Invasions and extinctions through the looking glass of evolutionary ecology

Multiple introductions of divergent lineages and admixture conferred the high invasiveness in a widespread weed (Hypochaeris radicata)

Biological invasion consists of spatially and temporally varying stages, accompanied by ecological and evolutionary changes. Understanding the genomics underlying invasion dynamics provides critical insights into the geographic sources and genetic diversity, contributing to successful invasions across space and time. Here, we used genomic data and model-based approaches to characterize the invasion dynamics of Hypochaeris radicata L., a noxious weed in Korea. Genetic diversity and assignment patterns were investigated using 3563 SNPs of 283 individuals sampled from 22 populations. We employed a coalescent-based simulation method to estimate demographic changes for each population and inferred colonization history using both phylogenetic and population genetic model-based approaches. Our data suggest that H. radicata has been repeatedly been introduced to Korea from multiple genetic sources within the last 50 years, experiencing weak population bottlenecks followed by subsequent population expansions. These findings highlight the potential for further range expansion, particularly in the presence of human-mediated dispersal. Our study represents the first population-level genomic research documenting the invasion dynamics of the successful worldwide invader, H. radicata, outside of Europe.

Severe Bottleneck Impacted the Genomic Structure of Egg-Eating Cichlids in Lake Victoria

Within 15,000 years, the explosive adaptive radiation of haplochromine cichlids in Lake Victoria, East Africa, generated 500 endemic species. In the 1980s, the upsurge of Nile perch, a carnivorous fish artificially introduced to the lake, drove the extinction of more than 200 endemic cichlids. The Nile perch predation particularly harmed piscivorous cichlids, including paedophages, cichlids eat eggs and fries, which is an example of the unique trophic adaptation seen in African cichlids. Here, aiming to investigate past demographic events possibly triggered by the invasion of Nile perch and the subsequent impacts on the genetic structure of cichlids, we conducted large-scale comparative genomics. We discovered evidence of recent bottleneck events in 4 species, including 2 paedophages, which began during the 1970s to 1980s, and population size rebounded during the 1990s to 2000s. The timing of the bottleneck corresponded to the historical records of endemic haplochromines" disappearance and later resurgence, which is likely associated with the introduction of Nile perch by commercial demand to Lake Victoria in the 1950s. Interestingly, among the 4 species that likely experienced bottleneck, Haplochromis sp. "matumbi hunter," a paedophagous cichlid, showed the most severe bottleneck signatures. The components of shared ancestry inferred by ADMIXTURE suggested a high genetic differentiation between matumbi hunter and other species. In contrast, our phylogenetic analyses highly supported the monophyly of the 5 paedophages, consistent with the results of previous studies. We conclude that high genetic differentiation of matumbi hunter occurred due to the loss of shared genetic components among haplochromines in Lake Victoria caused by the recent severe bottleneck.

A nature-positive future with biological invasions: theory, decision support and research needs

In 2050, most areas of biodiversity significance will be heavily influenced by multiple drivers of environmental change. This includes overlap with the introduced ranges of many alien species that negatively impact biodiversity. With the decline in biodiversity and increase in all forms of global change, the need to envision the desired qualities of natural systems in the Anthropocene is growing, as is the need to actively maintain their natural values. Here, we draw on community ecology and invasion biology to (i) better understand trajectories of change in communities with a mix of native and alien populations, and (ii) to frame approaches to the stewardship of these mixed-species communities. We provide a set of premises and actions upon which a nature-positive future with biological invasions (NPF-BI) could be based, and a decision framework for dealing with uncertain species movements under climate change. A series of alternative management approaches become apparent when framed by scale-sensitive, spatially explicit, context relevant and risk-consequence considerations. Evidence of the properties of mixed-species communities together with predictive frameworks for the relative importance of the ecological processes at play provide actionable pathways to a NPF in which the reality of mixed-species communities are accommodated and managed. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

Genomic Tools in Biological Invasions: Current State and Future Frontiers

Human activities are accelerating rates of biological invasions and climate-driven range expansions globally, yet we understand little of how genomic processes facilitate the invasion process. Although most of the literature has focused on underlying phenotypic correlates of invasiveness, advances in genomic technologies are showing a strong link between genomic variation and invasion success. Here, we consider the ability of genomic tools and technologies to (i) inform mechanistic understanding of biological invasions and (ii) solve real-world issues in predicting and managing biological invasions. For both, we examine the current state of the field and discuss how genomics can be leveraged in the future. In addition, we make recommendations pertinent to broader research issues, such as data sovereignty, metadata standards, collaboration, and science communication best practices that will require concerted efforts from the global invasion genomics community.

Alien plant fitness is limited by functional trade-offs rather than a long-term increase in competitive effects of native communities

Alien plants experience novel abiotic conditions and interactions with native communities in the introduced area. Intra- and interspecific selection on functional traits in the new environment may lead to increased population growth with time since introduction (residence time). However, selection regimes might differ depending on the invaded habitat. Additionally, in high-competition habitats, a build-up of biotic resistance of native species due to accumulation of eco-evolutionary experience to aliens over time may limit invasion success. We tested if the effect of functional traits and the population dynamics of aliens depends on interspecific competition with native plant communities. We conducted a multi-species experiment with 40 annual Asteraceae that differ in residence time in Germany. We followed their population growth in monocultures and in interspecific competition with an experienced native community (varying co-existence times between focals and community). To more robustly test our findings, we used a naïve community that never co-existed with the focals. We found that high seed mass decreased population growth in monocultures but tended to increase population growth under high interspecific competition. We found no evidence for a build-up of competition-mediated biotic resistance by the experienced community over time. Instead, population growth of the focal species was similarly inhibited by the experienced and naïve community. By comparing the effect of experienced and naïve communities on population dynamics over 2 years across a large set of species with a high variation in functional traits and residence time, this study advances the understanding of the long-term dynamics of plant invasions. In our study system, population growth of alien species was not limited by an increase of competitive effects by native communities (one aspect of biotic resistance) over time. Instead, invasion success of alien plants may be limited because initial spread in low-competition habitats requires different traits than establishment in high-competition habitats.

Demographic history and genomic consequences of 10,000 generations of isolation in a wild mammal

Increased human activities caused the isolation of populations in many species-often associated with genetic depletion and negative fitness effects. The effects of isolation are predicted by theory, but long-term data from natural populations are scarce. We show, with full genome sequences, that common voles (Microtus arvalis) in the Orkney archipelago have remained genetically isolated from conspecifics in continental Europe since their introduction by humans over 5,000 years ago. Modern Orkney vole populations are genetically highly differentiated from continental conspecifics as a result of genetic drift processes. Colonization likely started on the biggest Orkney island and vole populations on smaller islands were gradually split off, without signs of secondary admixture. Despite having large modern population sizes, Orkney voles are genetically depauperate and successive introductions to smaller islands resulted in further reduction of genetic diversity. We detected high levels of fixation of predicted deleterious variation compared with continental populations, particularly on smaller islands, yet the fitness effects realized in nature are unknown. Simulations showed that predominantly mildly deleterious mutations were fixed in populations, while highly deleterious mutations were purged early in the history of the Orkney population. Relaxation of selection overall due to benign environmental conditions on the islands and the effects of soft selection may have contributed to the repeated, successful establishment of Orkney voles despite potential fitness loss. Furthermore, the specific life history of these small mammals, resulting in relatively large population sizes, has probably been important for their long-term persistence in full isolation.

Adaptive plasticity and fitness costs of endangered, nonendangered, and invasive plants in response to variation in nitrogen and phosphorus availabilities

Global change drivers such as eutrophication and plant invasions will create novel environments for many plant species. Through adaptive trait plasticity plants may maintain their performance under these novel conditions and may outcompete those showing low-adaptive trait plasticity. In a greenhouse study, we determined if plasticity in traits is adaptive or maladaptive in endangered, nonendangered, and invasive plant species in response to variation of nitrogen (N) and phosphorus (P) availability (N:P ratios 1.7, 15, and 135) and whether plastic trait responses are adaptive and/or costly for fitness (i.e., biomass). Species choice comprised 17 species from three functional groups (legumes, nonlegume forbs, and grasses), either classified as endangered, nonendangered, or invasive. After 2 months, plants were harvested and nine traits related to carbon assimilation and nutrient uptake were measured (leaf area, SLA, LDMC, SPAD, RMR, root length, SRL, root surface area, and PME activity). We found more traits responding plastically to variation in P than in N. Plasticity only created costs when P was varied. Plasticity in traits was mostly adaptively neutral toward fitness, with plasticity in three traits being similarly adaptive across all species groups: SPAD (as a measure of chlorophyll content, adaptive to N and P limitation), leaf area, and root surface area (adaptive to P limitation). We found little differences in trait plasticity between endangered, nonendangered, and invasive species. Synthesis. Along a gradient from N limitation, balanced N:P supply, and P limitation, we found that the type of fluctuating nutrient (i.e., if N or P is varied) is decisive for the adaptive value of a trait. Variation in P availability (from balanced supply to P limitation) created both a stronger reduction in fitness as well as created plasticity costs in more traits than variation in N availability (from balanced supply to N limitation). However, the patterns observed in our study may change if nutrient availability is altered, either by nutrient inputs or by a shift in nutrient availabilities, for example, by decreasing N input as foreseen by European Legislation, but without simultaneously decreasing P input.

Shifting effects of host physiological condition following pathogen establishment

Understanding host persistence with emerging pathogens is essential for conserving populations. Hosts may initially survive pathogen invasions through pre-adaptive mechanisms. However, whether pre-adaptive traits are directionally selected to increase in frequency depends on the heritability and environmental dependence of the trait and the costs of trait maintenance. Body condition is likely an important pre-adaptive mechanism aiding in host survival, although can be seasonally variable in wildlife hosts. We used data collected over 7 years on bat body mass, infection and survival to determine the role of host body condition during the invasion and establishment of the emerging disease, white-nose syndrome. We found that when the pathogen first invaded, bats with higher body mass were more likely to survive, but this effect dissipated following the initial epizootic. We also found that heavier bats lost more weight overwinter, but fat loss depended on infection severity. Lastly, we found mixed support that bat mass increased in the population after pathogen arrival; high annual plasticity in individual bat masses may have reduced the potential for directional selection. Overall, our results suggest that some factors that contribute to host survival during pathogen invasion may diminish over time and are potentially replaced by other host adaptations.

Global invasion history and native decline of the common starling: insights through genetics

Few invasive birds are as globally successful as the Common or European Starling (Sturnus vulgaris). Native to the Palearctic, the starling has been intentionally introduced to North and South America, South Africa, Australia, and the Pacific Islands, enabling us to explore species traits that may contribute to its invasion success. Coupling the rich studies of life history and more recent explorations of genomic variation among invasions, we illustrate how eco-evolutionary dynamics shape the invasion success of this long-studied and widely distributed species. Especially informative is the comparison between Australian and North American invasions, because these populations colonized novel ranges concurrently and exhibit shared signals of selection despite distinct population histories. In this review, we describe population dynamics across the native and invasive ranges, identify putatively selected traits that may influence the starling’s spread, and suggest possible determinants of starling success world-wide. We also identify future opportunities to utilize this species as a model for avian invasion research, which will inform our understanding of species’ rapid evolution in response to environmental change.

Genetic diversity and structure of a recent fish invasion: Tench (Tinca tinca) in eastern North America

Introduced and geographically expanding populations experience similar eco-evolutionary challenges, including founder events, genetic bottlenecks, and novel environments. Theory predicts that reduced genetic diversity resulting from such phenomena limits the success of introduced populations. Using 1900 SNPs obtained from restriction-site-associated DNA sequencing, we evaluated hypotheses related to the invasion history and connectivity of an invasive population of Tench (Tinca tinca), a Eurasian freshwater fish that has been expanding geographically in eastern North America for three decades. Consistent with the reported history of a single introduction event, our findings suggest that multiple introductions from distinct genetic sources are unlikely as Tench had a small effective population size (~114 [95% CI = 106-123] individuals), no strong population subdivision across time and space, and evidence of a recent genetic bottleneck. The large genetic neighbourhood size (220 km) and weak within-population genetic substructure suggested high connectivity across the invaded range, despite the relatively large area occupied. There was some evidence for a small decay in genetic diversity as the species expanded northward, but not southward, into new habitats. As eradicating the species within a ~112 km radius would be necessary to prevent recolonization, eradicating Tench is likely not feasible at watershed-and possibly local-scales. Management should instead focus on reducing abundance in priority conservation areas to mitigate adverse impacts. Our study indicates that introduced populations can thrive and exhibit relatively high levels of genetic diversity despite severe bottlenecks (<1.5% of the ancestral effective population size) and suggests that landscape heterogeneity and population demographics can generate variability in spatial patterns of genetic diversity within a single range expansion.

Population genetic structure of the globally introduced big-headed ant in Taiwan

Global commerce and transportation facilitate the spread of invasive species. The African big-headed ant, Pheidole megacephala (Fabricius), has achieved worldwide distribution through globalization. Since the late 19th century, Taiwan has served as a major seaport because of its strategic location. The population genetic structure of P. megacephala in Taiwan is likely to be shaped by international trade and migration between neighboring islands. In this study, we investigated the population genetics of P. megacephala colonies sampled from four geographical regions in Taiwan and elucidated the population genetic structures of P. megacephala sampled from Taiwan, Okinawa, and Hawaii. We observed a low genetic diversity of P. megacephala across regions in Taiwan. Moreover, we noted low regional genetic differentiation and did not observe isolation by distance, implying that long-distance jump dispersal might have played a crucial role in the spread of P. megacephala. We sequenced the partial cytochrome oxidase I gene and observed three mitochondrial haplotypes (TW1-TW3). TW1 and TW3 most likely originated from populations within the species' known invasive range, suggesting that secondary introduction is the predominant mode of introduction for this invasive ant. TW2 represents a novel haplotype that was previously unreported in other regions. P. megacephala populations from Taiwan, Okinawa, and Hawaii exhibited remarkable genetic similarity, which may reflect their relative geographic proximity and the historical connectedness of the Asia-Pacific region.

Does Trypanosoma cruzi (Chagas, 1909) (Kinetoplastida: Trypanosomatidae) modify the antennal phenotype of Triatoma dimidiata (Latreille, 1811) (Hemiptera: Triatominae)?

Background

Triatoma dimidiata is a vector of the protozoan parasite Trypanosoma cruzi, the etiologic agent of Chagas disease. Phenotypic plasticity allows an organism to adjust its phenotype in response to stimuli or environmental conditions. Understanding the effect of T. cruzi on the phenotypic plasticity of its vectors, known as triatomines, has attracted great interest because of the implications of the parasite–triatomine interactions in the eco-epidemiology and transmission of the etiologic agent of Chagas disease. We investigated if the infection of the vector with T. cruzi may be associated with a change in the antennal phenotype of sylvatic, domestic, and laboratory-reared populations of T. dimidiata.

Methods

The abundance of each type of sensillum (bristles, basiconic, thick- and thin-walled trichoid) on the antennae of T.cruzi-infected and non-infected T.dimidiata reared in the laboratory or collected in sylvatic and domestic ecotopes were measured under light microscopy and compared using Kruskal–Wallis non-parametric tests and permutational multivariate analysis of variance.

Results

We found significant differences between sensilla patterns of infected and non-infected insects within sylvatic and domestic populations. Conversely, we found no significant differences between sensilla patterns of infected and non-infected insects within the laboratory-reared population. Besides, for sylvatic and domestic populations, sexual dimorphism tended to be increased in infected insects.

Conclusion

The differences observed in infected insects could be linked to higher efficiency in the perception of odor molecules related to the search for distant mates and hosts and the flight dispersal in search of new habitats. In addition, these insects could have a positive effect on population dynamics and the transmission of T.cruzi.

Graphical Abstract

Jack, master or both? The invasive ladybird Harmonia axyridis performs better than a native coccinellid despite divergent trait plasticity

The plasticity of performance traits can promote the success of biological invasions and therefore, precisely estimating trait reaction norms can help to predict the establishment and persistence of introduced species in novel habitats. Most studies focus only on a reduced set of traits and rarely include trait variability that may be vital to predicting establishment success. Here, using a split-brood full-sib design, we acclimated the globally invasive ladybird Harmonia axyridis and a native co-occurring and competing species Cheilomenes lunata to cold, medium and warm temperature regimes, and measured critical thermal limits, life-history traits, and starvation resistance. We used the conceptual framework of “Jack, Master or both” to test predictions regarding performance differences of these two species. The native C. lunata had a higher thermal plasticity of starvation resistance and a higher upper thermal tolerance than H. axyridis. By contrast, H. axyridis had a higher performance than C. lunata for preoviposition period, fecundity and adult emergence from pupae. We combined trait responses, transport duration and propagule pressure to predict the size of the populations established in a novel site following cold, medium and warm scenarios. Although C. lunata initially had a higher performance than the invasive species during transport, more individuals of H. axyridis survived in all simulated environments due to the combined life-history responses, and in particular, higher fecundity. Despite an increased starvation mortality in the warm scenario, given a sufficient propagule size, H. axyridis successfully established. This study underscores how the combination and plasticity of multiple performance traits can strongly influence establishment potential of species introduced into novel environments.

Populations of western North American monkeyflowers accrue niche breadth primarily via genotypic divergence in environmental optima

Niche breadth, the range of environments that individuals, populations, and species can tolerate, is a fundamental ecological and evolutionary property, yet few studies have examined how niche breadth is partitioned across biological scales. We use a published dataset of thermal performance for a single population from each of 10 closely related species of western North American monkeyflowers (genus Mimulus) to investigate whether populations achieve broad thermal niches through general purpose genotypes, specialized genotypes with divergent environmental optima, and/or variation among genotypes in the degree of generalization. We found the strongest relative support for the hypothesis that populations with greater genetic variation for thermal optimum had broader thermal niches, and for every unit increase in among‐family variance in thermal optimum, population‐level thermal breadth increased by 0.508°C. While the niche breadth of a single genotype represented up to 86% of population‐level niche breadth, genotype‐level niche breadth had a weaker positive effect on population‐level breadth, with every 1°C increase in genotypic thermal breadth resulting in a 0.062°C increase in population breadth. Genetic variation for thermal breadth was not predictive of population‐level thermal breadth. These findings suggest that populations of Mimulus species have achieved broad thermal niches primarily through genotypes with divergent thermal optima and to a lesser extent via general‐purpose genotypes. Future work examining additional biological hierarchies would provide a more comprehensive understanding of how niche breadth partitioning impacts the vulnerabilities of individuals, populations, and species to environmental change.

Genomics reveals the history of a complex plant invasion and improves the management of a biological invasion from the South African-Australian biotic exchange

Many plants exchanged in the global redistribution of species in the last 200 years, particularly between South Africa and Australia, have become threatening invasive species in their introduced range. Refining our understanding of the genetic diversity and population structure of native and alien populations, introduction pathways, propagule pressure, naturalization, and initial spread, can transform the effectiveness of management and prevention of further introductions. We used 20,221 single nucleotide polymorphisms to reconstruct the invasion of a coastal shrub, Chrysanthemoides monilifera ssp. rotundata (bitou bush) from South Africa, into eastern Australia (EAU), and Western Australia (WAU). We determined genetic diversity and population structure across the native and introduced ranges and compared hypothesized invasion scenarios using Bayesian modeling. We detected considerable genetic structure in the native range, as well as differentiation between populations in the native and introduced range. Phylogenetic analysis showed the introduced samples to be most closely related to the southern‐most native populations, although Bayesian analysis inferred introduction from a ghost population. We detected strong genetic bottlenecks during the founding of both the EAU and WAU populations. It is likely that the WAU population was introduced from EAU, possibly involving an unsampled ghost population. The number of private alleles and polymorphic SNPs successively decreased from South Africa to EAU to WAU, although heterozygosity remained high. That bitou bush remains an invasion threat in EAU, despite reduced genetic diversity, provides a cautionary biosecurity message regarding the risk of introduction of potentially invasive species via shipping routes.

Invasive Widow Spiders Perform Differently At Low Temperatures than Conspecifics from the Native Range

Temperature challenges are one of the leading abiotic causes of success or failure of non-native species in a novel environment, and this is particularly true for low temperatures. Establishing and reproducing in a novel thermal environment can alter survival, behaviour, and traits related to fitness. It has been proposed that plasticity or adaptation of thermal tolerance may allow an introduced species to thrive, or that successful invaders may be those with a thermal breadth in their native habitat that encompasses their new environment. Here, we tested these hypotheses using native and invasive populations of Australian redback spiders (Latrodectus hasselti). We measured how exposure to temperatures (exposure to 15°C and 25°C, respectively) common to invasive and native range habitats affected behavioural and life-history traits and tradeoffs that may underlie fitness in an invasive population detected in 1995 in Japan and a native population from Australia. We found that the critical thermal minimum (CTmin) was higher in the invasive population from Japan than in the native population, but critical thermal maximum (CTmax) did not differ between populations. Compared to the invasive population, eggs from the native population had a longer development time and lower hatching success at 15°C. Both populations performed equally well at 25 °C, as measured by egg development time and hatching success. Invasive juveniles that developed at 15 °C were slower to explore a novel environment and less bold when tested at 25 °C vs. 15 °C. In comparison, the native population showed faster average exploration, with no differences in response at the two development or testing temperatures. Overall, L. hasselti from Japan maintained hatching success and development across a wider temperature range than the native population, indicating greater thermal breadth and higher behavioural plasticity. These results support the importance of plasticity in thermal tolerance and behaviour for a successful invasion under novel environmental temperatures.

Invasive Alien Slug Arion vulgaris Moquin-Tandon, 1855 (Gastropoda: Pulmonata: Arionidae) in Moscow Parks and Its Co-introduced Parasite Alloionema appendiculatum Schneider, 1859

PURPOSE

The present study investigates the origin of Arion vulgaris slugs in the parks of Moscow city and their parasites.

METHODS

Snails and slugs inhabiting green areas of Moscow city were collected in the summer season of 2020 and examined on the presence of gastropod-associated nematodes and trematodes using morphological and molecular methods.

RESULTS

The presence of the alien slug species, Arion vulgaris, was recorded in several locations, and the mitochondrial gene-based analysis has shown that slug populations inhabited Moscow parks originated from West and Central Europe. Out of a total of 15 gastropod species examined, A. vulgaris was the only species infected by the nematode Alloionema appendiculatum Schneider, 1859, a larval parasite of molluscs. It is the first record of this nematode from the territory of the Russian Federation. COX1 mtDNA sequences of A. appendiculatum obtained from 3 populations of infected slugs were identical with those from Western and Central Europe similarly to their gastropod hosts thus indicating that the nematodes travelled with their hosts. No parasites dangerous for humans or animals were found.

CONCLUSION

The complex life cycle of A. appendiculatum includes a free-living stage in soil which offers a source of infection for other potentially susceptible gastropod species but the capacity of A. appendiculatum to change hosts in local conditions needs to be further investigated. The particular susceptibility and tolerance of A. vulgaris to nematodes in our study was in concordance with earlier data while in contradiction with the enemy release hypothesis.

Evidence for continent-wide convergent evolution and stasis throughout 150 y of a biological invasion

Adaptive evolution can help species to persist and spread in new environments, but it is unclear how the rate and duration of adaptive evolution vary throughout species ranges and on the decadal timescales most relevant to managing biodiversity for the 21st century. Using herbarium records, we reconstruct 150 y of evolution in an invasive plant as it spread across North America. Flowering phenology evolves to adapt to local growing seasons throughout the range but stalls after about a century. This punctuated, convergent evolution recapitulates long-term dynamics in the fossil record, implicating limits to evolutionary rates that are not evident for the first century of spread.

The extent to which evolution can rescue a species from extinction, or facilitate range expansion, depends critically on the rate, duration, and geographical extent of the evolutionary response to natural selection. Adaptive evolution can occur quickly, but the duration and geographical extent of contemporary evolution in natural systems remain poorly studied. This is particularly true for species with large geographical ranges and for timescales that lie between “long-term” field experiments and the fossil record. Here, we introduce the Virtual Common Garden (VCG) to investigate phenotypic evolution in natural history collections while controlling for phenotypic plasticity in response to local growing conditions. Reconstructing 150 y of evolution in Lythrum salicaria (purple loosestrife) as it invaded North America, we analyze phenology measurements of 3,429 herbarium records, reconstruct growing conditions from more than 12 million local temperature records, and validate predictions across three common gardens spanning 10° of latitude. We find that phenological clines have evolved repeatedly throughout the range, during the first century of evolution. Thereafter, the rate of microevolution stalls, recapitulating macroevolutionary stasis observed in the fossil record. Our study demonstrates that preserved specimens are a critical resource for investigating limits to evolution in natural populations. Our results show how natural selection and trade-offs measured in field studies predict adaptive divergence observable in herbarium specimens over 15 decades at a continental scale.

Geographic dispersion of invasive crop pests: the role of basal, plastic climate stress tolerance and other complementary traits in the tropics

Global pest invasions have significantly increased in recent years. These invasions together with climate warming directly impact agriculture. Tropical climates feature extreme weather events, including high temperatures and seasonal droughts. Thus, successful invasive pests in tropics have to adapt to these extreme climate features. The intrinsic factors relevant to tropical invasion of insects have been explored in many studies, but the knowledge is rather dispersed in contemporary literature. Here, we reviewed the potential biophysical characters of successful invasive pests' adaption to tropical environments including [1] inherent high basal stress tolerance and advanced life-history performances [2], phenotypic plasticity [3], rapid evolution to environmental stress, polyphagy, diverse reproductive strategies and high fecundity. We summarised how these traits and their interactive effects enhance pest invasions in the tropics. Comprehensive understanding of how these characters facilitate invasion improves models for predicting ecological consequences of climate change on invasive pest species for improved pest management.

The evolution of thermal performance in native and invasive populations of Mimulus guttatus

The rise of globalization has spread organisms beyond their natural range, allowing further opportunity for species to adapt to novel environments and potentially become invaders. Yet, the role of thermal niche evolution in promoting the success of invasive species remains poorly understood. Here, we use thermal performance curves (TPCs) to test hypotheses about thermal adaptation during the invasion process. First, we tested the hypothesis that if species largely conserve their thermal niche in the introduced range, invasive populations may not evolve distinct TPCs relative to native populations, against the alternative hypothesis that thermal niche and therefore TPC evolution has occurred in the invasive range. Second, we tested the hypothesis that clines of TPC parameters are shallower or absent in the invasive range, against the alternative hypothesis that with sufficient time, standing genetic variation, and temperature‐mediated selection, invasive populations would re‐establish clines found in the native range in response to temperature gradients. To test these hypotheses, we built TPCs for 18 native (United States) and 13 invasive (United Kingdom) populations of the yellow monkeyflower, Mimulus guttatus. We grew clones of multiple genotypes per population at six temperature regimes in growth chambers. We found that invasive populations have not evolved different thermal optima or performance breadths, providing evidence for evolutionary stasis of thermal performance between the native and invasive ranges after over 200 years post introduction. Thermal optimum increased with mean annual temperature in the native range, indicating some adaptive differentiation among native populations that was absent in the invasive range. Further, native and invasive populations did not exhibit adaptive clines in thermal performance breadth with latitude or temperature seasonality. These findings suggest that TPCs remained unaltered post invasion, and that invasion may proceed via broad thermal tolerance and establishment in already climatically suitable areas rather than rapid evolution upon introduction.

Propagule size and sex ratio influence colonisation dynamics after introduction of a non-native lizard

The composition of founding populations plays an important role in colonisation dynamics and can influence population growth during early stages of biological invasion. Specifically, founding populations with small propagules (i.e., low number of founders) are vulnerable to the Allee effect and have reduced likelihood of establishment compared to those with large propagules. The founding sex ratio can also impact establishment via its influence on mating success and offspring production. Our goal was to test the effects of propagule size and sex ratio on offspring production and annual population growth following introductions of a non-native lizard species (Anolis sagrei). We manipulated propagule composition on nine small islands, then examined offspring production, population growth, and survival rate of founders and their descendants encompassing three generations. By the third reproductive season, per capita offspring production was higher on islands seeded with a relatively large propagule size, but population growth was not associated with propagule size. Propagule sex ratio did not affect offspring production, but populations with a female-biased propagule had positive growth, whereas those with a male-biased propagule had negative growth in the first year. Populations were not affected by propagule sex ratio in subsequent years, possibly due to rapid shifts towards balanced (or slightly female-biased) population sex ratios. Overall, we show that different components of population fitness have different responses to propagule size and sex ratio in ways that could affect early stages of biological invasion. Despite these effects, the short lifespan and high fecundity of A. sagrei likely helped small populations to overcome Allee effects and enabled all populations to successfully establish. Our rare experimental manipulation of propagule size and sex ratio can inform predictions of colonisation dynamics in response to different compositions of founding populations, which is critical in the context of population ecology and invasion dynamics.

Neutral and adaptive drivers of genomic change in introduced brook trout (Salvelinus fontinalis) populations revealed by pooled sequencing

Understanding the drivers of successful species invasions is important for conserving native biodiversity and for mitigating the economic impacts of introduced species. However, whole-genome resolution investigations of the underlying contributions of neutral and adaptive genetic variation in successful introductions are rare. Increased propagule pressure should result in greater neutral genetic variation, while environmental differences should elicit selective pressures on introduced populations, leading to adaptive differentiation. We investigated neutral and adaptive variation among nine introduced brook trout (Salvelinus fontinalis) populations using whole-genome pooled sequencing. The populations inhabit isolated alpine lakes in western Canada and descend from a common source, with an average of ~19 (range of 7-41) generations since introduction. We found some evidence of bottlenecks without recovery, no strong evidence of purifying selection, and little support that varying propagule pressure or differences in local environments shaped observed neutral genetic variation differences. Putative adaptive loci analysis revealed nonconvergent patterns of adaptive differentiation among lakes with minimal putatively adaptive loci (0.001%-0.15%) that did not correspond with tested environmental variables. Our results suggest that (i) introduction success is not always strongly influenced by genetic load; (ii) observed differentiation among introduced populations can be idiosyncratic, population-specific, or stochastic; and (iii) conservatively, in some introduced species, colonization barriers may be overcome by support through one aspect of propagule pressure or benign environmental conditions.

Which factor explains the life-history of Xanthium strumarium L., an aggressive alien invasive plant species, along its altitudinal gradient?

Invasive biology acknowledges the concept of better performance by invasive plants in the introduced range. Xanthium strumarium L. is one of the successful invasive species in Khyber Pakhtunkhwa, Pakistan. The phenological pattern, vegetative and reproductive traits plasticity analysis of the species was explored to explain the invasive success across the altitudinal gradient in the current invaded habitats. Phenological patterns and timing (seedling, vegetative growth, flowering and fruiting, drying, and seed bank) were observed during a full year for two seasons. We also examine plant functional traits at altitudes of 500, 1000, and 1500 m a.s.l. to assess traits and biomass variations. The X. strumarium exhibits late vegetative and reproductive phenology at higher altitudes, enabling them to occupy an empty niche and benefit from decreased competition for resource acquisition. The lower altitude plants show a higher growth rate (stem size increase, number of leaves, and leaf area) due to the higher nutrient availability. Higher altitude plants have the highest reproductive biomass and biomass ratio revealing plant abilities to be reproductively adapted in the higher altitudes. Among climatic variables, mean yearly temperature, mean annual yearly humidity, and mean day length in hours, while in soil variables, organic matter and nitrogen percentage significantly affect the phenological and morphological stages. Therefore, we conclude that X. strumarium can invade higher altitudes with a shift in its phenological and morphological changes making the invasion process successful.

Into the weeds: new insights in plant stress

Weeds, plants that thrive in the face of disturbance, have eluded human's attempts at control for >12 000 years, positioning them as a unique group of extreme stress tolerators. The most successful weeds have a suite of traits that enable them to rapidly adapt to environments typified by stress, growing in hostile conditions or subject to massive destruction from agricultural practices. Through their ability to persist and adapt, weeds illuminate principles of evolution and provide insights into weed management and crop improvement. Here we highlight why the time is right to move beyond traditional model systems and leverage weeds to gain a deeper understanding of the mechanisms, adaptations, and genetic and physiological bases for stress tolerance.

The Genomic Processes of Biological Invasions: From Invasive Species to Cancer Metastases and Back Again

The concept of invasion is useful across a broad range of contexts, spanning from the fine scale landscape of cancer tumors up to the broader landscape of ecosystems. Invasion biology provides extraordinary opportunities for studying the mechanistic basis of contemporary evolution at the molecular level. Although the field of invasion genetics was established in ecology and evolution more than 50 years ago, there is still a limited understanding of how genomic level processes translate into invasive phenotypes across different taxa in response to complex environmental conditions. This is largely because the study of most invasive species is limited by information about complex genome level processes. We lack good reference genomes for most species. Rigorous studies to examine genomic processes are generally too costly. On the contrary, cancer studies are fortified with extensive resources for studying genome level dynamics and the interactions among genetic and non-genetic mechanisms. Extensive analysis of primary tumors and metastatic samples have revealed the importance of several genomic mechanisms including higher mutation rates, specific types of mutations, aneuploidy or whole genome doubling and non-genetic effects. Metastatic sites can be directly compared to primary tumor cell counterparts. At the same time, clonal dynamics shape the genomics and evolution of metastatic cancers. Clonal diversity varies by cancer type, and the tumors’ donor and recipient tissues. Still, the cancer research community has been unable to identify any common events that provide a universal predictor of “metastatic potential” which parallels findings in evolutionary ecology. Instead, invasion in cancer studies depends strongly on context, including order of events and clonal composition. The detailed studies of the behavior of a variety of human cancers promises to inform our understanding of genome level dynamics in the diversity of invasive species and provide novel insights for management.

Can functional genomic diversity provide novel insights into mechanisms of community assembly? A pilot study from an invaded alpine streambed

An important focus of community ecology, including invasion biology, is to investigate functional trait diversity patterns to disentangle the effects of environmental and biotic interactions. However, a notable limitation is that studies usually rely on a small and easy-to-measure set of functional traits, which might not immediately reflect ongoing ecological responses to changing abiotic or biotic conditions, including those that occur at a molecular or physiological level. We explored the potential of using the diversity of expressed genes-functional genomic diversity (FGD)-to understand ecological dynamics of a recent and ongoing alpine invasion. We quantified FGD based on transcriptomic data measured for 26 plant species occurring along adjacent invaded and pristine streambeds. We used an RNA-seq approach to summarize the overall number of expressed transcripts and their annotations to functional categories, and contrasted this with functional trait diversity (FTD) measured from a suite of characters that have been traditionally considered in plant ecology. We found greater FGD and FTD in the invaded community, independent of differences in species richness. However, the magnitude of functional dispersion was greater from the perspective of FGD than from FTD. Comparing FGD between congeneric alien-native species pairs, we did not find many significant differences in the proportion of genes whose annotations matched functional categories. Still, native species with a greater relative abundance in the invaded community compared with the pristine tended to express a greater fraction of genes at significant levels in the invaded community, suggesting that changes in FGD may relate to shifts in community composition. Comparisons of diversity patterns from the community to the species level offer complementary insights into processes and mechanisms driving invasion dynamics. FGD has the potential to illuminate cryptic changes in ecological diversity, and we foresee promising avenues for future extensions across taxonomic levels and macro-ecosystems.

Biological invasions in France: Alarming costs and even more alarming knowledge gaps

The ever-increasing number of introduced species profoundly threatens global biodiversity. While the ecological and evolutionary consequences of invasive alien species are receiving increasing attention, their economic impacts have largely remained understudied, especially in France. Here, we aimed at providing a general overview of the monetary losses (damages caused by) and expenditures (management of) associated with invasive alien species in France. This country has a long history of alien species presence, partly due to its long-standing global trade activities, highly developed tourism, and presence of overseas territories in different regions of the globe, resulting in a conservative minimum of 2,750 introduced and invasive alien species. By synthesizing for the first time the monetary losses and expenditures incurred by invasive alien species in Metropolitan France and French overseas territories, we obtained 1,583 cost records for 98 invasive alien species. We found that they caused a conservative total amount ranging between US$ 1,280 million and 11,535 million in costs over the period 1993–2018. We extrapolated costs for species invading France, for which costs were reported in other countries but not in France, which yielded an additional cost ranging from US$ 151 to 3,030 millions. Damage costs were nearly eight times higher than management expenditure. Insects, and in particular the Asian tiger mosquito Aedes albopictus and the yellow fever mosquito Ae. aegypti, totalled very high economic costs, followed by non-graminoid terrestrial flowering and aquatic plants (Ambrosia artemisiifolia, Ludwigia sp. and Lagarosiphon major). Over 90% of alien species currently recorded in France had no costs reported in the literature, resulting in high biases in taxonomic, regional and activity sector coverages. To conclude, we report alarming costs and even more alarming knowledge gaps. Our results should raise awareness of the importance of biosecurity and biosurveillance in France, and beyond, as well as the crucial need for better reporting and documentation of cost data.

Reacquisition of light-harvesting genes in a marine cyanobacterium confers a broader solar niche

The evolution of phenotypic plasticity, i.e., the environmental induction of alternative phenotypes by the same genotype, can be an important mechanism of biological diversification.^1,2 For example, an evolved increase in plasticity may promote ecological niche expansion as well as the innovation of novel traits;³ however, both the role of phenotypic plasticity in adaptive evolution and its underlying mechanisms are still poorly understood.^4,5 Here, we report that the Chlorophyll d-producing marine cyanobacterium Acaryochloris marina strain MBIC11017 has evolved greater photosynthetic plasticity by reacquiring light-harvesting genes via horizontal gene transfer. The genes, which had been lost by the A. marina ancestor, are involved in the production and degradation of the light-harvesting phycobiliprotein phycocyanin. A. marina MBIC11017 exhibits a high degree of wavelength-dependence in phycocyanin production, and this ability enables it to grow with yellow and green light wavelengths that are inaccessible to other A. marina. Consequently, this strain has a broader solar niche than its close relatives. We discuss the role of horizontal gene transfer for regaining a lost phenotype in light of Dollo's Law⁶ that the loss of a complex trait is irreversible.

Opposing macroevolutionary and trait-mediated patterns of threat and naturalisation in flowering plants

Due to expanding global trade and movement of people, new plant species are establishing in exotic ranges at increasing rates while the number of native species facing extinction from multiple threats grows. Yet, how species losses and gains globally may, together, be linked to traits and macroevolutionary processes is poorly understood. Here, we show that, adjusting for diversification rate and clade age, the proportion of threatened species across flowering plant families is negatively related to the proportion of naturalised species per family. Moreover, naturalisation is positively associated with range size, short generation time, autonomous seed production and interspecific hybridisation, but negatively with age and diversification, whereas threat is negatively associated with range size and hybridisation, and positively with biotic pollination, age and diversification rate. That we find such a pronounced signature of naturalisation and threat across plant families suggests that both trait syndromes have coexisted over deep evolutionary time and counter to intuition, that neither strategy is necessarily superior to the other over long evolutionary timespans.

Are All Conservation Measures for Endangered Species Legitimate? Lines of Thinking With the European Hamster

When dealing with the protection of an endangered species, it appears more and more important to address the ethical limits and the societal perception of the implemented conservation measures. This will be illustrated here through the example of conservation programs of the European hamster (Cricetus cricetus) in France. The main threats for this critically endangered rodent are the impoverishment and fragmentation of its habitat due to recent changes in agricultural practices and urbanization. Thus, the status of this species changed from harmful to endangered in only a few decades. This must lead to acceptance of the species by citizens and especially farmers paid to destroy this species until the 1990s while nowadays to protect it. To stem the decline, several measures have been taken through the last 20 years including population reinforcement, wild animal tracking, and implementation of suitable habitats. One can, therefore, discuss the efficiency of these measures and their integration in the entire socio-ecosystem. Population reinforcement and the questions that can arise from it will first be addressed. Secondly, in situ animal monitoring and implications of the methods used will be discussed. Third, we will deal with agricultural practices favorable to the species. Finally, we will highlight the links between European hamster conservation measures and wider problematics.

Directed Evolution. The Legacy of a Nobel Prize

This article is part of an anniversary issue of Journal of Molecular Evolution, commenting on a paper published on 1999 by the Nobel laureate Frances Arnold and her colleague Kentaro Miyazaki. The paper by Miyazaki and Arnold presented saturation mutagenesis as an alternative method to random mutagenesis for obtaining enzymes with increasing stability. Both techniques were conceived to accomplish directed evolution, an approach honoured by the Nobel Prize of Chemistry 2018. Here, I am commenting on the pros and cons of random and saturation mutagenesis, while also discussing important results from directed evolution. I conclude that molecular evolution is finding new applications in science and it is definitely an integral part of the genomic era's revolution.

Populations of a widespread invader and co-occurring native species vary in phenotypic plasticity

Phenotypic plasticity can promote plant invasions and enhance impacts on native species, but little is known about variation in plasticity among invader populations compared with native species. Variation in plasticity among invader populations could inform more precise predictions of invader spread and impacts across heterogeneous resource environments. We used a common garden experiment with sun and shade treatments to test for variation in plasticity among 12 populations of an invasive grass (Imperata cylindrica), and to determine whether the invader exhibited greater plasticity than six native species that co-occur in the Southeast USA. Principal component analysis revealed that invader populations from different native ranges consistently varied from each other and native species in traits linked to more favorable phenotypes under resource limitation. Overall, the invader exhibited greater plasticity than native species did, as demonstrated by higher plasticity index values for traits such as plant height, leaf mass ratio, and root : shoot ratio. Variation in phenotypic plasticity among invader populations suggests the potential for evolution of plasticity, and greater plasticity of invader populations than native species may underlie invader dominance. Differences in plasticity among populations appears to play an important role in predictions of the spread and potentially the impacts of invasive species.

Enduring extreme climate: Effects of severe drought on Triatoma brasiliensis populations in wild and man-made habitats of the Caatinga

Background

Triatoma brasiliensis, a triatomine-bug vector of Chagas disease, evolved in the semiarid Caatinga, where it occupies rocky outcrops, shrubby cacti, and human dwellings. Dwellings and rocks are considered high-quality microhabitats for this saxicolous species, whereas cacti probably represent secondary, lower-quality microhabitats. This ‘microhabitat-quality hierarchy’ hypothesis predicts that T. brasiliensis populations occupying dwellings or rocks should endure harsh environmental conditions better than their cactus-living relatives.

Methods/Findings

We tested this prediction by comparing T. brasiliensis infestation (proportion of microhabitats with bugs), density (bugs per microhabitat), and crowding (bugs per infested microhabitat) in dwellings, rocks, and cacti sampled before and during the extreme drought that ravaged the Caatinga in 2012–2016. We used random-intercepts generalized linear mixed models to account for microhabitat spatial clustering and for variations in bug-catch effort; we assessed model performance and computed model-averaged effect estimates using Akaike’s information criterion. Pre-drought infestation was similar across microhabitat types; during the drought, infestation remained stable in dwellings and rocks but dropped in cacti. Pre-drought bug density declined from dwellings to rocks to cacti; an additional decline associated with the drought was largely comparable across microhabitats, albeit perhaps somewhat larger in cacti. Finally, pre-drought bug crowding was higher in dwellings than in rocks or cacti and changed little during the drought–possibly with a downward trend in dwellings and an upward trend in cacti.

Conclusions

Triatoma brasiliensis populations fared better in dwellings and rocks than in cacti during extreme drought. Estimates of microhabitat and drought effects on infestation, density, and crowding suggest that only a few cacti (versus many rocks and dwellings) represent good-quality habitat under such extremely harsh conditions. Our findings provide empirical support to the microhabitat-quality hierarchy hypothesis, and imply that T. brasiliensis can endure extreme climate by exploiting high-quality microhabitats, whether wild or man-made, in the semiarid Caatinga.

Triatomine bugs, the vectors of Chagas disease, feed on vertebrate blood and occupy diverse microhabitats in close association with their hosts. Each bug species is adapted to particular, ‘primary’ microhabitats. Many species, however, also use ‘secondary’ microhabitats, sometimes including man-made structures. Secondary wild microhabitats are probably of lower overall quality than primary microhabitats, whereas human dwellings can be of higher overall quality if they provide a stable and abundant blood supply. This suggests that there may be a ‘hierarchy’ of triatomine microhabitat quality–top-quality dwellings, high-quality primary microhabitats, and low-quality secondary wild microhabitats. To test this hypothesis, we measured the impact of a severe drought on Triatoma brasiliensis populations living in dwellings and wild habitats, both primary (rocks) and secondary (shrubby cacti). Dwelling and rock bug populations remained largely stable during the drought, whereas most cactus-living populations either became extinct or shrunk to an undetectable size. A few cacti, however, supported relatively large bug colonies even during the drought, suggesting that they were of higher-than-average quality. Our findings provide support to the microhabitat-quality hierarchy hypothesis, and show that T. brasiliensis (and perhaps other triatomines) can endure extreme climate by exploiting (natural or man-made) high-quality microhabitats.

Genetic Diversity and Population Structure of Wild Sunflower (Helianthus annuus L.) in Argentina: Reconstructing Its Invasion History

Studying the levels and patterns of genetic diversity of invasive populations is important to understand the evolutionary and ecological factors promoting invasions and for better designing preventive and control strategies. Wild sunflower (Helianthus annuus L.) is native to North America and was introduced, and has become invasive, in several countries, including Argentina (ARG). Here, using classical population genetic analyses and approximate Bayesian computation (ABC) modeling, we studied the invasion history of wild sunflower in ARG. We analyzed 115 individuals belonging to 15 populations from ARG (invasive range) and United States (US, native range) at 14 nuclear and 3 chloroplast simple sequence repeat markers along with 23 phenotypic variables. Populations from ARG showed similar levels of nuclear genetic diversity to US populations and higher genetic diversity in the chloroplast genome, indicating no severe genetic bottlenecks during the invasion process. Bayesian clustering analysis, based on nuclear markers, suggests the presence of 3 genetic clusters, all present in both US and ARG. Discriminant analysis of principal components (DAPC) detected an overall low population structure between central US and ARG populations but separated 2 invasive populations from the rest. ABC modeling supports multiple introductions but also a southward dispersal within ARG. Genetic and phenotypic data support the central US as a source of introduction while the source of secondary introductions could not be resolved. Finally, using genetic markers from the chloroplast genome, we found lower population structure in ARG when compared with US populations, suggesting a role for seed-mediated gene flow in Argentina.

Range expansion is associated with increased survival and fecundity in a long-lived bat species

The speed and dynamics of range expansions shape species distributions and community composition. Despite the critical impact of population growth rates for range expansion, they are neglected in existing empirical studies, which focus on the investigation of selected life-history traits. Here, we present an approach based on non-invasive genetic capture-mark-recapture data for the estimation of adult survival, fecundity and juvenile survival, which determine population growth. We demonstrate the reliability of our method with simulated data, and use it to investigate life-history changes associated with range expansion in 35 colonies of the bat species Rhinolophus hipposideros. Comparing the demographic parameters inferred for 19 of those colonies which belong to an expanding population with those inferred for the remaining 16 colonies from a non-expanding population reveals that range expansion is associated with higher net reproduction. Juvenile survival was the main driver of the observed reproduction increase in this long-lived bat species with low per capita annual reproductive output. The higher average growth rate in the expanding population was not associated with a trade-off between increased reproduction and survival, suggesting that the observed increase in reproduction stems from a higher resource acquisition in the expanding population. Environmental conditions in the novel habitat hence seem to have an important influence on range expansion dynamics, and warrant further investigation for the management of range expansion in both native and invasive species.

Invasive Species Removal Promotes Habitat Restoration but Does Not Immediately Improve the Condition of a Threatened Plant Subspecies

Impacts of invasive species on rare species are relevant to conservation. We studied the response of Leedy's roseroot Rhodiola integrifolia subsp. leedyi, a subspecies listed as threatened under the Endangered Species Act, to removal of the invasive species Japanese knotweed Fallopia japonica. Japanese knotweed has invaded the largest known population of Leedy's roseroot, affecting about 10% of all Leedy's roseroot in the world. Japanese knotweed shaded Leedy's roseroot, but the two did not share belowground resources because of their position on cliffs. To study this interaction and, ultimately, to restore Leedy's roseroot habitat to an open, high-light condition, we removed Japanese knotweed in a three-treatment block design. We measured Leedy's roseroot abundance, growth, and reproduction in treatment blocks and in uninvaded areas before and after treatment. Compared with uninvaded areas, Japanese knotweed invasion negatively affected Leedy's roseroot abundance, growth, and reproduction. Light interception by Japanese knotweed degraded the habitat for Leedy's roseroot. Herbicide removal of Japanese knotweed resulted in increased light and temperature compared with untreated invaded plots but did not affect Leedy's roseroot abundance, growth, or reproduction over the 2 y of our study. These results show that invasive species removal is conducive to restoring Leedy's roseroot habitat, but recovery in the subspecies may lag behind restoration of the habitat, suggesting that additional action or time may be required to restore preinvasion performance of Leedy's roseroot. Results of this study may inform restoration efforts for other systems and contribute to the literature on interspecific interactions.

Expansion history and environmental suitability shape effective population size in a plant invasion

The margins of an expanding range are predicted to be challenging environments for adaptation. Marginal populations should often experience low effective population sizes (N_e ) where genetic drift is high due to demographic expansion and/or census population size is low due to unfavourable environmental conditions. Nevertheless, invasive species demonstrate increasing evidence of rapid evolution and potential adaptation to novel environments encountered during colonization, calling into question whether significant reductions in N_e are realized during range expansions in nature. Here we report one of the first empirical tests of the joint effects of expansion dynamics and environment on effective population size variation during invasive range expansion. We estimate contemporary values of N_e using rates of linkage disequilibrium among genome-wide markers within introduced populations of the highly invasive plant Centaurea solstitialis (yellow starthistle) in North America (California, USA), and within native Eurasian populations. As predicted, we find that N_e within the invaded range is positively correlated with both expansion history (time since founding) and habitat quality (abiotic climate). History and climate had independent additive effects with similar effect sizes, indicating an important role for both factors in this invasion. These results support theoretical expectations for the population genetics of range expansion, though whether these processes can ultimately arrest the spread of an invasive species remains an unanswered question.

Variance in biomass-allocation fractions is explained by distribution in European trees

Intraspecific variability in ecological traits confers the ability of a species to adapt to an ever-changing environment. Fractions of biomass allocation in plants (BAFs) represent both ecological traits and direct expressions of investment strategies and so have important implications on plant fitness, particularly under current global change. We combined data on BAFs of trees in > 10 000 forest plots with their distributions in Europe. We aimed to test whether plant species with wider distributions have more or less variable intraspecific variance of the BAFs foliage-woody biomass and shoot-root ratios than species with limited distribution. Irrespective of corrections for tree age and phylogenetic relatedness, the standard deviation in BAFs was up to three times higher in species with the most extensive distributions than in those with the least extensive distribution due to a higher genetic diversity. Variance in BAFs also increased with latitude. We show that a combination of 36% tree genetic diversity and 64% environmental variability explains variance in BAFs and implies that changes in genetic diversity occur quickly. Genetic diversity should thus play a key role in regulating species responses to future climate change. Loss of habitat, even if transient, could induce a loss of genetic diversity and hinder species survival.

Selection on structural allelic variation biases plasticity estimates

Wang and Althoff (2019) explored the capacity of Drosophila melanogaster to exhibit adaptive plasticity in a novel environment. In a full-sib, half-sib design, they scored the activity of the enzyme alcohol dehydrogenase (ADH) and plastic responses, measured as changes in ADH activity across ethanol concentrations in the range of 0-10% (natural variation) and 16% (the novel environment). ADH activity increased with alcohol concentration, and there was a positive association between larval viability and ADH activity in the novel environment. They also reported that families exhibiting greater plasticity had higher larval survival in the novel environment, concluding that ADH plasticity is adaptive. However, the four authors now concur that, since the study estimated plasticity from phenotypic differences across environments using full-sib families, it is not possible to disentangle the contributions of allele frequency changes at the Adh locus from regulatory control at loci known to influence ADH activity. Selective changes in allele frequencies may thus conflate estimates of plasticity; any type of "plasticity" (adaptive, neutral, or maladaptive) could be inferred depending on allele frequencies. The problem of scoring sib-groups after selection should be considered in any plasticity study that cannot use replicated genotypes. Researchers should monitor changes in allele frequencies as one mechanism to deal with this issue.

Release from natural enemies mitigates inbreeding depression in native and invasive Silene latifolia populations

Inbreeding and enemy infestation are common in plants and can synergistically reduce their performance. This inbreeding ×environment (I × E) interaction may be of particular importance for the success of plant invasions if introduced populations experience a release from attack by natural enemies relative to their native conspecifics. Here, we investigate whether inbreeding affects plant infestation damage, whether inbreeding depression in growth and reproduction is mitigated by enemy release, and whether this effect is more pronounced in invasive than native plant populations. We used the invader Silene latifolia and its natural enemies as a study system. We performed two generations of experimental out- and inbreeding within eight native (European) and eight invasive (North American) populations under controlled conditions using field-collected seeds. Subsequently, we exposed the offspring to an enemy exclusion and inclusion treatment in a common garden in the species' native range to assess the interactive effects of population origin (range), breeding treatment, and enemy treatment on infestation damage, growth, and reproduction. Inbreeding increased flower and leaf infestation damage in plants from both ranges, but had opposing effects on fruit damage in native versus invasive plants. Inbreeding significantly reduced plant fitness; whereby, inbreeding depression in fruit number was higher in enemy inclusions than exclusions. This effect was equally pronounced in populations from both distribution ranges. Moreover, the magnitude of inbreeding depression in fruit number was lower in invasive than native populations. These results support that inbreeding has the potential to reduce plant defenses in S. latifolia, which magnifies inbreeding depression in the presence of enemies. However, future studies are necessary to further explore whether enemy release in the invaded habitat has actually decreased inbreeding depression and thus facilitated the persistence of inbred founder populations and invasion success.