Differential effects of reproductive interference by an alien congener on native Taraxacum species

Reproductive interference (RI) has been suggested to play a critical role in native plant displacement by alien congeners. However, although co-existence of native and alien congeners may provide an opportunity to refute the RI hypothesis, few studies have examined such a case. Using a native Japanese dandelion, Taraxacum longeappendiculatum, and a co-existing alien congener, Taraxacum officinale, we tested the hypothesis that differences in RI by the alien between native recipient congeners explain whether a native will co-exist with or be displaced by an alien. We conducted a field survey to investigate the effects of alien relative abundance on T. longeappendiculatum seed set, and a hand-pollination experiment to identify the extent of pollen interference by the alien on T. longeappendiculatum. We compared these results with those obtained previously for another Japanese native species, Taraxacum japonicum, which was displaced by the alien. In our field survey, alien relative abundance had little effect on seed set in nearby T. longeappendiculatum, and hand-pollination with mixed pollen grains produced no substantial decrease in seed set of the native species. Model selection supported these tendencies; the effect of RI by the alien differed between the two native species. Other potential factors, such as resource competition or habitat changes, could not explain the co-existence of T. longeappendiculatum with and exclusion of T. japonicum by the alien in the same explanatory framework. Considering the consistent explanatory power, the findings suggest that RI is a critical mechanism that can determine both co-existence with and displacement of native dandelions by an alien congener.     

Alien pollen grains interfere with the reproductive success of native congener

The effects of invasive species on native species comprise important conservation issues. Determining the mechanisms by which invasives exclude natives is indispensable to efficiently control their impact, but most invasives remain poorly studied. The purpose of this study was to elucidate potentially important but neglected mechanisms, reproductive interference, in wild Taraxacum systems, in which invasive Taraxacum officinale has displaced its native congener T. japonicum in Japan. Hand-pollination of mixed pollen grains significantly reduced the native seed-set compared to conspecific-only pollination. Moreover, natives with a high ratio of invasive pollen on their stigmas suffered severe seed-set reduction, and the proportion of invasive pollen on native stigmas increased as frequencies of the alien neighbor increased. These results, combined with those of previous studies, revealed that depositing invasive pollen on native stigmas contributes to the observed alien-frequency-dependent reduction of native seed-set, and strongly suggest that reproductive interference was the primary cause of displacement in the Taraxacum systems.     

Effective range of reproductive interference exerted by an alien dandelion, <Emphasis Type="Italic">Taraxacum officinale</Emphasis>, on a native congener

Reproductive interference (RI), defined as the fitness cost of interspecific sexual interactions, such as interspecific pollen transfer (IPT) in plants, is ecologically important. Theoretically, RI could result in competitive exclusion, as it operates in a frequency-dependent manner. Additionally, IPT may have a greater range than resource competition, although information about the range of IPT is lacking. In the present study, we measured the range of IPT exerted by Taraxacum officinale (an alien species) on a native dandelion, T. japonicum. We used two approaches. In one, we analyzed the RI effect on a native seed set at three spatial scales. In the second, we tracked IPT from alien to native flower heads using fluorescent pigments as markers. We estimated that pollination distances were in the order of several meters. These distances exceeded the mean distance from each native plant to the nearest alien. As hypothesized, the effect of RI reached farther than neighboring individuals. These data indicate the spatial range from which alien dandelions should be removed to allow the conservation of natives.     

An invasive dandelion unilaterally reduces the reproduction of a native congener through competition for pollination

The impact of invasive alien species on native species is of increasing global concern. Invasive plants have various negative effects on natives through competition; however, relatively little is known about competition for pollination. The relationship between Japanese native dandelions (Taraxacum spp.) and invasive congeners may be a typical case of such an interaction. For example, native dandelions are being replaced by invasive congeners, especially in urban and suburban areas of Japan. To explain this phenomenon, we hypothesized that when natives are mixed with attractive invasives, natives may suffer from reduced seed set because invasives deprive natives of pollinators or because pollinators frequently move between species, resulting in interspecific pollen transfer. To test this hypothesis, we studied the effect of the invasive dandelion T. officinale on the pollination and seed set of the native T. japonicum using artificial arrays of monospecific and mixed-species plots as well as natural populations. Taraxacum officinale attracted more pollinator visits, perhaps because it produced more nectar than T. japonicum. The number of pollinator visits to T. japonicum was reduced when the congeners were grown together, and pollinators moved frequently between the two species. The proportion of seed set for T. japonicum was reduced in the presence of T. officinale in both artificial arrays and natural populations. These results support our hypothesis that interspecific competition for pollination plays an important role in the recent replacement of native dandelions by invasive congeners in Japan. Because invasive dandelions are apomicts, negative effects are incurred only by sexual natives. Thus, this system can be recognized as a rare case of interspecific interaction through pollination.     

Comparisons among populations and individuals to evaluate pollen–pistil interaction as a mechanism of reproductive interference in Taraxacum

Reproductive interference (RI), an interspecific mating interaction that reduces the fitness of at least one of the species involved, can lead to exclusive distributions in closely related species. A hypothesis previously proposed is that RI in plants may occur by ovule usurpation, in which pistils lack interspecific incompatibility and mistakenly accept heterospecific pollen, thereby losing an opportunity for conspecific pollen fertilization. However, few comparative studies have evaluated the consistency of the inferred mechanism within and among individuals and populations. We conducted hand-pollination experiments in six populations of three native Taraxacum species that suffered from different levels of RI from an alien congener, T. officinale, and compared pollen–pistil interactions among populations. We also investigated the interactions for eight individual T. japonicum plants whose response to heterospecific pollen deposition had been previously measured. Our results revealed that pollen tubes often penetrated native ovaries following heterospecific pollination in populations suffering from strong RI, whereas they seldom did in populations suffering from marginal RI. However, the relative frequency of the pollen tube penetration was not significantly related to the strength of alien RI. Not all pistils on an individual plant showed the same pollen receptivity following heterospecific pollination; rather, some accepted and some refused the pollen tubes. The relationship between pollen tube penetration following heterospecific pollination and the strength of the alien RI was also not significant among individuals. Our present results generally support the ovule usurpation hypothesis, but suggest that other factors, such as competition for pollinator services, variation in the effects of heterospecific pollen donors, and condition of the native inflorescences, might also affect the observed RI strength.

     

Nurse effect of the native cushion plant Azorella monantha on the invasive non-native Taraxacum officinale in the high-Andes of central Chile

Positive interactions among native plant species are common in alpine habitats, particularly those where one species (nurse plant) generates microclimatic conditions that are more benign than the surrounding environment, facilitating the establishment of other species. Nonetheless, these microclimatic conditions could facilitate the establishment of non-native species as well. A conspicuous component of the alien alpine flora of the central Chilean Andes is the perennial herb Taraxacum officinale agg. (dandelion). In contrast to other alien species that are restricted to human-disturbed sites, T. officinale is frequently observed growing within native plant communities dominated by cushion plants. In this study we evaluated if T. officinale is positively associated with the cushion plant Azorella monantha. Via seedling survival experiments and gas-exchange measurements we also assessed the patterns of facilitation between cushions and dandelions, and explore the potential mechanisms of invasion by dandelions. T. officinale grows spatially positively associated with cushions of A. monantha. Survival of seedlings, as well as their net-photosynthetic rates and stomatal conductance, were higher within cushions than in open areas away from them, suggesting that the microclimatic modifications generated by this native cushion facilitates the establishment and performance of a non-native invasive species. Our results, as well as other recent studies, highlight the role of native communities in facilitating rather than constraining non-native plant invasions, particularly in stressful habitats such as alpine environments.     

Hybridization rate and genotypic diversity of apomictic hybrids between native (<Emphasis Type="Italic">Taraxacum japonicum</Emphasis>) and introduced (<Emphasis Type="Italic">T. officinale</Emphasis>) dandelions in western Japan

Hybridization between the introduced and native plants may enhance invasiveness, especially in asexually reproducing species. Hybrid apomictic dandelions between native (Taraxacum platycarpum and T. japonicum) and exotic (T. officinale) species are distributed widely throughout Japan. To estimate the origin(s) and dispersal of the hybrids, we investigated the hybridization rate and genotypic diversity in mixed populations of T. japonicum, T. officinale and their hybrids at two green parks in western Japan. Among the plants identified as exotics from flower morphology, 86–96% were hybrids by genetic analysis. Genetic data with simple sequence repeat markers revealed a high clonal diversity of the hybrid both within and between populations, indicating multiple origins. A hybrid seed was found from among the 1891 seeds collected from T. japonicum in the parks, indicating ongoing hybridization in the field. T. officinale and hybrids were genetically differentiated between the two parks independent of the ploidy level; the allele frequency of T. officinale and tri- and tetraploid hybrids were similar within each park but different between the two parks. This suggests that the origins of hybrids were similar within the park but different between the parks. Overall, our results suggest that hybridization, including backcross, is an ongoing process, and that genetically diverse hybrids with various origins have been spreading in western Japan, probably because hybridization enhanced invasiveness at native habitat.     

Photonastic and thermonastic opening of capitulum in dandelion,Taraxacum officinale andTaraxacum japonicum

The capitula ofTaraxacum officinale andT. japonicum open in response to temperature rise at lower temperatures (thermonasty), and in response to light at higher temperatures (photonasty), as was the case inT. albidum. The capitula ofT. officinale could respond to the same temperature rise more sensitively than those ofT. albidum orT. japonicum. The minimum temperature for photonastic opening is as low as 13 C forT. officinale, while that forT. albidum andT. japonicum is about 18 C. That is why the capitula ofT. officinale opened earlier than those ofT. albidum andT. japonicum in the morning in April under natural conditions. The capitulum continued to be open for about 13–14 hr inT. officinale and about 8–11 hr inT. japonicum and inT. albidum both under natural conditions in April and even under constant light-temperature conditions, suggesting that the time of capitula-closing in these three species is not controlled by changes in environmental factors (light and/or temperature).     

Formation of a hybrid triploid agamosperm on a sexual diploid plant: evidence from progeny tests in Taraxacum platycarpum Dahlst.

Agamospermous dandelions of hybrid origin between a native sexual diploid species (Taraxacum platycarpum Dahlst. or T. japonicum Koidz.) and an alien agamospermous triploid [T. officinale Weber and T. laevigatum (Willd.) DC.] are now widely distributed throughout mainland Japan. These hybrid dandelions are known to be genetically variable. We hypothesized that this variability is maintained by repeated ongoing hybridization, based on the fact that triploid dandelions not only produce seeds agamospermously, but also produce some functional pollen grains that are able to sire seeds of sexual dandelions. To test this hypothesis, we examined whether heads of Japanese diploid dandelions produce new hybrid seeds after fertilization by pollen from triploid agamosperms under field conditions. One of the 430 tested plants grown from sexual dandelion seeds had morphological and molecular characteristics, which are consistent with a hybrid origin. The plant formed a hybrid surrounded by many individuals having recurved involucral bracts, in which frequency of T. officinale was very low (3.5 %). Cytological data and bagging experiments demonstrate triploidy and asexual seed production of the hybrid. Taken together, these results supported that the new hybrid is probably derived from a backcross of a hybrid to the native sexual species. Our findings provide evidence for the evolution of a new agamosperm through interspecific hybridization as a contemporary population process.     

Latitudinal variation in suppression of flower bud formation at high temperature in four native dandelions (Taraxacum spp.) with different distributions and genetic structures in Japan

The control of the response of flowering to temperature plays a key role in successful range‐expansion of plants. A previous study showed that the suppression of flower‐bud formation at high temperature in Taraxacum officinale decreases genetically with latitude from north to south in Japan. The present study investigated whether similar trait variation occurs among populations of native Taraxacum species in Japan. Seedlings of T. albidum (a low‐ and mid‐latitude allopolyploid), T. japonicum (a mid‐latitude diploid) and T. venustum (a high‐latitude autopolyploid) were grown at three temperatures. Time to flower‐bud appearance increased with temperature in T. japonicum and T. venustum, but did not increase in T. albidum. Time to flower‐bud appearance did not differ significantly among the three species at 14°C, but it was shorter in T. albidum than in the other two species at 19°C and 24°C. The early appearance of buds of T. albidum was confirmed by another experiment in which plants of 18 populations from the three species and T. platycarpum (a mid‐latitude diploid) grown at 19°C were used. The results clearly indicate that high‐temperature suppression of flower‐bud formation was lower in low‐latitude species than in high‐latitude species. This interspecific variation is analogous to the intraspecific variation in T. officinale. Time to bud appearance of five populations in T. albidum was homogeneous within and between the populations. The results suggest that the five populations are monoclonal and lack the sensitivity of suppression of flower‐bud formation to high temperature.     

Seed dispersal of alien and native plants by vertebrate herbivores

Seed dispersal is crucial for the success and spread of alien plants. Herbivores often establish a dual relationship with plants: antagonist, through herbivory, and mutualist, through seed dispersal. By consuming plants, herbivores may disperse large amounts of seeds, and can facilitate the spread of alien plants. However, seed dispersal of alien plants by herbivores has been largely uninvestigated. I studied factors associated with dispersal of alien and native seeds by the three most important vertebrate herbivores in SW Australia: emus (Dromaius novaehollandia), western grey kangaroos (Macropus fuliginosus) and European rabbits (Oryctolagus cuniculus). Overall frequencies of alien and native seeds dispersed by these herbivores were determined by differences among them in (1) the plant groups they predominantly disperse, that differed in frequencies of aliens versus natives, and (2) the predominant dispersal of aliens or natives within those plant groups. Emus and kangaroos (natives) tended to disperse predominantly alien seeds within plant groups (defined by life forms, dispersal syndromes, and diaspore size), whereas rabbits (alien) tended to disperse predominantly natives. This agrees with the hypothesis that herbivores will use predominantly plants that have evolved in different areas, because of less effective defences against new enemies. Overall frequencies were consistent with this pattern in kangaroos and rabbits, but not in emus. Kangaroos dispersed mostly plant groups that were mainly aliens (herbaceous species and small and medium sized dispersal units and seeds), which together with their predominant use of aliens over natives within groups resulted in the highest overall frequency of alien seeds (73%). Rabbits were similar to kangaroos in the type of plants dispersed, but their predominant use of natives over aliens within groups contributed to an overall predominance of native seeds in their pellets (88%). Emus dispersed mostly plant groups that were mainly natives (e.g. woody species with big diaspores), resulting in low overall frequency of alien seeds (11%), despite their predominant use of aliens over natives within plant groups. Thus, the within-groups trend pointed to a facilitative role of native herbivores of plant invasions through seed dispersal, but was obscured by the different use by herbivores of plant groups with different frequency of aliens.     

Ecophysiological plasticity and local differentiation help explain the invasion success of Taraxacum officinale (dandelion) in South America

Plasticity and local adaptation have been suggested as two main mechanisms that alien species use to successfully tolerate and invade broad geographic areas. In the present study, we try answer the question if the mechanism for the broad distributional range of T. officinale is for phenotypic plasticity, ecotypic adaptation or both. For this, we used individuals of T. officinale originated from seeds collected in five localities along its latitudinal distribution range in the southern‐hemisphere. Seedlings were acclimated at 5 and 25°C for one month. After the acclimation period we evaluated ecophysiological and cytogenetic traits. Additionally, we assessed the fitness at each temperature by recording the seed output of individuals from different localities. Finally, we performed a manipulative experiment in order to assess the tolerance to herbivory and competitive ability between T. officinale from all origins and Hypochaeris scorzonerae a co‐occurring native species. Overall, individuals of T. officinale showed high plasticity and ecotypic adaptation for all traits assessed in this study. Changes both in physiology and morphology observed in T. officinale from different origins were mostly correlated, enhancing their ecophysiological performance in temperatures similar to those of their origin. Additionally, all localities showed the same chromosome number and ploidy level. On the other hand, all individuals showed an increase the seed output at 25°C, but those from northern localities increased more. T. officinale from all origins was not significantly affected by herbivory while native showed a negative effect. On the other hand, T. officinale exerted a strong negative effect on the native species, but this former not effected significantly to the invasive T. officinale. High plasticity and local adaptation in all ecophysiological traits, seed‐set and the low cytogenetic variability in T. officinale suggests that both strategies are present in this invasive plant species and are not mutually exclusive. Finally, higher tolerance to herbivory and competitive ability suggests that T. officinale could perform successfully in environments with different climatic conditions, and thus colonize and invade South‐America.     

Higher plasticity in ecophysiological traits enhances the performance and invasion success of Taraxacum officinale (dandelion) in alpine environments

Phenotypic plasticity has long been suggested to facilitate biological invasions in changing environments, allowing a species to maintain a good ecophysiological performance. High-mountain habitats have been particularly useful for evaluation of the relative importance of environmental conditions in the colonization and invasion process, because they have heterogeneous and stressful climatic conditions, inducing photoinhibition. Light intensity is one of the most changing conditions along altitudinal gradients, showing more variability in higher altitudes. In this study, we analyzed the plasticity in photoprotective strategies and performance of the invasive Taraxacum officinale. Additionally, we tested whether higher plasticity enhances competitive ability in an alpine environment We conducted an experiment to evaluate plasticity with a second generation (F2) of T. officinale individuals from 1,600 to 3,600 m, in a greenhouse with variation in light intensity. Treatments consisted of transferring 120 individuals from each altitude to two conditions of light intensity. We then recorded concentrations of photoprotection pigment, de-epoxidation state of the xanthophyll cycle, foliar angles, photochemical efficiency by fluorescence of photosystem II, total dry biomass and flower production. Additionally, we compared plasticity in both photoprotective and performance traits between T. officinale and the co-occurring native species Hypochaeris thrincioides. Finally, we performed a manipulative experiment under two light regimes in order to assess the competitive outcome between the invasive T. officinale and the native H. thrincioides. Individuals from higher altitude showed significantly greater plasticity than individuals from lower altitude. Similarly, individuals under high light intensity showed higher levels of photoprotective pigments, biomass and flower production. On the other hand, the invasive plant species showed significantly greater plasticity than the co-occurring native species, and a strong negative impact on the biomass of the native plant. Phenotypic plasticity seems to be a successful strategy in T. officinale to compete with native species and may be positively associated with the success of invasions, being greater in individuals from more heterogeneous and stressful environments.     

Prezygotic barriers to gene flow between <Emphasis Type="Italic">Taraxacum ceratophorum</Emphasis> and the invasive <Emphasis Type="Italic">Taraxacum officinale</Emphasis> (Asteraceae)

Prezygotic reproductive barriers limit interspecific gene flow between congeners. Here, I examine the strength of floral isolation and interspecific pollen-pistil barriers between an invasive apomictic, Taraxacum officinale, and the indigenous sexual alpine dandelion, Taraxacum ceratophorum. Experimental arrays of either native inflorescences or a mixture of native and exotic inflorescences were used to examine insect preference and to track movement of a pollen analog. Using hand-pollinations, conspecific and heterospecific pollen germination success on native stigmas was compared. To additionally test for interspecific pollen competition, T. ceratophorum plants received one of three possible hand-pollinations: control conspecific pollination, concomitant conspecific and heterospecific pollination (mixed), or conspecific pollen followed by heterospecific pollen 15 min later (staggered). Floral isolation was negligible as no insect preference was detected. On a presence/absence basis, florets on native inflorescences received slightly less pollen analog from heterospecific donors than from conspecific donors; however, the amount of dye particles transferred from either Taraxacum species to stigmas on recipient T. ceratophorum inflorescences was equivalent. In contrast to weak floral isolation, strong pollen germination and pollen competition barriers should reduce the potential for hybridization. Heterospecific T. officinale pollen exhibited reduced germination success on T. ceratophorum stigmas in comparison to conspecific pollen. Furthermore, a significant pollen-competition effect on the percentage of hybrid offspring was detected only when T. officinale preceded T. ceratophorum pollen by 15 min. This result indicates that conspecific pollen out-competes heterospecific pollen but further suggests that biotic and abiotic factors reducing pollen accrual rates may partially remove barriers to natural hybridization.     

Differential effect of inter- and intraspecific competition on the performance of invasive and native Taraxacum species

Inter- and intraspecific competitive abilities are significant determinants of invasive success and the ecological impact of non-native plants. We tested two major hypotheses on the competitive ability of invasive species using invasive (Taraxacum officinale) and native (T. platycarpum) dandelions: differential interspecific competitive ability between invasive and native species and the kin recognition of invasive species. We collected seeds from two field sites where the two dandelion species occurred nearby. Plants were grown alone, with kin (plants from the same maternal genotype) or strangers (plants from different populations) of the same species, or with different species in a growth chamber, and the performance at the early developmental stage between species and treatments was compared. The invasive dandelions outcompeted the native dandelions when competing against each other, although no difference between species was detected without competition or with intraspecific competition. Populations of native species responded to interspecific competition differently. The effect of kinship on plant performance differed between the tested populations in both species. A population produced more biomass than the other populations when grown with a stranger, and this trend was manifested more in native species. Our results support the hypothesis that invasive plants have better competitive ability than native plants, which potentially contributes to the establishment and the range expansion of T. officinale in the introduced range. Although kin recognition is expected to evolve in invasive species, the competitive ability of populations rather than kinship seems to affect plant growth of invasive T. officinale under intraspecific competition.     

Alpine dandelions originated in the native and introduced range differ in their responses to environmental constraints

Few studies have compared the response of native and invasive populations under stressful conditions. Furthermore, there is little consensus as to whether a plastic response is related to invasiveness in stressful environments. Exotic species have recently been reported in the high Andes of central Chile, where individuals have to cope with drought and poor soils, in addition to extreme temperatures. We explored if the exotic species Taraxacum officinale (dandelion) has plastic responses to soil moisture and nutrient availability, and whether two sets of alpine populations derived from native and introduced populations can converge to similar plastic responses to environmental constraints. Using a common garden approach, we compared plants grown from seeds collected in alpine populations of its native range (Alps, France) and in alpine populations of its introduced range (Andes, Chile) under a drought experiment, a potassium gradient, and a nitrogen gradient. Plasticity was only found as a response to drought. Moreover, different responses were found between both origins. Andean individuals are drought-resistant, while individuals from the Alps were drought-sensitive. According to the nutrient experiments, Andean dandelions behave as a nitrogen demanding-potassium avoiding species, whereas individuals from the Alps did not show any particular dependency or repulsion tendency to either of these two nutrients. Results suggest that differences in life history traits of both derived sets of populations may have an important role in determining the response of dandelions under the evaluated conditions. However, the relative importance of genetic adaptation in these responses is still unclear. Although T. officinale is a cosmopolite weed, this is the first study that compares individuals coming from its native and invaded range under stressful conditions.     

Assessing the importance of disturbance, site conditions, and the biotic barrier for dandelion invasion in an Alpine habitat

Several factors have been identified as relevant in determining the abundance of non-native invasive species. Nevertheless, the relative importance of these factors will vary depending on the invaded habitat and the characteristics of the invasive species. Due to their harsh environmental conditions and remoteness, high-alpine habitats are often considered to be at low risk of plant invasion. However, an increasing number of reports have shown the presence and spread of non-native plant species in alpine habitats; thus, it is important to study which factors control the invasion process in these harsh habitats. In this study, we assessed the role of disturbance, soil characteristics, biotic resistance and seed rain in the establishment and abundance of the non-native invasive species Taraxacum officinale (dandelion) in the Andes of central Chile. By focusing on human-disturbed patches, naturally disturbed patches, and undisturbed patches, we did not find that disturbance per se, or its origin, affected the establishment and abundance of T. officinale. The abundance of this non-native invasive species was not negatively related to the diversity of native species at local scales, indicating no biotic resistance to invasion; instead, some positive relationships were found. Our results indicate that propagule pressure (assessed by the seed rain) and the abiotic soil characteristics are the main factors related to the abundance of this non-native invasive species. Hence, in contrast to what has been found for more benign habitats, disturbance and biotic resistance have little influence on the invasibility of T. officinale in this high-alpine habitat.     

Interspecific competition for seed production between two naturalized dandelions under different levels of fertilizer supply

Two naturalized dandelions,Taraxacum officinale andTaraxacum laevigatum, were grown in mixed stands at various plant densities and mixing ratios with various levels of fertilizer supply until seeds were produced. The effect of fertilizer supply on the competitive relationship for seed production between the species was analyzed. In pure stands, the simultaneous effects of plant density and fertilizer supply on mean plant weight were formulated for each species. By introducing the density conversion factor, the results of plant weight growth in mixed stands were formulated successfully.T. laevigatum produced more seeds thanT. officinale at the same plant weight. The relationship between mean plant weight and mean number of mature seeds produced per plant was also formulated for each species. Ratio diagrams were predicted with these formulas. The density conversion factors showed thatT. officinale was always dominant overT. laevigatum for plant weight growth within a generation. The ratio diagrams showed thatT. officinale became dominant overT. laevigatum for seed production with repeating generations at higher levels of fertilizer supply, but that the species with a larger ratio of plant density became dominant over the other species at lower levels of fertilizer supply.     

Germination characteristics of native Japanese dandelion autopolyploids and their putative diploid parent species

Germination characteristics of native Japanese Taraxacum lineages of Taraxacum platycarpum (diploid), T. venustum (triploid and tetraploid), and T. albidum (pentaploid) have been studied at different temperatures. Taraxacum platycarpum ssp. hondoense is the putative diploid parent of T. venustum. Diploid T. platycarpum ssp. hondoense and the polyploids T. venustum and T. albidum are found in different areas of Japan, and distribution differences may reflect divergent ecological and physiological traits among ploidy levels. In this study, to prevent mixing of seeds of different polyploidy we used flow cytometry to examine the ploidy level of the plants from which seeds were collected. Results from seed-germination experiments showed that dependence on temperature of final percentage germination was qualitatively similar for both autopolyploids T. venustum and diploids T. platycarpum—germination was suppressed at high and low temperatures. It was also shown that seed germination of autopolyploids was suppressed more than that of the ancestral diploid at low temperatures and that seed germination for polyploids was higher than for the diploid. Threshold variations at low temperatures might affect the distribution of native dandelions. Taraxacum venustum, which occurs in cool climates, might have developed a distinctly lower germination threshold at low temperatures whereas T. albidum, which is native to warm climates, might have developed an adaptive threshold at high temperatures.     

Host Identity Impacts Rhizosphere Fungal Communities Associated with Three Alpine Plant Species

Fungal diversity and composition are still relatively unknown in many ecosystems; however, host identity and environmental conditions are hypothesized to influence fungal community assembly. To test these hypotheses, we characterized the richness, diversity, and composition of rhizosphere fungi colonizing three alpine plant species, Taraxacum ceratophorum, Taraxacum officinale, and Polemonium viscosum. Roots were collected from open meadow and willow understory habitats at treeline on Pennsylvania Mountain, Colorado, USA. Fungal small subunit ribosomal DNA was sequenced using fungal-specific primers, sample-specific DNA tags, and 454 pyrosequencing. We classified operational taxonomic units (OTUs) as arbuscular mycorrhizal (AMF) or non-arbuscular mycorrhizal (non-AMF) fungi and then tested whether habitat or host identity influenced these fungal communities. Approximately 14% of the sequences represented AMF taxa (44 OTUs) with the majority belonging to Glomus groups A and B. Non-AMF sequences represented 186 OTUs belonging to Ascomycota (58%), Basidiomycota (26%), Zygomycota (14%), and Chytridiomycota (2%) phyla. Total AMF and non-AMF richness were similar between habitats but varied among host species. AMF richness and diversity per root sample also varied among host species and were highest in T. ceratophorum compared with T. officinale and P. viscosum. In contrast, non-AMF richness and diversity per root sample were similar among host species except in the willow understory where diversity was reduced in T. officinale. Fungal community composition was influenced by host identity but not habitat. Specifically, T. officinale hosted a different AMF community than T. ceratophorum and P. viscosum while P. viscosum hosted a different non-AMF community than T. ceratophorum and T. officinale. Our results suggest that host identity has a stronger effect on rhizosphere fungi than habitat. Furthermore, although host identity influenced both AMF and non-AMF, this effect was stronger for the mutualistic AMF community.