Genome-wide relaxation of selection and the evolution of the island syndrome in Orkney voles

Island populations often experience different ecological and demographic conditions than their counterparts on the continent, resulting in divergent evolutionary forces affecting their genomes. Random genetic drift and selection both may leave their imprints on island populations, although the relative impact depends strongly on the specific conditions. Here we address their contributions to the island syndrome in a rodent with an unusually clear history of isolation. Common voles (Microtus arvalis) were introduced by humans on the Orkney archipelago north of Scotland >5000 years ago and rapidly evolved to exceptionally large size. Our analyses show that the genomes of Orkney voles were dominated by genetic drift, with extremely low diversity, variable Tajima's D, and very high divergence from continental conspecifics. Increased d N/d S ratios over a wide range of genes in Orkney voles indicated genome-wide relaxation of purifying selection. We found evidence of hard sweeps on key genes of the lipid metabolism pathway only in continental voles. The marked increase of body size in Orkney-a typical phenomenon of the island syndrome-may thus be associated to the relaxation of positive selection on genes related to this pathway. On the other hand, a hard sweep on immune genes of Orkney voles likely reflects the divergent ecological conditions and possibly the history of human introduction. The long-term isolated Orkney voles show that adaptive changes may still impact the evolutionary trajectories of such populations despite the pervasive consequences of genetic drift at the genome level.

Climate match is key to predict range expansion of the world's worst invasive terrestrial vertebrates

Understanding the determinants of the range expansion of invasive alien species is crucial for developing effective prevention and control strategies. Nevertheless, we still lack a global picture of the potential factors influencing the invaded range expansion across taxonomic groups, especially for the world's worst invaders with high ecological and economic impacts. Here, by extensively collecting data on 363 distributional ranges of 19 of world's worst invasive terrestrial vertebrates across 135 invaded administrative jurisdictions, we observed remarkable variations in the range expansion across species and taxonomic groups. After controlling for taxonomic and geographic pseudoreplicates, model averaging analyses based on generalized additive mixed-effect models showed that species in invaded regions having climates more similar to those of their native ranges tended to undergo a larger range expansion. In addition, as proxies of propagule pressure and human-assisted transportation, the number of introduction events and the road network density were also important predictors facilitating the range expansion. Further variance partitioning analyses validated the predominant role of climate match in explaining the range expansion. Our study demonstrated that regions with similar climates to their native ranges could still be prioritized to prevent the spread of invasive species under the sustained global change.

Captivity induces large and population-dependent brain transcriptomic changes in wild-caught cane toads (Rhinella marina)

Gene expression levels are key molecular phenotypes at the interplay between genotype and environment. Mounting evidence suggests that short-term changes in environmental conditions, such as those encountered in captivity, can substantially affect gene expression levels. Yet, the exact magnitude of this effect, how general it is, and whether it results in parallel changes across populations are not well understood. Here, we take advantage of the well-studied cane toad, Rhinella marina, to examine the effect of short-term captivity on brain gene expression levels, and determine whether effects of captivity differ between long-colonized and vanguard populations of the cane toad's Australian invasion range. We compared the transcriptomes of wild-caught toads immediately assayed with those from toads captured from the same populations but maintained in captivity for seven months. We found large differences in gene expression levels between captive and wild-caught toads from the same population, with an over-representation of processes related to behaviour and the response to stress. Captivity had a much larger effect on both gene expression levels and gene expression variability in toads from vanguard populations compared to toads from long-colonized areas, potentially indicating an increased plasticity in toads at the leading edge of the invasion. Overall, our findings indicate that short-term captivity can induce large and population-specific transcriptomic changes, which has significant implications for studies comparing phenotypic traits of wild-caught organisms from different populations that have been held in captivity.

No evidence for cold-adapted life-history traits in cool-climate populations of invasive cane toads (Rhinella marina)

As an invasive organism spreads into a novel environment, it may encounter strong selective pressures to adapt to abiotic and biotic challenges. We examined the effect of water temperature during larval life on rates of survival and growth of the early life-history stages of cane toads (Rhinella marina) from two geographic regions (tropical vs. temperate) in the species’ invaded range in eastern Australia. If local adaptation at the southern (cool-climate) invasion front has extended the cold-tolerance of early life-stages, we would expect to see higher viability of southern-population toads under cooler conditions. Our comparisons revealed no such divergence: the effects of water temperature on rates of larval survival and growth, time to metamorphosis, size at metamorphosis and locomotor performance of metamorphs were similar in both sets of populations. In two cases where tropical and temperate-zone populations diverged in responses to temperature, the tropical animals performed better at low to medium temperatures than did conspecifics from cooler regions. Adaptation to low temperatures in the south might be constrained by behavioural shifts (e.g., in reproductive seasonality, spawning-site selection) that allow toads to breed in warmer water even in cool climates, by gene flow from warmer-climate populations, or by phylogenetic conservatism in these traits.

Brain transcriptome analysis reveals gene expression differences associated with dispersal behaviour between range-front and range-core populations of invasive cane toads in Australia

Understanding the mechanisms allowing invasive species to adapt to novel environments is a challenge in invasion biology. Many invaders demonstrate rapid evolution of behavioural traits involved in range expansion such as locomotor activity, exploration and risk‐taking. However, the molecular mechanisms that underpin these changes are poorly understood. In 86 years, invasive cane toads (Rhinella marina) in Australia have drastically expanded their geographic range westward from coastal Queensland to Western Australia. During their range expansion, toads have undergone extensive phenotypic changes, particularly in behaviours that enhance the toads’ dispersal ability. Common‐garden experiments have shown that some changes in behavioural traits related to dispersal are heritable. At the molecular level, it is currently unknown whether these changes in dispersal‐related behaviour are underlain by small or large differences in gene expression, nor is known the biological function of genes showing differential expression. Here, we used RNA‐seq to gain a better understanding of the molecular mechanisms underlying dispersal‐related behavioural changes. We compared the brain transcriptomes of toads from the Hawai'ian source population, as well as three distinct populations from across the Australian invasive range. We found markedly different gene expression profiles between the source population and Australian toads. By contrast, toads from across the Australian invasive range had very similar transcriptomic profiles. Yet, key genes with functions putatively related to dispersal behaviour showed differential expression between populations located at each end of the invasive range. These genes could play an important role in the behavioural changes characteristic of range expansion in Australian cane toads.

The accelerating anuran: evolution of locomotor performance in cane toads (Rhinella marina, Bufonidae) at an invasion front

As is common in biological invasions, the rate at which cane toads (Rhinella marina) have spread across tropical Australia has accelerated through time. Individuals at the invasion front travel further than range-core conspecifics and exhibit distinctive morphologies that may facilitate rapid dispersal. However, the links between these morphological changes and locomotor performance have not been clearly documented. We used raceway trials and high-speed videography to document locomotor traits (e.g. hop distances, heights, velocities, and angles of take-off and landing) of toads from range-core and invasion-front populations. Locomotor performance varied geographically, and this variation in performance was linked to morphological features that have evolved during the toads' Australian invasion. Geographical variation in morphology and locomotor ability was evident not only in wild-caught animals, but also in individuals that had been raised under standardized conditions in captivity. Our data thus support the hypothesis that the cane toad's invasion across Australia has generated rapid evolutionary shifts in dispersal-relevant performance traits, and that these differences in performance are linked to concurrent shifts in morphological traits.

Do Epigenetic Changes Drive Corticosterone Responses to Alarm Cues in Larvae of an Invasive Amphibian?

The developmental environment can exert powerful effects on animal phenotype. Recently, epigenetic modifications have emerged as one mechanism that can modulate developmentally plastic responses to environmental variability. For example, the DNA methylation profile at promoters of hormone receptor genes can affect their expression and patterns of hormone release. Across taxonomic groups, epigenetic alterations have been linked to changes in glucocorticoid (GC) physiology. GCs are metabolic hormones that influence growth, development, transitions between life-history stages, and thus fitness. To date, relatively few studies have examined epigenetic effects on phenotypic traits in wild animals, especially in amphibians. Here, we examined the effects of exposure to predation threat (alarm cues) and experimentally manipulated DNA methylation on corticosterone (CORT) levels in tadpoles and metamorphs of the invasive cane toad (Rhinella marina). We included offspring of toads sampled from populations across the species' Australian range. In these animals, exposure to chemical cues from injured conspecifics induces shifts in developmental trajectories, putatively as an adaptive response that lessens vulnerability to predation. We exposed tadpoles to these alarm cues, and measured changes in DNA methylation and CORT levels, both of which are mechanisms that have been implicated in the control of phenotypically plastic responses in tadpoles. To test the idea that DNA methylation drives shifts in GC physiology, we also experimentally manipulated methylation levels with the drug zebularine. We found differentially methylated regions (DMRs) between control tadpoles and their full-siblings exposed to alarm cues, zebularine, or both treatments. However, the effects of these manipulations on methylation patterns were weaker than clutch (e.g., genetic, maternal, etc.) effects. CORT levels were higher in larval cane toads exposed to alarm cues and zebularine. We found little evidence of changes in DNA methylation across the GC receptor gene (NR3C1) promoter region in response to alarm cue or zebularine exposure. In both alarm cue and zebularine-exposed individuals, we found differentially methylated DNA in the suppressor of cytokine signaling 3 gene (SOCS3), which may be involved in predator avoidance behavior. In total, our data reveal that alarm cues have significant impacts on tadpole physiology, but show only weak links between DNA methylation and CORT levels. We also identify genes containing DMRs in tadpoles exposed to alarm cues and zebularine, particularly in range-edge populations, that warrant further investigation.

Stress and immunity: Field comparisons among populations of invasive cane toads in Florida

Cane toads (Rhinella marina) were introduced worldwide and have become invasive in multiple locations, representing a major driver of biodiversity loss through competition (food, shelter, territory), predation, and the poisoning of native species. These toads have been used in Australia as a model for studies concerning invasion biology and ecoimmunology, as longer-established (core) and invasion front (edge) populations show altered stress and immune response profiles. Although cane toads were also introduced into the United States in the 1950s, these patterns have yet to be evaluated for the populations spanning Florida. Toads introduced into Florida have dispersed primarily northward along a latitudinal gradient, where they encounter cooler temperatures that may further impact stress and immune differences between core and edge populations. In this study, we sampled cane toads from nine different locations spanning their invasion in Florida. Cane toads from southern populations showed higher plasma bacterial killing ability and natural antibody titers than the toads from the northern populations, indicating they have a better immune surveillance system. Also, southern toads were more responsive to a novel stressor (1 hr restraint), showing a higher increase in corticosterone levels. These results indicate that possible trade-offs have occurred between immune and stress responses as these toads have become established in northern cooler areas in Florida.

Diurnal activity in cane toads (Rhinella marina) is geographically widespread

Although adult cane toads (Rhinella marina) are generally active only at night, a recent study reported that individuals of this species switched to diurnal activity in response to encountering a novel habitat type (deeply shaded gorges) in the course of their Australian invasion. Our sampling over a broader geographic scale challenges the idea that this behaviour is novel; we documented diurnal behaviour both in the species’ native range and in several sites within the invaded range, in multiple habitat types. Diurnal activity was most common in the tropics and in areas where toads attain high population densities and are in poor body condition, suggesting that the expansion of activity times may be induced by intraspecific competition for food.

Tradeoffs between dispersal and reproduction at an invasion front of cane toads in tropical Australia

Individuals at the leading edge of a biological invasion experience novel evolutionary pressures on mating systems, due to low population densities coupled with tradeoffs between reproduction and dispersal. Our dissections of >1,200 field-collected cane toads (Rhinella marina) at a site in tropical Australia reveal rapid changes in morphological and reproductive traits over a three-year period after the invaders first arrived. As predicted, individuals with dispersal-enhancing traits (longer legs, narrower heads) had reduced reproductive investment (lower gonad mass). Post-invasion, the population was increasingly dominated by individuals with less dispersive phenotypes and a higher investment into reproduction (including, increased expression of sexually dimorphic traits in males). These rapid shifts in morphology and reproductive biology emphasise the impacts of the invasion process on multiple, interlinked aspects of organismal biology.

Resource-dependent evolution of female resistance responses to sexual conflict

Sexual conflict can promote the evolution of dramatic reproductive adaptations as well as resistance to its potentially costly effects. Theory predicts that responses to sexual conflict will vary significantly with resource levels—when scant, responses should be constrained by trade‐offs, when abundant, they should not. However, this can be difficult to test because the evolutionary interests of the sexes align upon short‐term exposure to novel environments, swamping any selection due to sexual conflict. What is needed are investigations of populations that are well adapted to both differing levels of sexual conflict and resources. Here, we used this approach in a long‐term experimental evolution study to track the evolution of female resistance to sexual conflict in the fruit fly Drosophila melanogaster. In resource‐rich regimes, high‐conflict females evolved resistance to continual exposure to males. There was no difference in baseline survival, consistent with the idea that responses evolving under nutritional abundance experienced no trade‐offs with resistance. In the poor resource regimes, the ability of high‐conflict females to evolve resistance to males was severely compromised and they also showed lower baseline survival than low‐conflict females. This suggested high‐conflict females traded off somatic maintenance against any limited resistance they had evolved in response to sexual conflict. Overall, these findings provide experimental support for the hypothesis that evolutionary responses to sexual conflict are critically dependent upon resource levels.

At the invasion front, male cane toads (Rhinella marina) have smaller testes

As a colonizing species expands its range, individuals at the invasion front experience different evolutionary pressures than do those at the range-core. For example, low densities at the edge of the range mean that males should rarely experience intense sperm competition from rivals; and investment into reproduction may trade-off with adaptations for more rapid dispersal. Both of these processes are predicted to favour a reduction in testis size at the invasion front. To explore effects of invasion stage in Australian cane toads (Rhinella marina), we collected and dissected 214 adult males from three regions: one in the species' range-core (northeastern Australia), and two from invasion fronts (one in northwestern Australia and one in southeastern Australia). Despite the brief duration of separation between toads in these areas (approx. 85 years), testis masses averaged greater than 30% higher (as a proportion of body mass) in range-core males than in conspecifics sampled from either vanguard of the invasion. Previous work has documented low reproductive frequencies in female cane toads at the invasion front also, consistent with the hypothesis that evolutionary and ecological pressures unleashed by an invasion can favour relatively low resource allocation to reproduction in both sexes.

Conserved growth rate and age structure of Xenopus laevis in the edge and core of an expanding population

Invasive species represent a unique opportunity to study the evolutionary mechanisms driving range expansions. Although range expansion is expected to be associated with increased reproduction and dispersal at the range edge, Xenopus laevis seems to decrease its reproductive investment and to enhance dispersal at the range edge. Evidence of increased dispersal at the edge of expanding populations occurring simultaneously with a faster growth rate has been reported for other organisms. Here, we focused on the growth rate and age structure at the range edge vs. the range core in an expanding population of X. laevis. We used skeletochronology to characterize the age of 250 individuals captured at the range core and edge of this expanding population. Using the Von Bertalanffy equation, we then compared individual growth rates between locations. We found no significant changes in growth rate or age structure between edge and core samples. This result suggests that the reduced investment in reproduction recorded in another study at the range edge might compensate for the increased dispersal without impacting growth in this population. This implies that the resource allocation in an expanding population might thus be more diverse than commonly assumed.