Thomas Steinger

Long-term persistence in a changing climate: DNA analysis suggests very old ages of clones of alpine Carex curvula

Carex curvula is a very slow-growing rhizomatous sedge that forms extensive stands in the European an alpine belt. The recruitment of sexual progeny is extremely rare and propagation occurs predominantly through clonal growth. The randomly amplified polymorphic DNA (RAPD) technique was used to analyse clonal structure in a small patch (2.0x0.4 m sampling transect plus some additional samples) of a high-alpine population of the species. Amplification of the DNA of 116 tiller samples from the patch with eight ten-base primers yielded a total of 95 bands, of which 73 were polymorphic. Based on the RAPD amplification profiles a total of 15 multilocus genotypes (putative clones) were identified. Due to the high number of polymorphic loci the number of genetic markers delineating individual clones was high (range: 16–39 markers) which suggests that our estimates of clonal diversity are precise. More than half of the sampled tillers were identified as belonging to a single clone which formed a relatively homogeneous disc intermingling with other clones only at its margin. Based on the maximum diameter of this large clone of more than 7000 tillers and estimates of annual expansion growth of rhizomes (0.4 mm year-1), the age of the clone was calculated to be around 2000 years. This demonstrates that clones of C. curvula may persist on a single spot over long periods with quite diverse alpine climates ranging from rather mild periods in the Middle Ages to cool periods during the so called “little ice age” in the last century. Our results suggest caution with plant migration scenarios based on shifting isotherms where late-successional clonal species, which dominate the alpine vegetation all over the world, are concerned.

Shift in cytotype frequency and niche space in the invasive plant Centaurea maculosa

Polyploidy is often assumed to increase the spread and thus the success of alien plant species, but few empirical studies exist. We tested this hypothesis with Centaurea maculosa Lam., a species native to Europe and introduced into North America approximately 120 years ago where it became highly invasive. We analyzed the ploidy level of more than 2000 plants from 93 native and 48 invasive C. maculosa populations and found a pronounced shift in the relative frequency of diploid and tetraploid cytotypes. In Europe diploid populations occur in higher frequencies than tetraploids and only four populations had both cytotypes, while in North America diploid plants were found in only one mixed population and thus tetraploids clearly dominated. Our results showed a pronounced shift in the climatic niche between tetraploid populations in the native and introduced range toward drier climate in North America and a similar albeit smaller shift between diploids and tetraploids in the native range. The field data indicate that diploids have a predominately monocarpic life cycle, while tetraploids are often polycarpic. Additionally, the polycarpic life-form seems to be more prevalent among tetraploids in the introduced range than among tetraploids in the native range. Our study suggests that both ploidy types of C. maculosa were introduced into North America, but tetraploids became the dominant cytotype with invasion. We suggest that the invasive success of C. maculosa is partly due to preadaptation of the tetraploid cytotype in Europe to drier climate and possibly further adaptation to these conditions in the introduced range. The potential for earlier and longer seed production associated with the polycarpic life cycle constitutes an additional factor that may have led to the dominance of tetraploids over diploids in the introduced range.

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

SummaryCentaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.

Climate vs. soil factors in local adaptation of two common plant species

Evolutionary theory suggests that divergent natural selection in heterogeneous environments can result in locally adapted plant genotypes. To understand local adaptation it is important to study the ecological factors responsible for divergent selection. At a continental scale, variation in climate can be important while at a local scale soil properties could also play a role. We designed an experiment aimed to disentangle the role of climate and (abiotic and biotic) soil properties in local adaptation of two common plant species. A grass (Holcus lanatus) and a legume (Lotus corniculatus), as well as their local soils, were reciprocally transplanted between three sites across an Atlantic-Continental gradient in Europe and grown in common gardens in either their home soil or foreign soils. Growth and reproductive traits were measured over two growing seasons. In both species, we found significant environmental and genetic effects on most of the growth and reproductive traits and a significant interaction between the two environmental effects of soil and climate. The grass species showed significant home site advantage in most of the fitness components, which indicated adaptation to climate. We found no indication that the grass was adapted to local soil conditions. The legume showed a significant home soil advantage for number of fruits only and thus a weak indication of adaptation to soil and no adaptation to climate. Our results show that the importance of climate and soil factors as drivers of local adaptation is species-dependent. This could be related to differences in interactions between plant species and soil biota.

Does natural selection promote population divergence? A comparative analysis of population structure using amplified fragment length polymorphism markers and quantitative traits

Divergent natural selection is considered an important force in plant evolution leading to phenotypic differentiation between populations exploiting different environments. Extending an earlier greenhouse study of population differentiation in the selfing annual plant Senecio vulgaris, we estimated the degree of population divergence in several quantitative traits related to growth and life history and compared these estimates with those based on presumably neutral molecular markers (amplified fragment length polymorphisms; AFLPs). This approach allowed us to disentangle the effects of divergent selection from that of other evolutionary forces (e.g. genetic drift). Five populations were examined from each of two habitat types (ruderal and agricultural habitats). We found a high proportion of total genetic variance to be among populations, both for AFLP markers (φST = 0.49) and for quantitative traits (range of QST: 0.26–0.77). There was a strong correlation between molecular and quantitative genetic differentiation between pairs of populations (Mantels r = 0.59). However, estimates of population differentiation in several quantitative traits exceeded the neutral expectation (estimated from AFLP data), suggesting that divergent selection contributed to phenotypic differentiation, especially between populations from ruderal and agricultural habitats. Estimates of within‐population variation in AFLP markers and quantitative genetic were poorly correlated, indicating that molecular marker data may be of limited value to predict the evolutionary potential of populations of S. vulgaris.

Potential contribution of natural enemies to patterns of local adaptation in plants.

Genetic differentiation among plant populations and adaptation to local environmental conditions are well documented. However, few studies have examined the potential contribution of plant antagonists, such as insect herbivores and pathogens, to the pattern of local adaptation. Here, a reciprocal transplant experiment was set up at three sites across Europe using two common plant species, Holcus lanatus and Plantago lanceolata. The amount of damage by the main above-ground plant antagonists was measured: a rust fungus infecting Holcus and a specialist beetle feeding on Plantago, both in low-density monoculture plots and in competition with interspecific neighbours. Strong genetic differentiation among provenances in the amount of damage by antagonists in both species was found. Local provenances of Holcus had significantly higher amounts of rust infection than foreign provenances, whereas local provenances of Plantago were significantly less damaged by the specialist beetle than the foreign provenances. The presence of surrounding vegetation affected the amount of damage but had little influence on the ranking of plant provenances. The opposite pattern of population differentiation in resistance to local antagonists in the two species suggests that it will be difficult to predict the consequences of plant translocations for interactions with organisms of higher trophic levels.

Predicting evolutionary change in invasive, exotic plants and its consequences for plant-herbivore interactions.

Invasion ecology, the study of the distribution and spread of organisms in habitats to which they are not native, has received considerable attention during past decades (Groves and Burdon, 1986; Drake et al., 1989; Vitousek et al., 1996; Williamson, 1996; Lonsdale, 1999; Walker, 1999; Alpert et al., 2000; Mack et al., 2000). This is mainly a consequence of the increased awareness of the major threats posed by invasions to biodiversity, ecosystem integrity, agriculture and human health (Lonsdale, 1999; Mack et al., 2000). Two questions have dominated most of the studies in this context: which species are most likely to become invasive, and which habitats are most susceptible to invasion (Alpert et al., 2000; Kolar and Lodge, 2001). Surprisingly, the evolutionary genetics of invasive species remained relatively unexplored despite the profound effect of genetic characteristics of populations on their capacity for range expansion (Ellstrand and Schierenbeck, 2000; Tsutsui et al., 2000) and on species interactions (Carroll et al., 2001; Siemann and Rogers, 2001). In fact, in a recent review on this topic, Lee (2002) concluded that ‘the invasion success of many species might depend more heavily on their ability to respond to natural selection than on broad physiological tolerance or plasticity’. Natural selection and genetic drift can alter the genetic structure of invading populations, and hence affect not only the process of adaptation to the new physical environment, but also the plentiful biotic interaction encountered in the new habitat. The invasion process is generally divided into two phases: the initial introduction and establishment, and the spread into the new environment. Many of

Interaction between the endophytic fungus Epichloë bromicola and the grass Bromus erectus: effects of endophyte infection, fungal concentration and environment on grass growth and flowering

Epichloë bromicola is an endophytic fungal species that systemically and perennially colonizes intercellular spaces of leaf blades, leaf sheaths and culms of Bromus grass species. E. bromicola causes choke disease in B. erectus, suppressing maturation of most, if not all, host inflorescences. In an investigation of the interaction between fungus and host, we used a quantitative polymerase chain reaction technique to estimate the amount of fungal DNA, and thereby fungal concentration, in host plants. Fungal concentration was directly correlated with vegetative vigour of the plant, as measured by longest leaf length, number of tillers and vegetative above‐ground biomass, suggesting that, during vegetative growth, the endophytic fungus is most beneficial for the plant when present in high concentrations. In contrast, the reproduction of the plant, as measured by the number of functional inflorescences, was inversely correlated with fungal concentration: the majority of infected plants, and all that were associated with high concentrations of fungi, were diseased. Thus, the benefit of endophyte infection for the plant is coupled with the disadvantages of infertility. Fungal concentration was shown to be at least in part genetically determined because fungal concentration differed significantly in different plant–endophyte genotype combinations (symbiotum). In a field experiment with normal and CO2‐enriched environments, elevated CO2 levels favoured fungal reproductive vigour over host reproductive vigour, suggesting that these plant endophytes would be at a selective advantage in a corresponding environmental‐change scenario. We conclude that a dynamic and complex relationship between fungal endophyte infection, fungal concentration, genotype and environment affects growth and fecundity of B. erectus and should contribute to the evolution of these plant–fungal interactions.

Maternal and direct effects of elevated CO2 on seed provisioning, germination and seedling growth in Bromus erectus.

Abstract Elevated CO2 can affect plant fitness not only through its effects on seed production but also by altering the quality of seeds and therefore germination and seedling performance. We collected seeds from mother plants of Bromus erectus grown in field plots at ambient and elevated CO2 (m-CO2, maternal CO2) and germinated them in the greenhouse in a reciprocal design under ambient and elevated CO2 (o-CO2, offspring CO2). This design allowed us to examine both the direct effects of elevated CO2 on germination and seedling growth and the indirect (maternal) effects via altered seed quality. Elevated m-CO2 significantly increased seed mass and increased the C:N ratio of seeds from field-grown plants. Percentage and rate of germination were not affected by the m-CO2 or o-CO2 treatments. Similarly, elevated m-CO2 had no significant effect on seedling size as estimated by the total leaf length. When differences in seed mass were adjusted by using seed mass as a covariate in ANOVA, a negative effect of m-CO2 on seedling size appeared which increased with increasing seed mass (significant covariate×m-CO2 interaction). This may indicate that the advantage of increased seed mass at elevated m-CO2 was offset by the reduced concentration of nitrogen (and possibly other nutrients) in these seeds. In contrast to m-CO2, elevated o-CO2 greatly increased seedling size, and this stimulatory effect of elevated o-CO2 was found to increase with increasing seed mass (significant covariate×o-CO2 interaction). Taken together, these results suggest that in B. erectus transgenerational effects of elevated CO2 are relatively small. However, other factors (genetic and environmental) that contribute to variation in seed provisioning can critically influence the responsiveness of seedlings to elevated CO2.

Low genetic differentiation among seasonal cohorts in Senecio vulgaris as revealed by amplified fragment length polymorphism analysis

Common groundsel, Senecio vulgaris (Asteraceae), is a highly selfing semelparous ephemeral weed that belongs to the few plant species in central Europe capable of growing, flowering and fruiting all year round. In temperate climates, flowering S. vulgaris cohorts were found to appear up to three times per year. Using amplified fragment length polymorphism (AFLP) molecular markers we examined temporal genetic differentiation among spring, summer and autumn cohorts at each of seven sites located in two regions in Switzerland. Strong genetic differentiation among cohorts may indicate the existence of seasonal races of S. vulgaris, reproductively isolated by nonoverlapping flowering phenologies. Analysis of molecular variance (amova) revealed that < 2.5% of the AFLP variation resided among cohorts within sites, whereas there was significant genetic differentiation among plants from different sites (15.6%) and among individuals within cohorts (81.9%). Significant genetic differentiation was also observed between the two regions. Isolation‐by‐distance was found on a regional scale, but not on a local scale. Gene flow was estimated to be ≈ 15‐fold higher among cohorts within sites than among sites. We further found, on average, similar levels of genetic diversity within the three seasonal cohorts. The results of this study demonstrate that season of growth represents a weak barrier for genetic exchange among S. vulgaris populations and does not affect molecular variance. Therefore, there is no evidence for the existence of seasonally specialized races of S. vulgaris. We discuss some implications of the results for the biological control of S. vulgaris using a native rust fungus.