Jacintha Ellers

Trait plasticity in species interactions: a driving force of community dynamics

Evolutionary community ecology is an emerging field of study that includes evolutionary principles such as individual trait variation and plasticity of traits to provide a more mechanistic insight as to how species diversity is maintained and community processes are shaped across time and space. In this review we explore phenotypic plasticity in functional traits and its consequences at the community level. We argue that resource requirement and resource uptake are plastic traits that can alter fundamental and realised niches of species in the community if environmental conditions change. We conceptually add to niche models by including phenotypic plasticity in traits involved in resource allocation under stress. Two qualitative predictions that we derive are: (1) plasticity in resource requirement induced by availability of resources enlarges the fundamental niche of species and causes a reduction of vacant niches for other species and (2) plasticity in the proportional resource uptake results in expansion of the realized niche, causing a reduction in the possibility for coexistence with other species. We illustrate these predictions with data on the competitive impact of invasive species. Furthermore, we review the quickly increasing number of empirical studies on evolutionary community ecology and demonstrate the impact of phenotypic plasticity on community composition. Among others, we give examples that show that differences in the level of phenotypic plasticity can disrupt species interactions when environmental conditions change, due to effects on realized niches. Finally, we indicate several promising directions for future phenotypic plasticity research in a community context. We need an integrative, trait-based approach that has its roots in community and evolutionary ecology in order to face fast changing environmental conditions such as global warming and urbanization that pose ecological as well as evolutionary challenges.

Plasticity versus environmental canalization: population differences in thermal responses along a latitudinal gradient in Drosophila serrata.

The phenotypic plasticity of traits, defined as the ability of a genotype to express different phenotypic values of the trait across a range of environments, can vary between habitats depending on levels of temporal and spatial heterogeneity. Other traits can be insensitive to environmental perturbations and show environmental canalization. We tested levels of phenotypic plasticity in diverse Drosophila serrata populations along a latitudinal cline ranging from a temperate, variable climate to a tropical, stable climate by measuring developmental rate and size-related traits at three temperatures (16°C, 22°C, and 28°C). We then compared the slopes of the thermal reaction norms among populations. The 16–22°C part of the reaction norms for developmental rate was flatter (more canalized) for the temperate populations than for the tropical populations. However, slopes for the reaction norms of the two morphological traits (wing size, wing:thorax ratio), were steeper (more plastic) in the temperate versus the tropical populations over the entire thermal range. The different latitudinal patterns in plasticity for developmental rate and the morphological traits may reflect contrasting selection pressures along the tropical-temperate thermal gradient.

Genetic structure in Orchesella cincta (Collembola): strong subdivision of European populations inferred from mtDNA and AFLP markers.

Population genetic structure is determined both by current processes and historical events. Current processes include gene flow, which is largely influenced by the migration capacity of a species. Historical events are, for example, glaciation periods, which have had a major impact on the distribution of many species. Species with a low capacity or tendency to move about or disperse often exhibit clear spatial genetic structures, whereas mobile species mostly show less spatial genetic differentiation. In this paper we report on the genetic structure of a small, wingless arthropod species (Orchesella cincta: Collembola) in Europe. For this purpose we used mtDNA COII sequences and AFLP markers. We show that large genetic differences exist between populations of O. cincta, as expected from O. cinctas winglessness and sedentary lifestyle. Despite the fact that most variability was observed within populations (59%), a highly significant amount of AFLP variation (25%) was observed between populations from northwestern Europe, central Europe and Italy. This suggests that gene flow among regions is extremely low, which is additionally supported by the lack of shared mtDNA alleles between regions. Based on the genetic variation and sequence differences observed we conclude that the subdivision occurred long before the last glaciation periods. Although the populations still interbreed in the lab, we assume that in the long term the genetic isolation of these regions may lead to speciation processes.

Reference genes for QRT-PCR tested under various stress conditions in Folsomia candida and Orchesella cincta (Insecta, Collembola)

BackgroundGenomic studies measuring transcriptional responses to changing environments and stress currently make their way into the field of evolutionary ecology and ecotoxicology. To investigate a small to medium number of genes or to confirm large scale microarray studies, Quantitative Reverse Transcriptase PCR (QRT-PCR) can achieve high accuracy of quantification when key standards, such as normalization, are carefully set. In this study, we validated potential reference genes for their use as endogenous controls under different chemical and physical stresses in two species of soil-living Collembola, Folsomia candida and Orchesella cincta. Treatments for F. candida were cadmium exposure, phenanthrene exposure, desiccation, heat shock and pH stress, and for O. cincta cadmium, desiccation, heat shock and starvation.ResultsEight potential reference genes for F. candida and seven for O. cincta were ranked by their stability per stress factor using the programs geNorm and Normfinder. For F. candida the succinate dehydrogenase (SDHA) and eukaryotic transcription initiation factor 1A (ETIF) genes were found the most stable over the different treatments, while for O. cincta, the beta actin (ACTb) and tyrosine 3-monooxygenase (YWHAZ) genes were the most stable.ConclusionWe present a panel of reference genes for two emerging ecological genomic model species tested under a variety of treatments. Within each species, different treatments resulted in differences in the top stable reference genes. Moreover, the two species differed in suitable reference genes even when exposed to similar stresses. This might be attributed to dissimilarity of physiology. It is vital to rigorously test a panel of reference genes for each species and treatment, in advance of relative quantification of QRT-PCR gene expression measurements.

Genotypic richness and phenotypic dissimilarity enhance population performance

Increases in biodiversity can result from an increase in species richness, as well as from a higher genetic diversity within species. Intraspecific genetic diversity, measured as the number of genotypes, can enhance plant primary productivity and have cascading effects at higher trophic levels, such as an increase in herbivore and predator richness. The positive effects of genotypic mixtures are not only determined by additive effects, but also by interactions among genotypes, such as facilitation or inhibition. However, so far there has been no effort to predict the extent of such effects. In this study, we address the question of whether the magnitude of the effect of genotype number on population performance can be explained by the extent of dissimilarity in key traits among genotypes in a mixture. We examine the relative contribution of genotype number and phenotypic dissimilarity among genotypes to population performance of the soil arthropod, Orchesella cincta. Nearly homogeneous genotypes were created from inbred isofemale lines. Phenotypic dissimilarity among genotypes was assessed in terms of three life-history traits that are associated with population growth rate, i.e., egg size, egg development time, and juvenile growth rate. A microcosm experiment with genotype mixtures consisting of one, two, four, and eight genotypes, showed that genotypic richness strongly increased population size and biomass production and was associated with greater net diversity effects. Most importantly, there was a positive log-linear relationship between phenotypic dissimilarity in a mixture and the net diversity effects for juvenile population size and total biomass. In other words, the degree of phenotypic dissimilarity among genotypes determined the magnitude of the genotypic richness effect, although this relationship leveled off at higher values of phenotypic dissimilarity. Although the exact mechanisms responsible for these effects are currently unknown, similar advantages of trait dissimilarity have been found among species. Hence, to better understand population performance, genotype number and phenotypic dissimilarity should be considered collectively.

Do fungivores trigger the transfer of protective metabolites from host plants to arbuscular mycorrhizal hyphae

A key objective in ecology is to understand how cooperative strategies evolve and are maintained in species networks. Here, we focus on the tri-trophic relationship between arbuscular mycorrhizal (AM) fungi, host plants, and fungivores to ask if host plants are able to protect their mutualistic mycorrhizal partners from being grazed. Specifically, we test whether secondary metabolites are transferred from hosts to fungal partners to increase their defense against fungivores. We grew Plantago lanceolata hosts with and without mycorrhizal inoculum, and in the presence or absence of fungivorous springtails. We then measured fungivore effects on host biomass and mycorrhizal abundance (using quantitative PCR) in roots and soil. We used high-performance liquid chromatography to measure host metabolites in roots, shoots, and hyphae, focusing on catalpol, aucubin, and verbascoside. Our most striking result was that the metabolite catalpol was consistently found in AM fungal hyphae in host plants exposed to fungivores. When fungivores were absent, catalpol was undetectable in hyphae. Our results highlight the potential for plant-mediated protection of the mycorrhizal hyphal network.

Genetic correlation between temperature-induced plasticity of life-history traits in a soil arthropod

Temperature is considered one of the most important mediators of phenotypic plasticity in ectotherms. However, the costs and benefits shaping the evolution of different thermal responses are poorly elucidated. One of the possible constraints to phenotypic plasticity is its intrinsic genetic cost, such as genetic linkage or pleiotropy. Genetic coupling of the thermal response curves for different life history traits may significantly affect the evolution of thermal sensitivity in thermally fluctuating environments. We used the collembolan Orchesella cincta to study if there is genetic variation in temperature-induced phenotypic plasticity in life history traits, and if the degree of temperature-induced plasticity is correlated across traits. Egg development rate, juvenile growth rate and egg size of 19 inbred isofemale lines were measured at two temperatures. Our results show that temperature was a highly significant factor for all three traits. Egg development rate and juvenile growth rate increased with increasing temperature, while egg size decreased. Line by temperature interaction was significant for all traits tested; indicating that genetic variation for temperature-induced plasticity existed. The degree of plasticity was significantly positively correlated between egg development rate and growth rate, but plasticity in egg size was not correlated to the other two plasticity traits. The findings suggest that the thermal plasticities of egg development rate and growth rate are partly under the control of the same genes or genetic regions. Hence, evolution of the thermal plasticity of traits cannot be understood in isolation of the response of other traits. If traits have similar and additive effects on fitness, genetic coupling between these traits may well facilitate the evolution of optimal phenotypes. However, for this we need to know the selective forces under field conditions.

Allelic diversity of metallothionein in Orchesella cincta (L): traces of natural selection by environmental pollution.

The advances made in statistical methods to detect selection from DNA sequence variation has resulted in an enormous increase in the number of studies reporting positive selection. However, a disadvantage of such statistical tests is that often no insight into the actual source of selection is obtained. Finer understanding of evolution can be obtained when those statistical tests are combined with field observations on allele frequencies. We assessed whether the metallothionein (mt) gene of Orchesella cincta (Collembola), which codes for a metal-binding protein, is subject to selection, by investigating alleles and allele frequencies among European metal-stressed and reference populations. Eight highly divergent alleles were resolved in Northwest Europe. At the nucleotide level, a total of 51 polymorphic sites (five of them implying amino-acid changes) were observed. Although statistical tests applied to the sequences alone showed no indication of selection, a G-test rejected the null hypothesis that alleles are homogeneously distributed over metal-stressed and reference populations. Analysis of molecular variance assigned a small, but significant amount of the total variance to differences between metal-stressed and non-stressed populations. In addition, it was shown that metal-stressed populations tend to be more genetically diversified at this locus than non-stressed ones. These results suggest that the mt gene and its surrounding DNA region are affected by environmental metal contamination. This study illustrates that, in addition to statistical tests, field observations on allele frequencies are needed to gain understanding of selection and adaptive evolution.

Effects of exposure to short-term heat stress on male reproductive fitness in a soil arthropod

Ambient temperature is a key environmental factor influencing a variety of aspects of the ecology and evolution of ectotherms. Reproductive traits have been suggested to be more sensitive to thermal stress than other life history traits. This study investigated the direct and indirect effects of heat shock on male reproductive success in the widespread springtail Orchesella cincta. Male springtails were exposed to four temperature treatments: heat hardening (35.2°C for 1h), heat shock (37.2°C for 1h), heat hardening+heat shock (35.2°C for 1h, followed 15h later by 37.2°C for 1h), and control (20°C). The heat shock gene Hsp70 showed high expression in all the heat treatments, indicating that the treatments indeed induced thermal stress. Significant mortality was only found in the treatment with heat shock, both with and without heat hardening. A direct effect of heat treatment was found on time to first reproduction, which was significantly longer after heat shock (with or without heat hardening) than in the control treatment. There was no difference among treatments in the number of spermatophores produced in the first reproductive instar. Heat treatment also had indirect effects on male reproductive success. Females chose significantly more spermatophores from control males than from males that received heat shock, heat hardening or both. A high percentage of spermatophores produced by heat shocked males caused reproductive failure in females, but no significant differences among treatments were found. Our results suggest that not all traits were equally affected by the heat stress. Heat hardening did not protect reproductive traits against the negative effects of heat shock. The indirect effects of heat shock on reproduction may be equally important as the direct effects.

Frontiers in phenotypic plasticity research: new questions about mechanisms, induced responses and ecological impacts.

Phenotypic plasticity of organisms in response to environmental variability is a welldeveloped concept in ecology and evolutionary biology. It is currently seen as one of the prime mechanisms with which organisms can respond adaptively to environmental change. Despite its importance for evolutionary processes and the long standing history of empirical and theoretical research on plasticity, we are still far from fully understanding the causes and consequences of phenotypic plasticity. This issue of Evolutionary Ecology contains seven papers that employ the concept of phenotypic plasticity to tackle new questions at their respective fields of research. While introducing new concepts in an existing field may improve understanding of relevant phenomena, the reverse is equally valuable: studying phenotypic plasticity in new contexts may improve our understanding of mechanisms and evolution of phenotypic plasticity. To highlight these new frontiers in phenotypic plasticity research we briefly emphasize the importance of each paper, and its contributions to general plasticity theory. One of the facets of phenotypic plasticity that have remained resistant to research is the underlying mechanisms allowing for the production of different phenotypes from a single genotype (Schlichting and Smith 2002). A critical step in the relationship between genotype and phenotype is transcription and translation. Environmental signals are important modulators of the transcriptional activity of genes, and altered gene expression has been linked to environmentally-induced phenotypes (Kent et al. 2009; Sumner et al. 2006). The presence of specific DNA sequences upstream of the coding region of a gene encodes binding sites for transcription factors that can promote or inhibit gene expression. Despite the ubiquity of this view, we do not know what reaction norms for gene expression look like and what the selective pressures shaping those reaction norms really are. Roelofs et al. (2010) review a possible mechanism responsible for transcriptional plasticity in stress responses within single or multiple generations. Gene promoters containing a core