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.     

Distribution and ecology of cytotypes of the Aster amellus aggregates in the Czech Republic

BACKGROUND AND AIMS: Polyploidy is viewed as an important mechanism of sympatric speciation, but only a few studies have documented patterns of distribution and ecology of different cytotypes in their contact zone. Aster amellus agg. (Asteraceae) is one of the species with documented multiple ploidy levels. The aim of this study was to determine spatial distribution and ecology of two cytotypes, diploid (2n = 18) and hexaploid (2n = 54), of Aster amellus agg. at their contact zone in the Czech Republic. METHODS: Root-tip squashes and flow cytometry were used to determine the ploidy of 2175 individuals from 87 populations. To test whether some differences in ecology between the two ploidy levels exist, in each locality relevés were recorded and abiotic conditions of the sites were studied by estimating potential direct solar radiation, Ellenberg indicator values and above-ground biomass. KEY RESULTS: Together with diploid and hexaploids, minorite cytotypes (triploid, pentaploid and nonaploid) were found. No significant ecological differences between diploid and hexaploid cytotypes were found. In spite of this, no population consisting of both of the two basic cytotypes was found. CONCLUSIONS: The results of this study show that the contact zone of diploid and hexaploid cytotypes in the Czech Republic is much more diffuse than indicated in previous records. Although populations of both cytotypes occur in close proximity (the closest populations of different cytotypes were 500 m apart), each individual population consists of only one basic ploidy level. This was unexpected since there are no clear differences in abiotic conditions between populations. Taken together with the absence of an intermediate tetraploid cytotype and with reference to published world distributional patterns of different ploidy levels, this suggests a secondary contact zone. Detailed genetic study is, however, necessary to confirm this.     

Niche differentiation between diploid and hexaploid <Emphasis Type="Italic">Aster amellus</Emphasis>

The maintenance of separated diploid and polyploid populations within a contact zone is possible due to both prezygotic and postzygotic isolation mechanisms. Niche differentiation between two cytotypes may be an important prezygotic isolating mechanism and can be studied using reciprocal transplant experiments. We investigated niche differentiation between diploid and hexaploid Aster amellus in their contact zone in the Czech Republic. Diploid populations are confined to habitats with low productivity, whereas hexaploid populations occur in habitats with both low and high productivity. Thus, we chose three diploid populations and six hexaploid populations, three in each of the two different habitat types. We analyzed habitat characteristics and carried out reciprocal transplant experiments in the field using both seeds and adult plants. Sites of diploid and hexaploid populations differed significantly in vegetation and soil properties. The mean number of juveniles was higher at sites of home ploidy level than at sites of foreign ploidy level, suggesting niche differentiation between the two cytotypes. On the other hand, transplanted adult plants survived at all sites and juvenile plants were able to establish at some sites of the foreign cytotype. Furthermore, the mean number of juveniles, survival, and flowering percentages were higher at home sites than at foreign sites, indicating local adaptation. We conclude that niche differentiation between the two cytotypes and local adaptation within each cytotype may contribute to the maintenance of diploid and hexaploid populations of A. amellus in their contact zone. Moreover, further factors, such as differences in flowering phenology and exclusion of minority cytotypes, should also be considered.     

Mycorrhizal Symbiosis and Local Adaptation in Aster amellus: A Field Transplant Experiment

Many plant populations have adapted to local soil conditions. However, the role of arbuscular mycorrhizal fungi is often overlooked in this context. Only a few studies have used reciprocal transplant experiments to study the relationships between soil conditions, mycorrhizal colonisation and plant growth. Furthermore, most of the studies were conducted under controlled greenhouse conditions. However, long-term field experiments can provide more realistic insights into this issue. We conducted a five-year field reciprocal transplant experiment to study the relationships between soil conditions, arbuscular mycorrhizal fungi and plant growth in the obligate mycotrophic herb Aster amellus. We conducted this study in two regions in the Czech Republic that differ significantly in their soil nutrient content, namely Czech Karst (region K) and Ceske Stredohori (region S). Plants that originated from region S had significantly higher mycorrhizal colonisation than plants from region K, indicating that the percentage of mycorrhizal colonisation has a genetic basis. We found no evidence of local adaptation in Aster amellus. Instead, plants from region S outperformed the plants from region K in both target regions. Similarly, plants from region S showed more mycorrhizal colonisation in all cases, which was likely driven by the lower nutrient content in the soil from that region. Thus, plant aboveground biomass and mycorrhizal colonisation exhibited corresponding differences between the two target regions and regions of origin. Higher mycorrhizal colonisation in the plants from region with lower soil nutrient content (region S) in both target regions indicates that mycorrhizal colonisation is an adaptive trait. However, lower aboveground biomass in the plants with lower mycorrhizal colonisation suggests that the plants from region K are in fact maladapted by their low inherent mycorrhizal colonization. We conclude that including mycorrhizal symbiosis in local adaptation studies may increase our understanding of the mechanisms by which plants adapt to their environment.     

Biology of the polyploid geophyte Allium oleraceum (Amaryllidaceae): Variation in size,sexual and asexual reproduction and germination within and between tetra-, penta- and hexaploid cytotypes

It has been proposed that plant species cytotypes commonly exhibit altered morphology, reproduction, geographic and ecological distributions. We studied phenotypic variation in height, sexual (flowers, seeds) and asexual (aerial bulbils) reproduction in natural populations and in the conditions of a common garden of three cytotypes (2n = 4x, 5x, 6x) of the bulbous geophyte Allium oleraceum in the Czech Republic. Additionally, we compared the germination and dormancy of seeds and bulbils to determine whether propagules have different ecological roles. The pattern of morphological differentiation observed between cytotypes under natural conditions was similar to that observed under common garden conditions, suggesting that variability in morphological characteristics appears to be associated with ploidy levels. We revealed differences in size and sexual and asexual production between A. oleraceum cytotypes, but with wide overlap among cytotypes, suggesting a limited possibility of the studied traits to reliably distinguish between cytotypes. Tetraploids and pentaploids were rather similar; they were taller and produced more flowers than hexaploids, which were mostly flowerless (mean <0.7 flower/plant). All cytotypes were able to produce viable seeds, but their numbers were extremely low, usually less than 3 seeds per inflorescence; clonal reproduction via aerial bulbils dominates in all cytotypes (flower:bulbil ratio <0.5), with tetraploids producing more but lighter bulbils than other cytotypes. The seed: ovule ratio was low (<0.1) in all cytotypes, although hexaploids reached higher values than other cytotypes. Bulbils germinated better (means >80%) than seeds in all cytotypes, with pentaploid bulbils showing the highest germination (mean 90.5%). The cytotypes did not differ in seed germination (range of means 73.4%–76.3%). About 6% of seeds did not germinate but were still viable at the end of the experiment, while all non-germinated bulbils of all cytotypes had rotten away. Seeds, but not bulbils, can likely form a short-term persistent propagule soil bank. We found no evidence of a phenotypic trade-off between the production of flowers (seeds) and the production of bulbils within the inflorescence of all studied cytotypes. Together, inter-cytotypic differences in fitness-related traits cannot completely explain the different geographic and ecological distributions of cytotypes in the Czech Republic found by previous research.     

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.     

Breeding barriers at a diploid–hexaploid contact zone in <Emphasis Type="Italic">Aster amellus</Emphasis>

Polyploidization is an important mechanism of sympatric speciation, but few studies have addressed breeding barriers between polyploids and their diploid progenitors in the field, and the available data have been mainly obtained from diploid-tetraploid contact zones. In contrast to diploid-tetraploid complexes, hybridization between diploid and hexaploid individuals may lead to viable fertile tetraploid offspring, and thus the interactions between these ploidy levels can be more complex. We investigated the breeding barriers operating between diploid and hexaploid individuals of Aster amellus at a contact zone in Central Europe to understand the absence of hybrids (i.e., tetraploids) and mixed populations. Phenological segregation, assortative mating mediated by pollinators and crossing ability were assessed under natural and controlled conditions in diploid and hexaploid populations growing in close proximity. The results revealed low levels of reproductive isolation (RI) due to flowering phenology (RI = 11–45%) and pollinator behavior (RI = 17%), so that pollen transfer between diploids and hexaploids is possible. In contrast, almost complete reproductive isolation was observed due to a series of post-pollination barriers that significantly reduced the production of offspring from inter-cytotype crosses (RI = 99.9%), even though some tetraploids were detected in seeds and seedlings. We conclude that the absence of tetraploids at the contact zone is probably due to a combination of several factors, including spatial segregation, strong post-pollination barriers (such as gametic isolation, low viability of tetraploid seeds and/or inability of tetraploid plants to reach the flowering stage), and to a lesser extent, temporal and behavioral segregation. Future studies should explore the fitness of tetraploids and the effect of different traits on the reproductive success and fitness of each cytotype. This will enable a fuller understanding of the dynamics and mechanisms acting in contact zones.     

Neopolyploidy in Spartina pectinata Link: 1. Morphological analysis of tetraploid and hexaploid plants in a mixed natural population

Prairie cordgrass has been reported as a multi-polyploidy species having three cytotypes: tetra- (2n?=?4x?=?40), hexa- (2n?=?6x?=?60), and octoploid (2n?=?8x?=?80). A mixed-ploidy population comprising tetraploids and hexaploids was recently found at a single location in Illinois. However, adaptation and morphological differences between tetra- and hexaploids occurring in natural conditions as well as the contact zones of these cytotypes have yet to be determined. In this study, the cytotypes of 147 individuals of prairie cordgrass collected across the contact zone (4x?+?6x) were determined by flow cytometry using somatic G1 nuclei, and the results were confirmed by chromosome counts. Nineteen morphological characteristics were compared between the cytotypes. Tetra- and hexaploid plants have 2C genome sizes of 1.57 and 2.36?pg with chromosome counts of 40 and 60, respectively. This increase in polyploidy resulted in a greater variability of morphological expression in Illinois prairie cordgrass. Substantial differences in the flowering time, stomatal size, and plant morphological characteristics were observed between tetra- and hexaploids. The results indicate that the increasing of ploidy level in prairie cordgrass resulted in increased plant size in ploidy mixtures. The recent event of ploidy mixtures in prairie cordgrass natural populations offers unique opportunities for studying the formation and establishment of neopolyploidy.     

Sympatric occurrences of various cytotypes of Vaccinium sect. Oxycoccus (Ericaceae)

Flow cytometry was employed to estimate ploidy levels in Vaccinium sect. Oxycoccus. Twelve localities in the Czech Republic, Sweden and Denmark were studied in detail in order to investigate the potential sympatric growth of various cytotypes. Diploid V. microcarpum and tetraploid, pentaploid and hexaploid individuals of V. oxycoccos were found among the 296 accessions analyzed. Both species as well as various (up to all three) ploidy levels of V. oxycoccos co-occurred at the same locality. Mixed accessions (containing various cytotypes) were even ascertained on a very fine spatial scale (20 × 20 cm). Pentaploid plants of probable hybrid origin have been found in Sweden for the first time. The population growing on peat bog near Lyngby, Copenhagen (Denmark) consisted of both hexaploids and tetraploids. This site refers to the discovery of hexaploid cranberries in the late twentieths of the last century. The results represent further support for merging of all polyploid cytotypes (4x—6x) into a single taxon.     

An experimental investigation of life history and plasticity in two cytotypes of Polygonum aviculare L. subsp. aviculare that coexist in an abandoned arable field

Polygonum aviculare subsp. aviculare is an annual selfing weed common in abandoned arable fields where it occurs as a widespread hexaploid cytotype (6x=60) and a rarer tetraploid cytotype (4x=40). The basis of phenological differentiation between the two cytotypes observed in a natural population where they coexist was examined in a greenhouse experiment comprising six soil conditions consisting of factorial combinations of two levels of fertility and three pot sizes. The environmental and genetic component of variation in 11 life history and morphological traits was quantified. Even though all traits except life span were plastic the two cytotypes appear to have evolved contrasting life history strategies and it is inferred that this can account for the temporal niche differentiation observed in the abandoned field during the first year of dereliction. Tetraploids are short-lived plants allocating a high proportion of their biomass to reproduction and completing their life cycle before July when the plant cover is sparse. Hexaploids are larger, later flowering, longer lived, plants with a lower reproductive effort and a higher final seed yield; it is inferred that these traits enable the hexaploids to compete successfully with the dense herbaceous layer of summer annuals that develops in the course of the first year of secondary succession. Differentiation in phenotypic plasticity between the two cytotypes was interpreted as indicative of higher opportunism and lower tolerance of poor soils and restricted rooting space in the hexaploid compared to the tetraploid cytotype.     

Genome size discriminates between closely related taxaElytrigia repens andE. intermedia (Poaceae: Triticeae) and their hybrid

Flow cytometric and karyological investigations were performed on the closely related taxaElytrigia repens andE. intermedia (Poaceae: Triticeae) from the Czech Republic. DNA-hexaploids clearly prevailed among 238 examined plants and amounted to 96.2% of all samples. 2C-values ± s.d. for hexaploidElytrigia repens andE. intermedia were estimated at 23.27 ± 0.20 pg and 27.04 ± 0.24 pg respectively. Genome size thus allowed reliable separation of the two species (difference ca. 16%) as well as the identification of hybrid individuals. Natural hybridization inE. repens — E. intermedia alliance seems to be quite a common phenomenon as indicated from a large proportion (one sixth) of hexaploid samples with intermediate 2C-values. Previously, the crosses were most probably overlooked or misidentified due to their weak morphological differentiation. New nonaploid cytotypes (2n=9x=63) were revealed for both species as well as for the hybrid (determined on the basis of morphological characters only), representing the first records from the field. Fusion of unreduced and reduced gametes of the hexaploids is the most plausible mode of nonaploid origin.     

Ecological differentiation,lack of hybrids involving diploids,and asymmetric gene flow between polyploids in narrow contact zones of Senecio carniolicus (syn. Jacobaea carniolica,Asteraceae)

Areas of immediate contact of different cytotypes offer a unique opportunity to study evolutionary dynamics within heteroploid species and to assess isolation mechanisms governing coexistence of cytotypes of different ploidy. The degree of reproductive isolation of cytotypes, that is, the frequency of heteroploid crosses and subsequent formation of viable and (partly) fertile hybrids, plays a crucial role for the long‐term integrity of lineages in contact zones. Here, we assessed fine‐scale distribution, spatial clustering, and ecological niches as well as patterns of gene flow in parental and hybrid cytotypes in zones of immediate contact of di‐, tetra‐, and hexaploid Senecio carniolicus (Asteraceae) in the Eastern Alps. Cytotypes were spatially separated also at the investigated microscale; the strongest spatial separation was observed for the fully interfertile tetra‐ and hexaploids. The three main cytotypes showed highly significant niche differences, which were, however, weaker than across their entire distribution ranges in the Eastern Alps. Individuals with intermediate ploidy levels were found neither in the diploid/tetraploid nor in the diploid/hexaploid contact zones indicating strong reproductive barriers. In contrast, pentaploid individuals were frequent in the tetraploid/hexaploid contact zone, albeit limited to a narrow strip in the immediate contact zone of their parental cytotypes. AFLP fingerprinting data revealed introgressive gene flow mediated by pentaploid hybrids from tetra‐ to hexaploid individuals, but not vice versa. The ecological niche of pentaploids differed significantly from that of tetraploids but not from hexaploids.     

Parental Ploidy Strongly Affects Offspring Fitness in Heteroploid Crosses among Three Cytotypes of Autopolyploid Jacobaea carniolica (Asteraceae)

Reproductive interactions among cytotypes in their contact zones determine whether these cytotypes can co-exist and form stable contact zones or not. In autopolyploids, heteroploid cross-compatibilities might depend on parental ploidy, but tests of this hypothesis in autopolyploid systems with more than two ploidies are lacking. Here, we study Jacobaea carniolica, which comprises diploid, tetraploid, and hexaploid individuals regularly forming contact zones. Seeds obtained from in situ cross-pollinations within and among cytotypes were subjected to DNA flow cytometry and greenhouse germination experiments. Hybrid fitness and parental effects on hybrid fitness were tested with regression models comparing fitness parameters of early life stages. Irrespective of the direction of crosses, seed viability and seedling survival in diploid-polyploid crosses were substantially lower than in tetraploid-hexaploid crosses. In contrast, seedling growth traits indicated neither transgressive character expression nor any selection against hybrid offspring. Congruent with a model of genome dosage effects, these traits differed between reciprocal crosses, especially of diploids and tetraploids, where trait values resembled those of the maternal parent. The strong effect of parental ploidy on offspring fitness in heteroploid crosses may cause contact zones involving exclusively polyploid cytotypes to be less stable over longer terms than those involving diploids and polyploids.     

Occurrence of tetraploid and octoploid cytotypes in Senecio jacobaea ssp. jacobaea (Asteraceae) in Pannonia and the Carpathians

Four different cytotypes have been reported for Senecio jacobaea L. ssp. jacobaea throughout Europe, with the most common occurrence of tetraploids (2 n  = 40). Here we present a survey of previously published chromosome number data on this subspecies and its geographical distribution, and focus on populations from Pannonia and the Carpathians. Two ploidy levels have been determined in the study area, using chromosome counting and flow cytometry: tetraploid (2 n  = 40) and octoploid (2 n  = 80). Fifty-one populations originating from Slovakia, Austria, the Czech Republic, Hungary, Ukraine and Romania have been analysed. Multivariate morphometric analyses have been performed on 39 populations to study morphological differentiation between these two cytotypes. Despite slight morphological tendencies expressed on the level of populations, tetraploid and octoploids cannot be reliably distinguished morphologically and they are not taxonomically classified formally here.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153 , 231–242.     

No effect of ploidy level in plant response to competition in a common garden experiment

One of the important aspects explaining the evolutionary success of polyploid plants is that polyploids often, although not always, occupy a wider range of environments than their diploid ancestors. The two most likely explanations for this pattern are a wider plasticity in polyploid species and the existence of a range of locally adapted types. Most studies on patterns of distribution of different ploidy levels are only observational, and do not distinguish between these alternative explanations. The present study investigated the performance and plasticity of diploid and hexaploid cytotypes of a perennial plant, Aster amellus . The hexaploid plants occur in habitats with a wider range of competition intensity. Nine different populations of the species were selected: three diploid (from low competition habitats) and six hexaploid (from both low and high competition habitats). Plants were grown from seeds from these populations with and without competition in a common garden. Competition had strong effect on plant performance. There was, however, little effect of the ploidy levels and home environment. There was also almost no interaction between competition and ploidy level/home environment. The results of this study provide no support for any of the two suggested explanations for the wider range of habitats occupied by hexaploid plants. Other explanations thus must be thought. Generally, the results indicate that, although the higher plasticity of higher ploidy levels is often suggested, this may not be true. We therefore should attempt to assemble more experimental data to support or reject this assumption.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 211–219.     

Variation in DNA-ploidy Levels of Reynoutria Taxa in the Czech Republic

The genus Reynoutria is represented by four taxa in the Czech Republic: Reynoutria japonica var. japonica, R. japonica var. compacta, R. sachalinensis and R. xbohemica. By using flow cytometry, cytological variability within the genus is described based on 257 Reynoutria samples. The varieties of R. japonica are cytologically uniform, var. japonica is exclusively octoploid (2n = 8x = 88) and var. compacta occurs only as a tetraploid (2n = 4x = 44), but R. sachalinensis and R. xbohemica exhibit some variation in chromosome numbers. Reynoutria sachalinensis is predominantly tetraploid (2n = 4 x = 44), but also occurs occasionally as hexaploid and octoploid cytotypes. The most common ploidy level in R. xbohemica is hexaploid (2n = 6x = 66), but tetraploid and octoploid clones were also found. The four taxa occurring in the Czech Republic are described briefly and the possible origins of the cytotypes discussed.     

Cytotype segregation on regional and microgeographic scales in snow buttercups (Ranunculus adoneus: Ranunculaceae)

Polyploid speciation is an important source of angiosperm diversity. Insights into the origin and establishment of new polyploid species may be gained by studying the distributions of ancestral and derivative cytotypes at multiple spatial scales. Diploid (2n = 16) and tetraploid (2n = 32) snow buttercups (Ranunculus adoneus: Ranunculaceae) occur in the alpine of the central and southern Rocky Mountains. Root-tip squashes and flow cytometry were used to determine the ploidy of 1618 individuals from 35 populations. Samples from 31 of the 35 sites were entirely of one cytotype, either diploid or tetraploid. Diploid and tetraploid snow buttercups have nonoverlapping regional distributions. Where both cytotypes occur on the same site, the two are spatially segregated despite no apparent change in habitat. Triploid snow buttercups were only found at a diploid/tetraploid contact zone, while two hexaploid plants were found in tetraploid populations. Tetraploid establishment once or twice in the history of the species complex could account for the regional distribution of the two cytotypes. Habitat differentiation between cytotypes or reproductive exclusion of minority cytotypes may explain the observed segregation at both microgeographic and regional scales.     

A new cytotype of Jacobaea vulgaris (Asteraceae): frequency,morphology and origin

Jacobaea vulgaris subsp. vulgaris (syn. Senecio jacobaea subsp. jacobaea) constitutes an intricate polyploid complex distributed in Europe. Four cytotypes have been reported in this species, three with euploid (diploid, tetraploid and octoploid; 2n=20, 40 and 80) and one with aneuploid (2n=32) chromosome numbers. Here we report that the diploid chromosome number (2n=20) reported from Bulgaria is due to misidentification with Jacobaea aquatica. On the other hand, we have discovered a new, hexaploid (2n=6x=60) cytotype within J. vulgaris subsp. vulgaris using flow cytometry. The new cytotype occurs within four sympatric populations of otherwise tetraploid and octoploid plants in Pannonia (one locality in the eastern Czech Republic and two localities in southwestern Slovakia) and in Podillya (one locality in western Ukraine). The frequency of hexaploid individuals within 76 studied populations is very low (only 10 of 693 analysed plants), and hexaploids probably represent hybrids between tetraploid and octoploid plants. Three mixed populations with hexaploid plants were subjected to detailed morphological and pollen fertility analyses. Multivariate morphometric analysis reveals partial separation of tetraploid and octoploid plants, whereas hexaploid individuals are similar in morphology to octoploids. In comparison with tetraploids, octoploids and hexaploids exhibit slightly longer ray florets, involucral bracts and tubular florets and more hairy outer achenes. Hexaploid plants display larger pollen grains and lower pollen fertility compared to tetraploids and octoploids.     

PATTERNS OF VARIATION IN THE STELLARIA LONGIPES COMPLEX: EFFECTS OF POLYPLOIDY AND NATURAL SELECTION

Patterns of morphological variation were studied in herbarium specimens of Stellaria longipes, an herbaceous perennial and subsequently in a growth chamber experiment using three cytotypes (4x, 6x, 8x) of S. longipes and diploids of its proposed progenitor S. longifolia. Despite extensive phenotypic plasticity in many traits, patterns of variation resulting from ecotypic differentiation within S. longipes could be detected in the field. A distinct form of S. longipes, which is restricted primarily to arctic and alpine tundra locations, shows genetic differentiation for the following traits: few flowers per ramet, a low proportion of flowering ramets, and ovate leaves. The three cytotypes of S. longipes could be distinguished by their mean genotypic value for leaf length and number of flowers per ramet. The extent of phenotypic plasticity in these traits makes it unlikely that the cytotypes could be distinguished in the field. The direction and extent of morphological divergence between S. longifolia and S. longipes suggest an alloploid origin for S. longipes. Variational trends (among-habitat types and cytotypes) in trait means are similar to those reported previously for the pattern of plasticity. This supports the argument that similar forces guide evolution of the mean and pattern of plasticity of a trait.     

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.