Jeff R. Powell

Invasive Plant Suppresses the Growth of Native Tree Seedlings by Disrupting Belowground Mutualisms

The impact of exotic species on native organisms is widely acknowledged, but poorly understood. Very few studies have empirically investigated how invading plants may alter delicate ecological interactions among resident species in the invaded range. We present novel evidence that antifungal phytochemistry of the invasive plant, Alliaria petiolata, a European invader of North American forests, suppresses native plant growth by disrupting mutualistic associations between native canopy tree seedlings and belowground arbuscular mycorrhizal fungi. Our results elucidate an indirect mechanism by which invasive plants can impact native flora, and may help explain how this plant successfully invades relatively undisturbed forest habitat.

Phylogenetic trait conservatism and the evolution of functional trade-offs in arbuscular mycorrhizal fungi

The diversity of functional and life-history traits of organisms depends on adaptation as well as the legacy of shared ancestry. Although the evolution of traits in macro-organisms is well studied, relatively little is known about character evolution in micro-organisms. Here, we surveyed an ancient and ecologically important group of microbial plant symbionts, the arbuscular mycorrhizal (AM) fungi, and tested hypotheses about the evolution of functional and life-history traits. Variation in the extent of root and soil colonization by AM fungi is constrained to a few nodes basal to the most diverse groups within the phylum, with relatively little variation associated with recent divergences. We found no evidence for a trade-off in biomass allocated to root versus soil colonization in three published glasshouse experiments; rather these traits were positively correlated. Partial support was observed for correlated evolution between fungal colonization strategies and functional benefits of the symbiosis to host plants. The evolution of increased soil colonization was positively correlated with total plant biomass and shoot phosphorus content. Although the effect of AM fungi on infection by root pathogens was phylogenetically conserved, there was no evidence for correlated evolution between the extent of AM fungal root colonization and pathogen infection. Variability in colonization strategies evolved early in the diversification of AM fungi, and we propose that these strategies were influenced by functional interactions with host plants, resulting in an evolutionary stasis resembling trait conservatism.

Abrupt rise in atmospheric CO2 overestimates community response in a model plant-soil system.

Attempts to understand the ecological effect of increasing atmospheric CO2 concentration, [CO2], usually involve exposing todays ecosystems to expected future [CO2] levels. However, a major assumption of these approaches has not been tested—that exposing ecosystems to a single-step increase in [CO2] will yield similar responses to those of a gradual increase over several decades. We tested this assumption on a mycorrhizal fungal community over a period of six years. [CO2] was either increased abruptly, as is typical of most [CO2] experiments, or more gradually over 21 generations. The two approaches resulted in different structural and functional community responses to increased [CO2]. Some fungi were sensitive to the carbon pulse of the abrupt [CO2] treatment. This resulted in an immediate decline in fungal species richness and a significant change in mycorrhizal functioning. The magnitude of changes in fungal diversity and functioning in response to gradually increasing [CO2] was smaller, and not significantly different to those with ambient [CO2]. Our results suggest that studies may overestimate some community responses to increasing [CO2] because biota may be sensitive to ecosystem changes that occur as a result of abrupt increases.

Microsatellite analysis of genetic divergence among populations of giant Galápagos tortoises

Giant Galápagos tortoises represent an interesting model for the study of patterns of genetic divergence and adaptive differentiation related to island colonization events. Recent mitochondrial DNA work elucidated the evolutionary history of the species and helped to clarify aspects of nomenclature. We used 10 microsatellite loci to assess levels of genetic divergence among and within island populations. In particular, we described the genetic structure of tortoises on the island of Isabela, where discrimination of different taxa is still subject of debate. Individual island populations were all genetically distinct. The island of Santa Cruz harboured two distinct populations. On Isabela, populations of Volcan Wolf, Darwin and Alcedo were significantly different from each other. On the other hand, Volcan Wolf showed allelic similarity with the island of Santiago. On Southern Isabela, lower genetic divergence was found between Northeast Sierra Negra and Volcan Alcedo, while patterns of gene flow were recorded among tortoises of Cerro Azul and Southeast Sierra Negra. These tortoises have endured heavy exploitation during the last three centuries and recently attracted much concern due to the current number of stochastic and deterministic threats to extant populations. Our study complements previous investigation based on mtDNA diversity and provides further information that may help devising tortoise management plans.

The Effect of Heterogeneous Environments and a Competitor on Genetic Variation in Drosophila

Thirty-six homokaryotypic laboratory populations of Drosophila pseudoobscura, all of which originated from the same freshly collected population, were maintained for 2 yr. The populations had zero, one, two, or three of the following factors variable: medium, temperature, and/or presence of a competitor, D. persimilis. Twelve, 18, and 24 mo after the populations started, estimates of levels of heterozygosity of each population were made by measuring the amount of electrophoretically detectable genetic variation at nine polymorphic enzyme loci. In going from zero to one factor varying, all three factors increased the level of heterozygosity observed; i.e., populations experiencing two temperatures maintained more genetic variation than populations held at one temperature; populations which had two types of media present had higher levels of genetic variation than populations with only one medium; populations to which D. persimilis were added maintained more genetic variation than when D. pseudoobscura was alone. In going from one to two or two to three variables, increased genetic variation sometimes could and sometimes could not be detected. Estimates of population sizes were made. From these it was concluded that genetic drift cannot account for the pattern observed. Therefore, it is concluded that selection was operating; at least some of the electrophoretically detectable alleles may themselves be parts of the units of selection. It is argued that such diversifying selection is important in maintaining high levels of genic variation in natural population, especially in organisms such as Drosophila.

Evolutionary criteria outperform operational approaches in producing ecologically relevant fungal species inventories

Analyses of the structure and function of microbial communities are highly constrained by the diversity of organisms present within most environmental samples. A common approach is to rely almost entirely on DNA sequence data for estimates of microbial diversity, but to date there is no objective method of clustering sequences into groups that is grounded in evolutionary theory of what constitutes a biological lineage. The general mixed Yule‐coalescent (GMYC) model uses a likelihood‐based approach to distinguish population‐level processes within lineages from processes associated with speciation and extinction, thus identifying a distinct point where extant lineages became independent. Using two independent surveys of DNA sequences associated with a group of ubiquitous plant‐symbiotic fungi, we compared estimates of species richness derived using the GMYC model to those based on operational taxonomic units (OTUs) defined by fixed levels of sequence similarity. The model predicted lower species richness in these surveys than did traditional methods of sequence similarity. Here, we show for the first time that groups delineated by the GMYC model better explained variation in the distribution of fungi in relation to putative niche‐based variables associated with host species identity, edaphic factors, and aspects of how the sampled ecosystems were managed. Our results suggest the coalescent‐based GMYC model successfully groups environmental sequences of fungi into clusters that are ecologically more meaningful than more arbitrary approaches for estimating species richness.

Genetic divergence, phylogeography and conservation units of giant tortoises from Santa Cruz and Pinzón, Galápagos Islands

Island radiations can offer challenging systemsfor the implementation of conservation policiesbecause descendent populations may exhibitdifferent levels of adaptive divergence,reproductive isolation, and phylogeneticdistinctiveness. This seems particularly truefor the endangered Galápagos gianttortoises (Geochelone nigra), whichcomprise a lineage that radiated rapidly andconcomitantly with the evolution of thearchipelago. We used mitochondrial DNAsequences and microsatellite markers toinvestigate the genetic structure, and toreconstruct genealogical relationships and thehistory of population colonization of gianttortoises from the Islands of Santa Cruz andPinzón, including samples of a basal taxonfrom the Island of San Cristóbal.Populations displayed marked geneticdivergence, contrasting demographic histories,and deep phylogeographic structure. The patternof diversification among populations wasconsistent with geological and biogeographichistory, and to some extent, with adaptive andmorphological divergence. Results stronglyindicate the presence of a minimum of fourconservation units with long-standingevolutionary separation: two in Santa Cruz, onein Pinzón, and one in San Cristóbal. Wepropose that these findings be effectivelyintegrated with other existing data by theappropriate environmental agencies to evaluatecurrent conservation efforts and implement newstrategies aimed at protecting the integrityand diversity of giant tortoise populations.

A cryptic taxon of Galapagos tortoise in conservation peril

As once boldly stated, ‘bad taxonomy can kill’, highlighting the critical importance of accurate taxonomy for the conservation of endangered taxa. The concept continues to evolve almost 15 years later largely because most legal protections aimed at preserving biological diversity are based on formal taxonomic designations. In this paper we report unrecognized genetic divisions within the giant tortoises of the Galápagos. We found three distinct lineages among populations formerly considered a single taxon on the most populous and accessible island of Santa Cruz; their diagnosability, degree of genetic divergence and phylogenetic placement merit the recognition of at least one new taxon. These results demonstrate the fundamental importance of continuing taxonomic investigations to recognize biological diversity and designate units of conservation, even within long-studied organisms such as Galápagos tortoises, whose evolutionary heritage and contribution to human intellectual history warrant them special attention.

Mycorrhizal responsiveness trends in annual crop plants and their wild relatives—a meta-analysis on studies from 1981 to 2010

Background and aimsYear of release of a cultivar reflects the agricultural and breeding practices of its time; we hypothesize that there are differences in mycorrhizal responsiveness of new high yielding and old crop plants and landraces. We evaluated the importance of the year of release on mycorrhizal responsiveness, arbuscular mycorrhizal (AM) fungal root colonization and P efficiency. We also analyzed the effect of experimental treatments, P efficiency (P acquisition and P utilization efficiency) and AM fungal root colonization on a potential mycorrhizal responsiveness trend for year of release.MethodsWe conducted a meta-analysis on 39 publications working on 320 different crop plant genotypes.ResultsNew cultivars were less intensely colonized but were more mycorrhiza-responsive (and possibly dependent) compared to ancestral genotypes. This trend was potentially influenced by the moderator variables density, pre-germination, plant, plant type and AMF species. AM root colonization was also important for the mycorrhizal responsiveness trend for year of release, but P efficiency was not.ConclusionsWith the data available we could find no evidence that new crop plant genotypes lost their ability to respond to mycorrhiza due to agricultural and breeding practices.

Distinguishing defensive characteristics in the phloem of ash species resistant and susceptible to emerald ash borer.

We examined the extent to which three Fraxinus cultivars and a wild population that vary in their resistance to Emerald Ash Borer (EAB) could be differentiated on the basis of a suite of constitutive chemical defense traits in phloem extracts. The EAB-resistant Manchurian ash (F. mandshurica, cv. Mancana) was characterized by having a rapid rate of wound browning, a high soluble protein concentration, low trypsin inhibitor activities, and intermediate levels of peroxidase activity and total soluble phenolic concentration. The EAB-susceptible white ash (F. americana, cv. Autumn Purple) was characterized by a slow wound browning rate and low levels of peroxidase activity and total soluble phenolic concentrations. An EAB-susceptible green ash cultivar (F. pennsylvanica, cv. Patmore) and a wild accession were similar to each other on the basis of several chemical defense traits, and were characterized by high activities of peroxidase and trypsin inhibitor, a high total soluble phenolic concentration, and an intermediate rate of wound browning. Lignin concentration and polyphenol oxidase activities did not differentiate resistant and susceptible species. Of 33 phenolic compounds separated by HPLC and meeting a minimum criterion for analysis, nine were unique to Manchurian ash, five were shared among all species, and four were found in North American ashes and not in the Manchurian ash. Principal components analysis revealed clear separations between Manchurian, white, and green ashes on the basis of all phenolics, as well as clear separations on the basis of quantities of phenolics that all species shared. Variation in some of these constitutive chemical defense traits may contribute to variation in resistance to EAB in these species.