Invasive genotypes are opportunistic specialists not general purpose genotypes

Population influences desiccation tolerance in an invasive aquatic snail, Potamopyrgus antipodarum (Tateidae, Mollusca)

Traits in species that influence invasion success may vary in populations across its invaded range. The aquatic New Zealand mud snail, Potamopyrgus antipodarum, reproduces parthenogenetically in its invaded range, and a few distinct clonal genotypes have been identified in North America. Much of the spread of the snail in North America has been the result of unintentional overland transport by recreational water users. Thus, desiccation tolerance may play an important role in the invasion success of this species. The primary goal of these experiments is to determine if variation in desiccation tolerance exists between populations of this species. Here we compared multiple multi-locus genotypes (MLGs) and populations within those genotypes with regard to their desiccation tolerance. We conducted three experiments. The first compared the survival rate over time of snails from three populations and two MLGs with regard to their ability to survive being completely removed from water. The second experiment examined different size classes of snails from the same population, and the third experiment compared four different populations and two MLGs genotypes with respect to their survival rate over time when removed from water but being kept in moist conditions. We found larger snails tolerate desiccation longer than smaller snails with snails between 4 and 4.6 mm surviving at a rate of more than 50% after 12 h while smaller snails survived at a less than 5% rate after 12 h. We also found significant variation both between and within MLGs in their survival rate when completely removed from water and dried with the MLG from the western US having a more than 50% greater survival probability than the eastern MLGs at both 18 and 24 h out of water. When removed from water and kept moist all MLGs had a near 100% survival rate at 60 days at 7 °C, and most survived at a greater than 90% rate at 60 days at 17 °C, while no MLG's survived past 30 days at 27 °C. The results demonstrate that variation for desiccation tolerance exists between populations of this invader which could influence the invasiveness of different populations.

Environmental Adaptation of Genetically Uniform Organisms with the Help of Epigenetic Mechanisms-An Insightful Perspective on Ecoepigenetics

Organisms adapt to different environments by selection of the most suitable phenotypes from the standing genetic variation or by phenotypic plasticity, the ability of single genotypes to produce different phenotypes in different environments. Because of near genetic identity, asexually reproducing populations are particularly suitable for the investigation of the potential and molecular underpinning of the latter alternative in depth. Recent analyses on the whole-genome scale of differently adapted clonal animals and plants demonstrated that epigenetic mechanisms such as DNA methylation, histone modifications and non-coding RNAs are among the molecular pathways supporting phenotypic plasticity and that epigenetic variation is used to stably adapt to different environments. Case studies revealed habitat-specific epigenetic fingerprints that were maintained over subsequent years pointing at the existence of epigenetic ecotypes. Environmentally induced epimutations and corresponding gene expression changes provide an ideal means for fast and directional adaptation to changing or new conditions, because they can synchronously alter phenotypes in many population members. Because microorganisms inclusive of human pathogens also exploit epigenetically mediated phenotypic variation for environmental adaptation, this phenomenon is considered a universal biological principle. The production of different phenotypes from the same DNA sequence in response to environmental cues by epigenetic mechanisms also provides a mechanistic explanation for the "general-purpose genotype hypothesis" and the "genetic paradox of invasions".

Primera cita en África del caracol del cieno de Nueva Zelanda Potamopyrgus antipodarum (J.E. Gray, 1843) (Tateidae, Mollusca)

Incluido recientemente entre las “cien peores” especies invasoras y siendo el tercero de los “peores moluscos invasores” en Europa, el caracol del cieno de Nueva Zelanda ha llegado a ser cosmopolita. Se comunica el primer hallazgo en el continente africano, en Marruecos. Se encontraron poblaciones establecidas de Potamopyrgus antipodarum en el humedal Low Moulouya, un Sitio Ramsar y un Sitio de Interés Biológico y Ecológico (SIBE), donde ocupa hábitats naturales y antropogénicos (manantial, río y canal artificial). El proceso de invasión parece estar en sus primeras etapas, lo que exige medidas drásticas para poder controlar su progreso.

Cool birds: facultative use by an introduced species of mechanical air conditioning systems during extremely hot outdoor conditions

Rapid climate change across the globe is having dramatic effects on wildlife. Responses of organisms to shifting thermal conditions often include physiological and behavioural accommodations, but to date these have been largely viewed and studied as naturally evolved phenomena (e.g. heat avoidance, sweating, panting) and not necessarily as strategies where animals exploit other anthropogenic conditions or resources. Moreover, the degree to which native versus introduced species show thermal plasticity has generated much conservation and ecological interest. We previously have observed introduced rosy-faced lovebirds (Agapornis roseicollis) perching in the relief-air vents on building faces in the Phoenix, Arizona, USA, metropolitan area, but doing so only during summer. Here, we show that such vent-perching events are significantly associated with extreme outdoor summer temperatures (when daily local highs routinely exceed 40°C). In fact, the temperature threshold at which we detected lovebirds starting to perch in cool air vents mirrors the upper range of the thermoneutral zone for this species. These results implicate novel, facultative use of an anthropogenic resource-industrial air-conditioning systems-by a recently introduced species (within the last 35 years) to cool down and survive extremely hot conditions in this urban 'heat-island' environment.

Effects of osmotic and thermal shock on the invasive aquatic mudsnail Potamopyrgus antipodarum: mortality and physiology under stressful conditions

Invasive freshwater species, such as the exotic mollusc Potamopyrgus antipodarum (New Zealand mudsnail), can frequently survive under harsh conditions, including brackish and hypoxic environments. We experimentally assessed the effects of osmotic (0, 10, 20, 25 and 30 psu) and thermal (20 °C) shock on mortality, activity and physiology of P. antipodarum collected at Capitol Lake, Olympia, Washington, USA, during winter and spring seasons when environmental temperature was 5 and 10 °C respectively. We measured standard metabolic rate and enzymatic activities (malate dehydrogenase, lactate dehydrogenase, alanopine dehydrogenase) in snails after a 10-day acclimation period at high salinity. Significantly higher mortalities were observed at higher salinities; the strongest effects occurred on snails collected at the end of winter, and exposed to 30 psu and 20 °C (100% mortality in 3 days). When snails were collected during the spring, 100% mortality was observed after 40 days at 30 psu and 20 °C. Standard metabolic rates were significantly lower when snails were exposed to salinities of 25 and 30 psu, even after 10 days of acclimation. Enzymatic activities showed small but significant declines after 10 days at 30 psu reflecting the declines observed in overall metabolism. The physiological tolerances to temperature and salinity displayed by this population of P. antipodarum make its eradication from Capital Lake difficult to achieve.

Response of the mollusc communities to environmental factors along an anthropogenic salinity gradient

Anthropogenic salinisation of freshwater ecosystems is frequent across the world. The scale of this phenomenon remains unrecognised, and therefore, monitoring and management of such ecosystems is very important. We conducted a study on the mollusc communities in inland anthropogenic ponds covering a large gradient of salinity located in an area of underground coal mining activity. A total of 14 gastropod and 6 bivalve species were noted. No molluscs were found in waters with total dissolved solids (TDS) higher than 17.1 g L^-1. The share of alien species in the communities was very high in waters with elevated salinity and significantly lower in the freshwaters. Canonical correspondence analysis (CCA) showed that TDS, pH, alkalinity, nitrate nitrogen, ammonium nitrogen, iron, the content of organic matter in sediments, the type of substrate and the content of sand and gravel in sediments were the variables that were significantly associated with the distribution of molluscs. The regression analysis revealed that total mollusc density was positively related to alkalinity and negatively related to nitrate nitrogen. The taxa richness was negatively related to TDS, which is consistent with previous studies which indicated that a high salinity level is a significant threat to freshwater malacofauna, causing a loss of biodiversity and contributing to the colonisation and establishment of alien species in aquatic ecosystems.

Regional epigenetic variation in asexual snail populations among urban and rural lakes

Epigenetic variation has the potential to influence environmentally dependent development and contribute to phenotypic responses to local environments. Environmental epigenetic studies of sexual organisms confirm the capacity to respond through epigenetic variation. An epigenetic response could be even more important in a population when genetic variation is lacking. A previous study of an asexual snail, Potamopyrgus antipodarum, demonstrated that different populations derived from a single clonal lineage differed in both shell phenotype and methylation signature when comparing lake versus river populations. Here, we examine methylation variation among lakes that differ in environmental disturbance and pollution histories. Snails were collected from a more pristine rural Lake 1 (Lake Lytle), and two urban lakes, Lake 2 (Capitol Lake) and Lake 3 (Lake Washington) on the Northwest Pacific coast. DNA methylation was assessed for each sample population using methylated DNA immunoprecipitation, MeDIP, followed by next-generation sequencing. The differential DNA methylation regions (DMRs) identified among the different lake comparisons suggested a higher number of DMRs and variation between rural Lake 1 and one urban Lake 2, and between the two urban Lakes 2 and 3, but limited variation between the rural Lake 1 and urban Lake 3. DMR genomic characteristics and gene associations were investigated. The presence of site-specific differences between each of these lake populations suggest an epigenetic response to varied environmental factors. The results do not support an effect of geographic distance in these populations. The role of dispersal distance among lakes, population history, environmental pollution and stably inherited methylation versus environmentally triggered methylation in producing the observed epigenetic variation are discussed. Observations support the proposal that epigenetic alterations may associate with phenotypic variation and environmental factors and history of the different lakes.

Genetic and epigenetic regulation of phenotypic variation in invasive plants – linking research trends towards a unified framework

Phenotypic variation in the introduced range of an invasive species can be modified by genetic variation, environmental conditions and their interaction, as well as stochastic events like genetic drift. Recent studies found that epigenetic modifications may also contribute to phenotypic variation being independent of genetic changes. Despite gaining profound ecological insights from empirical studies, understanding the relative contributions of these molecular mechanisms behind phenotypic variation has received little attention for invasive plant species in particular.

This review therefore aimed at summarizing and synthesizing information on the genetic and epigenetic basis of phenotypic variation of alien invasive plants in the introduced range and their evolutionary consequences. Transgenerational inheritance of epigenetic modifications was highlighted focusing on its influence on microevolution of the invasive plant species. We presented a comprehensive account of epigenetic regulation of phenotypic variation and its role in plant invasion in the presence of reduced standing genetic variation, inbreeding depression and associated genomic events which have often been observed during introduction and range expansion of an invasive alien species. Finally, taking clues from the studies conducted so far, we proposed a unified framework of future experimental approaches to understand ecological and evolutionary aspects of phenotypic variation. This holistic approach, being aligned to the invasion process in particular (introduction-establishment-spread), was intended to understand the molecular mechanisms of phenotypic variation of an invasive species in its introduced range and to disentangle the effects of standing genetic variation and epigenetic regulation of phenotypic variation.

Variation and Evolution of Function-Valued Traits

Function-valued traits—phenotypes whose expression depends on a continuous index (such as age, temperature, or space)—occur throughout biology and, like any trait, it is important to understand how they vary and evolve. Although methods for analyzing variation and evolution of function-valued traits are well developed, they have been underutilized by evolutionists, especially those who study natural populations. We seek to summarize advances in the study of function-valued traits and to make their analyses more approachable and accessible to biologists who could benefit greatly from their use. To that end, we explain how curve thinking benefits conceptual understanding and statistical analysis of functional data. We provide a detailed guide to the most flexible and statistically powerful methods and include worked examples (with R code) as supplemental material. We review ways to characterize variation in function-valued traits and analyze consequences for evolution, including constraint. We also discuss how selection on function-valued traits can be estimated and combined with estimates of heritable variation to project evolutionary dynamics.

Diet breadth modulates preference - performance relationships in a phytophagous insect community

In most phytophagous insects, larvae are less mobile than adults and their fitness depends on the plant chosen by their mother. To maximize fitness, adult preference and larval performance should thus be correlated. This correlation is not always apparent and seems to increase with the level of specialisation, i.e. specialists have a stronger preference for high quality host plant species compared to generalists. The aim of this study was to test whether the relationship between female preference and larval performance was stronger for specialists than for generalists within a community of fruit flies (Diptera: Tephritidae). A total of six fruit fly species was used, including four generalists, and two specialists co-existing in La Reunion island (France). We estimated oviposition preference through the number of eggs laid and larval performance through the larval survival on 29 different host plants species belonging to 15 families in the laboratory and evaluated the relationship between these two traits. Preference-performance relationship differed according to the degree of specialisation with a strong positive correlation for specialists and no relationship for generalists. These results substantiate the theory that choosing high quality hosts is more important for specialists that are adapted to survive on fewer host plants than for generalists.

Sensitivity to dietary phosphorus limitation in native vs. invasive lineages of a New Zealand freshwater snail

Why some species and lineages are more likely to be invasive than others is one of the most important unanswered questions in basic and applied biology. In particular, the relative contributions to the invasion process of factors like pre-adaptation to invasiveness in the native range, evolution post-colonization, and random vs. non-random sampling of colonist lineages remain unclear. We use a powerful common garden approach to address the potential for a role for sensitivity to nutrient limitation in determining the invasiveness of particular lineages of Potamopyrgus antipodarum, a New Zealand freshwater snail that has become globally invasive. We quantified specific growth rate (SGR), an important fitness-related trait in this species, under high phosphorus (P) vs. low-P conditions for a diverse set of native and invasive P. antipodarum. This experiment revealed that native-range P. antipodarum experience a more severe decline in SGR in low-P conditions relative to SGR in high-P conditions than their invasive range counterparts. Although these results suggest resilience to P limitation in invasive lineages, the absence of significant absolute differences in SGR between native and invasive lineages indicates that a straightforward connection between response to P limitation and invasiveness in P. antipodarum is unlikely. Regardless, our data demonstrate that invasive vs. native lineages of P. antipodarum exhibit consistently different responses to an important environmental variable that is rarely studied in the context of invasion success. Further studies directed at exploring and disentangling the roles of sampling effects, selection on preexisting variation, and evolution after colonization will be required to provide a comprehensive picture of the role (or lack thereof) of nutrient limitation in the global invasion of P. antipodarum, as well for as other invasive taxa.

A population of sexual Daphnia pulex resists invasion by asexual clones

Asexual reproduction avoids the costs associated with sex, predicting that invading asexual clones can quickly replace sexual populations. Daphnia pulex populations in the Great Lakes area are predominately asexual, but the elimination of sexual populations by invading clones is poorly understood. Asexual clones were detected at low frequency in one rare sexual population in 1995, with some increase in frequency during 2003 and 2004. However, these clones remained at low frequency during further yearly sampling (2005-2013) with no evidence that the resident sexual population was in danger of elimination. There was evidence for hybridization between rare males produced by asexual clones and sexual females with the potential to produce new asexual genotypes and spread the genetic factors for asexuality. In a short-term laboratory competition experiment, the two most common asexual clones did not increase in frequency relative to a genetically diverse sexual population due in part to a greater investment in diapausing eggs that trades-off current population growth for increased contribution to the egg bank. Our results suggest that a successful invasion can be prolonged, requiring a combination of clonal genotypes with high fitness, persistence of clones in the egg bank and negative factors affecting the sexual population such as inbreeding depression resulting from population bottlenecks.

Phenotypic plasticity of the introduced New Zealand mud snail, Potamopyrgus antipodarum, compared to sympatric native snails

Phenotypic plasticity is likely to be important in determining the invasive potential of a species, especially if invasive species show greater plasticity or tolerance compared to sympatric native species. Here in two separate experiments we compare reaction norms in response to two environmental variables of two clones of the New Zealand mud snail, Potamopyrgus antipodarum, isolated from the United States, (one invasive and one not yet invasive) with those of two species of native snails that are sympatric with the invader, Fossaria bulimoides group and Physella gyrina group. We placed juvenile snails in environments with high and low conductivity (300 and 800 mS) in one experiment, and raised them at two different temperatures (16°C and 22°C) in a second experiment. Growth rate and mortality were measured over the course of 8 weeks. Mortality rates were higher in the native snails compared to P. antipodarum across all treatments, and variation in conductivity influenced mortality. In both experiments, reaction norms did not vary significantly between species. There was little evidence that the success of the introduced species is a result of greater phenotypic plasticity to these variables compared to the sympatric native species.

Responses to nitrogen pulses and growth under low nitrogen availability in invasive and native tree species with differing successional status

Invasive species are frequently found in recently disturbed sites. To examine how these disturbance-dependent invasive species exploit resource pulses resulting from disturbance, twelve physiological and morphological traits, including age-dependent responsiveness in leaf traits to nitrogen pulse, were compared between Bischofia javanica, an invasive tree species in Ogasawara islands, and three native Ogasawara species, each having a different successional status. When exposed to a nitrogen pulse, invasive B. javanica showed higher increases in photosynthetic capacity, leaf area, epidermal cell number and cell size in leaves of broad age classes, and root nitrogen absorption ability than two native mid-/late or late-successional species, but showed no particular superiority to a native pioneer species in these responses. Under low nitrogen, however, it showed the largest relative growth rate among the four species, while the native pioneer showed the lowest growth. From these results, we concluded that the combination of moderately high responsiveness to resource pulses and the ability to maintain steady growth under resource limitations may give B. javanica a competitive advantage over a series of native species with different successional status from early to late-successional stages.

Niche differentiation and colonization of a novel environment by an asexual parasitic wasp

How do asexual taxa become adapted to a diversity of environments, and how do they persist despite changing environmental conditions? These questions are linked by their mutual focus on the relationship between genetic variation, which is often limited in asexuals, and the ability to respond to environmental variation. Asexual taxa originating from a single ancestor present a unique opportunity to assess rates of phenotypic and genetic change when access to new genetic variation is limited to mutation. Diachasma muliebre is an asexual Hymenopteran wasp that is geographically and genetically isolated from all sexual relatives. D. muliebre attack larvae of the western cherry fruit fly (Rhagoletis indifferens), which in turn feed inside bitter cherry fruit (Prunus emarginata) in August and September. R. indifferens has recently colonized a new host plant with an earlier fruiting phenology (June/July), domesticated sweet cherries (P. avium), and D. muliebre has followed its host into this temporally earlier niche. We tested three hypotheses: 1) that all D. muliebre lineages originate from a single asexual ancestor; 2) that different D. muliebre lineages (as defined by unique mtDNA haplotypes) have differentiated on their ancestral host in an important life-history trait, eclosion timing; and 3) that early-eclosing lineages have preferentially colonized the new sweet cherry niche. We find that mitochondrial COI and microsatellite data provide strong support for a single ancestral origin for all lineages. Furthermore, COI sequencing revealed five mitochondrial haplotypes among D. muliebre, and individual wasps possessing one distinctive mitochondrial haplotype (haplotype II) eclosed as reproductive adults significantly earlier than wasps with all other haplotypes. In addition, this early-eclosing lineage of D. muliebre is one of two lineages that have colonized the P. avium habitat, consistent with the preferential colonization hypothesis. These data suggest that D. muliebre has evolved adaptive phenotypic variation despite limited genetic variation, and that this variation has subsequently allowed an expansion of some wasps into a novel habitat. The D. muliebre system may allow for in-depth study of adaptation and long-term persistence of asexual taxa.

Adaptive responses and invasion: the role of plasticity and evolution in snail shell morphology

Invasive species often exhibit either evolved or plastic adaptations in response to spatially varying environmental conditions. We investigated whether evolved or plastic adaptation was driving variation in shell morphology among invasive populations of the New Zealand mud snail (Potamopyrgus antipodarum) in the western United States. We found that invasive populations exhibit considerable shell shape variation and inhabit a variety of flow velocity habitats. We investigated the importance of evolution and plasticity by examining variation in shell morphological traits 1) between the parental and F₁ generations for each population and 2) among populations of the first lab generation (F₁) in a common garden, full-sib design using Canonical Variate Analyses (CVA). We compared the F₁ generation to the parental lineages and found significant differences in overall shell shape indicating a plastic response. However, when examining differences among the F₁ populations, we found that they maintained among-population shell shape differences, indicating a genetic response. The F₁ generation exhibited a smaller shell morph more suited to the low-flow common garden environment within a single generation. Our results suggest that phenotypic plasticity in conjunction with evolution may be driving variation in shell morphology of this widespread invasive snail.

Genetics and evolution of function-valued traits: understanding environmentally responsive phenotypes

Many central questions in ecology and evolutionary biology require characterizing phenotypes that change with time and environmental conditions. Such traits are inherently functions, and new 'function-valued' methods use the order, spacing, and functional nature of the data typically ignored by traditional univariate and multivariate analyses. These rapidly developing methods account for the continuous change in traits of interest in response to other variables, and are superior to traditional summary-based analyses for growth trajectories, morphological shapes, and environmentally sensitive phenotypes. Here, we explain how function-valued methods make flexible use of data and lead to new biological insights. These approaches frequently offer enhanced statistical power, a natural basis of interpretation, and are applicable to many existing data sets. We also illustrate applications of function-valued methods to address ecological, evolutionary, and behavioral hypotheses, and highlight future directions.

Androgenesis: a review through the study of the selfish shellfish Corbicula spp

Among the asexual reproductive modes, androgenesis is probably one of the most astonishing and least studied mechanisms. In this 'paternal monopolization', the maternal nuclear genome fails to participate in zygote development and offspring are paternal nuclear clones. Obligate androgenesis is known in only a few organisms, including multiple species of clam in the genus Corbicula. Corbicula is a good system to review the evolutionary consequences of this 'all-male asexuality' because the cytological mechanisms of androgenetic reproduction have been described. In Corbicula, sperm are unreduced and, after fertilization, the maternal nuclear chromosomes are extruded as two polar bodies. Hermaphroditic lineages of Corbicula have a worldwide distribution and seem to reproduce through androgenesis, whereas their sexual relatives have restricted ranges. The invasive success of these androgenetic Corbicula lineages may be linked to their asexual mode of reproduction. We review the phenomenon of androgenesis, focusing on evolutionary perspectives, using the genus Corbicula as an exemplar system.

General-purpose genotype or how epigenetics extend the flexibility of a genotype

This project aims at investigating the link between individual epigenetic variability (not related to genetic variability) and the variation of natural environmental conditions. We studied DNA methylation polymorphisms of individuals belonging to a single genetic lineage of the clonal diploid fish Chrosomus eos-neogaeus sampled in seven geographically distant lakes. In spite of a low number of informative fragments obtained from an MSAP analysis, individuals of a given lake are epigenetically similar, and methylation profiles allow the clustering of individuals in two distinct groups of populations among lakes. More importantly, we observed a significant pH variation that is consistent with the two epigenetic groups. It thus seems that the genotype studied has the potential to respond differentially via epigenetic modifications under variable environmental conditions, making epigenetic processes a relevant molecular mechanism contributing to phenotypic plasticity over variable environments in accordance with the GPG model.