The conditions for the coexistence of diploids, triploids and tetraploids in a single population were investigated with a deterministic model under the assumptions that diploids might produce 2 n gametes, and that triploids had a lower fitness than other cytotypes and generated equal proportions of haploid and diploid gametes. When diploids produced only haploid gametes, the dynamics of the cytotypes were similar to that of heterozygote disadvantage with two alleles at a single locus, with triploids being equivalent to the heterozygotes. Production of 2 n gametes by diploids increased the pool of diploid gametes and created a stable equilibrium involving a majority of diploids and a minority of polyploids. When the fitness of tetraploids was equal to or higher than that of diploids, increased triploid fitness decreased the threshold of 2 n gametes necessary to deterministically fix tetraploids in the population. Conversely, when tetraploids were less fit than diploids, the rate of 2 n gamete production leading to the exclusion of diploids first decreases and then increased with increasing triploid fitness. Triploids are repeatedly found in diploid-tetraploid hybridizations and are rarely totally sterile. They might play a determinant role in the future of multiple cytotype populations. The effect of triploids depends on the relative fitness of diploids and tetraploids and is also a function of their fitness.     

Asexuality and the coexistence of cytotypes

Reproductive isolation via apomixis is one way for newly created cytotypes to persist and coexist with other cytotypes. Arnica cordifolia (Asteraceae) has both triploid and tetraploid cytotypes co-occurring in many locations. The rate of apomixis in each cytotype was explored as a mechanism for the maintenance of sympatric cytotypes. Flow cytometry was used on both adults and seeds from mixed cytotype populations to estimate reproductive mode and to evaluate the relationship between cytotype frequency and reproductive success. Flowering time was surveyed to look for temporal reproductive isolation between cytotypes. Both triploids and tetraploids can be asexual. Apomixis in A. cordifolia is usually autonomous, not pseudogamous as previously thought. Sexual reproduction appears to be uncommon. The minority cytotype in each population does not produce fewer seeds, confirming that minority cytotype exclusion is unlikely to occur via reproductive disadvantage. Triploids flowered earlier than tetraploids, but with much overlap. Asexual reproduction is an important factor promoting the coexistence of cytotypes in this system. Other mechanisms maintaining populations of sympatric cytotypes are not well studied or understood and warrant further investigation.     

Difference in reproductive mode rather than ploidy explains niche differentiation in sympatric sexual and apomictic populations of Potentilla puberula

Apomicts tend to have larger geographical distributional ranges and to occur in ecologically more extreme environments than their sexual progenitors. However, the expression of apomixis is typically linked to polyploidy. Thus, it is a priori not clear whether intrinsic effects related to the change in the reproductive mode or rather in the ploidy drive ecological differentiation. We used sympatric sexual and apomictic populations of Potentilla puberula to test for ecological differentiation. To distinguish the effects of reproductive mode and ploidy on the ecology of cytotypes, we compared the niches (a) of sexuals (tetraploids) and autopolyploid apomicts (penta‐, hepta‐, and octoploids) and (b) of the three apomictic cytotypes. We based comparisons on a ploidy screen of 238 populations along a latitudinal transect through the Eastern European Alps and associated bioclimatic, and soil and topographic data. Sexual tetraploids preferred primary habitats at drier, steeper, more south‐oriented slopes, while apomicts mostly occurred in human‐made habitats with higher water availability. Contrariwise, we found no or only marginal ecological differentiation among the apomictic higher ploids. Based on the pronounced ecological differences found between sexuals and apomicts, in addition to the lack of niche differentiation among cytotypes of the same reproductive mode, we conclude that reproductive mode rather than ploidy is the main driver of the observed differences. Moreover, we compared our system with others from the literature, to stress the importance of identifying alternative confounding effects (such as hybrid origin). Finally, we underline the relevance of studying ecological parthenogenesis in sympatry, to minimize the effects of differential migration abilities.     

Why only tetraploid Solidago gigantea (Asteraceae) became invasive: a common garden comparison of ploidy levels

Many studies have compared the growth of plants from native and invasive populations, but few have considered the role of ploidy. In its native range in North America, Solidago gigantea Aiton (Asteraceae) occurs as a diploid, tetraploid and hexaploid, with considerable habitat differentiation and geographic separation amongst these ploidy levels. In the introduced range in Europe, however, only tetraploid populations are known. We investigated the growth performance and life history characteristics of plants from 12 European and 24 North American (12 diploid, 12 tetraploid) populations in a common garden experiment involving two nutrient and two calcium treatments. Twelve plants per population were grown in pots for two seasons. We measured 24 traits related to leaf nutrients, plant size, biomass production and phenology as well as sexual and vegetative reproduction. Native diploid plants had a higher specific leaf area and higher leaf nutrient concentrations than native tetraploids, but tetraploids produced many more shoots and rhizomes. Diploids grown with additional calcium produced less biomass, whereas tetraploids were not affected. European plants were less likely to flower and produced smaller capitulescences than North American tetraploids, but biomass production and shoot and rhizome number did not differ. We conclude that a knowledge of ploidy level is essential in comparative studies of invasive and native populations. While clonal growth is important for the invasion success of tetraploid S. gigantea, its potential was not acquired by adaptation after introduction but by evolutionary processes in the native range.     

Are tetraploids more successful? Floral signals,reproductive success and floral isolation in mixed-ploidy populations of a terrestrial orchid

Background and Aims Polyploidization, the doubling of chromosome sets, is common in angiosperms and has a range of evolutionary consequences. Newly formed polyploid lineages are reproductively isolated from their diploid progenitors due to triploid sterility, but also prone to extinction because compatible mating partners are rare. Models have suggested that assortative mating and increased reproductive fitness play a key role in the successful establishment and persistence of polyploids. However, little is known about these factors in natural mixed-ploidy populations. This study investigated floral traits that can affect pollinator attraction and efficiency, as well as reproductive success in diploid and tetraploid Gymnadenia conopsea (Orchidaceae) plants in two natural, mixed-ploidy populations.Methods Ploidy levels were determined using flow cytometry, and flowering phenology and herbivory were also assessed. Reproductive success was determined by counting fruits and viable seeds of marked plants. Pollinator-mediated floral isolation was measured using experimental arrays, with pollen flow tracked by means of staining pollinia with histological dye.Key Results Tetraploids had larger floral displays and different floral scent bouquets than diploids, but cytotypes differed only slightly in floral colour. Significant floral isolation was found between the two cytotypes. Flowering phenology of the two cytotypes greatly overlapped, and herbivory did not differ between cytotypes or was lower in tetraploids. In addition, tetraploids had higher reproductive success compared with diploids.Conclusions The results suggest that floral isolation and increased reproductive success of polyploids may help to explain their successful persistence in mixed-ploidy populations. These factors might even initiate transformation of populations from pure diploid to pure tetraploid.     

Reproductive isolation between diploid and tetraploid cytotypes of Libidibia ferrea (= Caesalpinia ferrea) (Leguminosae): ecological and taxonomic implications

Polyploidy is the most common chromosomal mechanism involved in the evolution of plants. However, the emergence of polyploid individuals does not guarantee the establishment of a new lineage, and the relationship between polyploidy and reproductive biology is therefore relevant. Libidibia ferrea is a legume tree that has diploid and tetraploid populations. In this work we analyse the reproductive biology of the species to verify the degree of reproductive isolation between the two cytotypes. Observations on phenology, floral morphology, biology, and visitors, breeding system, reproductive success and reproductive isolation were made for both cytotypes in two municipalities of northeastern Brazil. Cytotypes differed for all morphometric parameters analysed, with tetraploids exhibiting higher mean values than diploids. Both cytotypes had the same effective pollinators (native bees from the genera Centris and Xylocopa, and the introduced Apis mellifera). However, since stamens of diploids and tetraploids were of different size, it is possible that spatial separation occurs when pollen of each cytotype is deposited on a bee’s body. Diploids were self-incompatible (ISI?=?0) and exhibited high fruit-set after intraploidy crossing (20?%), whereas tetraploids were self-compatible (ISI?=?0.47) and set only 3.9?% of fruits after intraploidy crossing. Both cytotypes showed low fruit- and seed-set under natural conditions, and there was no fruit-set after crosses between them. Data are discussed in relation to the establishment of polyploids, to the pattern of distribution of the species and to the taxonomic implications. The cytotypes of L. ferrea should be considered as distinct species, since they are reproductively isolated from each other.     

Polyploidy and habitat differentiation in Dactylis glomerata L. from Galicia (Spain)

Summary The microdistribution of diploid and tetraploid plants of Dactylis glomerata L. was examined and related to their immediate environment in several sites in central Galicia, where morphologically indistinguishable individuals of both ploidies grow in sympatry. The two related cytotypes differed in habitat preference. Diploids were mainly confined to the low-density forest-floor habitat in woodlands of mostly ancient origin, whereas tetraploids were widespread in varied habitats but clearly predominant in open areas, particularly in disturbed anthropic sites. The in situ comparison of plant performance showed that where plants of each ploidy were more common they produced more tillers, panicles and seeds. This habitat preference closely reflected differences in life-history characteristics. The tetraploids had an early and short flowering time almost always completed before the aestival drought, whereas the diploids began to flower several weeks later and flowered throughout the drought. Comparisons along artificial gradients of soil water availability and light transmittance indicated that the cytotypes had distinct physiological requirements which probably originated in metabolic and more general genetic differentiation and could be directly attributable to ploidy. Habitat differentiation increases the species' colonizing ability. It also amplifies divergence in reproductive strategy between diploids and tetraploids, which reduces ineffective crossing between cytotypes and thereby permits them to coexist in sympatry. The effect of hybridization at the polyploid level on the differentiation between cytotypes was assessed from the recent introduction of a foreign tetraploid entity into the study area. Hybridization between the two distinct tetraploids was found to increase habitat differentiation between the diploids and the tetraploids, but the major part of this differentiation is probably attributable to ploidy itself.     

PATTERNS OF ALLOZYME VARIATION IN DIPLOID AND TETRAPLOID CENTAUREA JACEA AT DIFFERENT SPATIAL SCALES

Abstract.— The extent and spatial patterns of genetic variation at allozyme markers were investigated within and between diploid and autotetraploid knapweeds (Centaurea jacea L. sensu lato, Asteraceae) at contrasted geographic scales: (1) among populations sampled from a diploid‐tetraploid contact zone in the northeastern part of the Belgian Ardennes, and (2) within mixed populations from that zone where diploids and tetraploids coexist. Our data were also compared with a published dataset by Sommer (1990) describing allozyme variation in separate diploid and tetraploid knapweeds populations collected throughout Europe. Genetic diversity was higher in tetraploids. In the Belgian Ardennes and within the mixed populations, both cytotypes had similar levels of spatial genetic structure, they were genetically differentiated, and their distributions of allele frequencies were not spatially correlated. In contrast, at the European scale, diploids and tetraploids did not show differentiated gene pools and presented a strong correlation between their patterns of spatial genetic variation. Numerical simulations showed that the striking difference in patterns observed at small and large geographic scales could be accounted for by a combination of (1) isolation by distance within cytotypes; and (2) partial reproductive barriers between cytotypes and/or recurrent formation of tetraploids. We suggest that this may explain the difficulty of the taxonomic treatment of knapweeds and of polyploid complexes in general.     

Origins and widespread distribution of co-existing Polyploids in Arnica cordifolia (Asteraceae)

BACKGROUND AND AIMS: Polyploidy is a central force structuring genetic diversity in angiosperms, but its ecological significance and modes of origin are not fully understood. This work investigated the patterns of coexistence and molecular relatedness of polyploids in the perennial herb, Arnica cordifolia. METHODS: The local- and broad-scale distributions of cytotypes were analysed using flow cytometry. Samples were collected from both roadside and understorey habitats to test the hypothesis of niche separation between triploids and tetraploids. The nuclear rDNA internal transcribed spacer (ITS) and plastid rpl16 spacer, trnL intron plus trnL-trnF spacer and trnK 3' intron regions were sequenced. KEY RESULTS: Broad-scale sampling established that both triploids and tetraploids were common throughout the range of the species, pentaploids were rare, and diploids were not found. Local-scale sampling revealed coexistence of both triploids and tetraploids within the majority of sites. Triploids and tetraploids were equally represented in the understorey and roadside habitat. Triploids were more variable than tetraploids, but both cytotypes shared polymorphisms in ITS. CONCLUSIONS: Coexistence of cytotypes appears to be the norm in A. cordifolia, but habitat differentiation (roadside vs. understorey) is not supported as a coexistence mechanism. Molecular analyses supported multiple events creating triploids but revealed a lack of variation in the tetraploids. Additionally, sequence polymorphisms in ITS suggested a hybridization event prior to polyploidization.     

Chloroplast DNA phylogeography and cytotype geography in autopolyploid Plantago media

In order to gain insight into the causes of parapatric diploid and tetraploid distributions in Plantago media chloroplast DNA (cpDNA) restriction site polymorphism was studied in 36 European populations. Parapatric distributions are often explained by adaptive differences between cytotypes to an underlying heterogeneity in environmental factors. Alternatively, such distribution patterns may be explained nonadaptively, through frequency-dependant production of hybrids with low fitness. However, nonadaptive explanations have been neglected in polyploid literature. In this study nine chloroplast haplotypes were found. Their phylogeny suggests that tetraploids arose at least three times from diploids. In general, related haplotypes were also geographically clustered, although there were some marked geographical discontinuities. In the Pyrenees, diploids and tetraploids carried diverged haplotypes throughout their parapatric ranges. At the contact zone the level of cpDNA introgression in a mixed diploid-tetraploid population was low. It is discussed that the cpDNA phylogeography supports the nonadaptive hypothesis that parapatric cytotype distributions may be explained by postPleistocene range expansions followed by mutual minority cytotype exclusion, due to hybrid unfitness.     

Diversity and evolution of the Hordeum murinum polyploid complex in Algeria

Population diversity and evolutionary relationships in the Hordeum murinum L. polyploid complex were explored in contrasted bioclimatic conditions from Algeria. A multidisciplinary approach based on morphological, cytogenetic, and molecular data was conducted on a large population sampling. Distribution of diploids (subsp. glaucum) and tetraploids (subsp. leporinum) revealed a strong correlation with a North-South aridity gradient. Most cytotypes exhibit regular meiosis with variable irregularities in some tetraploid populations. Morphological analyses indicate no differentiation among taxa but high variability correlated with bioclimatic parameters. Two and three different nuclear sequences (gene coding for an unspliced genomic protein kinase domain) were isolated in tetraploid and hexaploid cytotypes, respectively, among which one was identical with that found in the diploid subsp. glaucum. The tetraploids (subsp. leporinum and subsp. murinum) do not exhibit additivity for 5S and 45S rDNA loci comparative with the number observed in the related diploid (subsp. glaucum). The subgenomes in the tetraploid taxa could not be differentiated using genomic in situ hybridization (GISH). Results support an allotetraploid origin for subsp. leporinum and subsp. murinum that derives from the diploid subsp. glaucum and another unidentified diploid parent. The hexaploid (subsp. leporinum) has an allohexaploid origin involving the two genomes present in the allotetraploids and another unidentified third diploid progenitor.     

Polyploidy and invasion success: trait trade-offs in native and introduced cytotypes of two Asteraceae species

Invasion success is favoured by the introduction of pre-adapted genotypes. In addition, novel pressures in the introduced range may lead to phenotypic changes related to fitness or competitive ability of introduced plants. Polyploidy appears to be over-represented in invasive plants, but differences between cytotypes in growth strategies including trade-offs among plant traits have received little attention so far in the context of biological invasions. We grew Centaurea stoebe L. and Senecio inaequidens D.C. in a greenhouse experiment to test for differences in fitness (shoot biomass, reproductive output) and competitive ability (vegetative size, specific leaf area, leaf dry matter content, root–shoot ratio) between diploid and polyploid cytotypes as well as between native and introduced plants. For both species, diploid and tetraploid genotypes occur in the native range, whereas only tetraploids are present in the introduced range. In the native range of both species, diploid and tetraploid genotypes had different growth strategies. Tetraploid genotypes of C. stoebe and S. inaequidens had, respectively, higher specific leaf area and stem height than diploid ones. Thus, for both species, native tetraploids appeared more competitive than native diploids, which could explain, at least partially, the invasion success of the pre-adapted tetraploid genotypes. The comparison of native and introduced tetraploid genotypes revealed differences in traits linked to competitive ability, which could be linked to novel selection in the new environment. In S. inaequidens, we found evidence for a competition-colonisation trade-off, whereas persistence of C. stoebe in the new range seemed to be linked to a competition-defence trade-off.