New Weapons in the Toad Toolkit: A Review of Methods to Control and Mitigate the Biodiversity Impacts of Invasive Cane Toads (Rhinella Marina)

A cane toad (Rhinella marina) N-methyltransferase converts primary indolethylamines to tertiary psychedelic amines

Psychedelic indolethylamines have emerged as potential medicines to treat several psychiatric pathologies. Natural sources of these compounds include 'magic mushrooms' (Psilocybe spp.), plants used to prepare ayahuasca, and toads. The skin and parotid glands of certain toads accumulate a variety of specialized metabolites including toxic guanidine alkaloids, lipophilic alkaloids, poisonous steroids, and hallucinogenic indolethylamines such as DMT, 5-methoxy-DMT, and bufotenin. The occurrence of psychedelics has contributed to the ceremonial use of toads, particularly among Mesoamerican peoples. Yet, the biosynthesis of psychedelic alkaloids has not been elucidated. Herein, we report a novel indolethylamine N-methyltransferase (RmNMT) from cane toad (Rhinella marina). The RmNMT sequence was used to identify a related NMT from the common toad, Bufo bufo. Close homologs from various frog species were inactive, suggesting a role for psychedelic indolethylamine biosynthesis in toads. Enzyme kinetic analyses and comparison with functionally similar enzymes showed that recombinant RmNMT was an effective catalyst and not product inhibited. The substrate promiscuity of RmNMT enabled the bioproduction of a variety of substituted indolethylamines at levels sufficient for purification, pharmacological screening, and metabolic stability assays. Since the therapeutic potential of psychedelics has been linked to activity at serotonergic receptors, we evaluated binding of derivatives at 5-HT1A and 5-HT2A receptors. Primary amines exhibited enhanced affinity at the 5-HT1A receptor compared with tertiary amines. With the exception of 6-substituted derivatives, N,N-dimethylation also protected against catabolism by liver microsomes.

Spatial ecology of the invasive Asian common toad in Madagascar and its implications for invasion dynamics

Invasion dynamics are determined, among other aspects, by the spatial behaviour of invasive populations. The invasive toad Duttaphrynus melanostictus is spreading inland from the eastern coast of Madagascar, causing considerable ecological impacts. Understanding the basic factors determining the spread dynamics can inform management strategies and provide insights into spatial evolutionary processes. We radio-tracked 91 adult toads in three localities along the invasion gradient to determine whether spatial sorting of dispersive phenotypes is occurring, and investigate intrinsic and extrinsic determinants of spatial behaviour. Overall, toads in our study appeared as habitat generalists, and their sheltering behaviour was tied to water proximity, with toads changing shelter more frequently closer to waterbodies. Toads showed low displacement rates (mean = 4.12 m/day) and quite a philopatric behaviour but were able to perform daily movements of over 50 m. We did not detect any spatial sorting of dispersal-relevant traits nor sex- or size-biased dispersal. Our results suggest that toads are more likely to expand their range during the wet season, and that the range expansion is probably dominated by short-distance dispersal at this stage of the invasion, although a future increase in invasion speed is expected, due to the capacity for long-distance movements of this species.

Challenges in eDNA detection of the invasive European green crab, Carcinus maenas

The early detection of invasive species is essential to cease the spread of the species before it can cause irreversible damage to the environment. The analysis of environmental DNA (eDNA) has emerged as a non-harmful method to detect the presence of a species before visual detection and is a promising approach to monitor invasive species. Few studies have investigated the use of eDNA for arthropods, as their exoskeleton is expected to limit the release of eDNA into the environment. We tested published primers for the invasive European green crab, Carcinus maenas, in the Gulf of Maine and found them not species-specific enough for reliable use outside of the area for which they were designed for. We then designed new primers, tested them against a broad range of local faunal species, and validated these primers in a field study. We demonstrate that eDNA analyses can be used for crustaceans with an exoskeleton and suggest that primers and probe sequences must be tested on local fauna at each location of use to ensure no positive amplification of these other species.

Understanding the drivers of dispersal evolution in range expansions and their ecological consequences

Research has conclusively demonstrated the potential for dispersal evolution in range expansions and shifts, however the degree of dispersal evolution observed has varied substantially among organisms. Further, it is unknown how the factors influencing dispersal evolution might impact other ecological processes at play. We use an individual-based model to investigate the effects of the underlying genetics of dispersal and mode of reproduction in range expansions and shifts. Consistent with predictions from stationary populations, dispersal evolution increases with sexual reproduction and loci number. Contrary to our predictions, however, increased dispersal does not always improve a population’s ability to track changing conditions. The mate finding Allee effect inherent to sexual reproduction increases extinction risk during range shifts, counteracting the beneficial effect of increased dispersal evolution. Our results demonstrate the importance of considering both ecological and evolutionary processes for understanding range expansions and shifts.

Impacts of invasive cane toads on an Endangered marsupial predator and its prey

Recent research has revealed that impacts of some invasive species are chronic. Invasive cane toads Rhinella marina have apparently caused rapid and severe population-level declines of the Endangered northern quoll Dasyurus hallucatus across tropical Australia; however, more targeted, quantitative impact data are needed to disentangle this from other threats such as fire regimes, disease, feral cats and dingos. Moreover, repeatable counts before, during, after and long after toad invasion are needed in order to determine if short-term impacts are chronic vs. transitory. We used game cameras to monitor 2 quoll populations and their prey over a 5 yr period spanning the invasion of the toxic cane toads in 2 gorges in northwestern Australia. We predicted severe declines in quolls with the toad invasion, and predatory release of 2 prey species of quolls, a rodent and a smaller marsupial. Quolls declined quickly upon arrival of toads, becoming undetectable in one gorge and barely detectable in the other. Identification of individuals via unique spot patterns confirmed that the declines in detection rates were due to changes in relative abundance rather than decreases in activity. Despite quoll declines we found no evidence of mesopredator release; small mammals generally declined as toads arrived. Our research confirmed rapid population-level declines of quolls, and possibly smaller mammals, associated with arrival of invasive cane toads. Importantly, our surveys provide a baseline for future surveys to determine whether these short-term impacts are chronic or transitory, and whether recovery requires assistance from managers.

Enhancing the ecological realism of evolutionary mismatch theory

Following rapid environmental change, why do some animals thrive, while others struggle? We present an expanded, cue-response framework for predicting variation in behavioral responses to novel situations. We show how signal detection theory can be used when individuals have three behavioral options (approach, avoid, or ignore). Based on this theory, we outline predictions about which animals are more likely to make mistakes around novel conditions (i.e., fall for a trap or fail to use an undervalued resource) and the intensity of that mismatch (i.e., severe versus moderate). Explicitly considering three options provides a more holistic perspective and allows us to distinguish between severe and moderate traps, which could guide management strategies in a changing world.

Discovery of Novel Viruses Associated With the Invasive Cane Toad (Rhinella marina) in Its Native and Introduced Ranges

Cane toads (Rhinella marina) are notoriously successful invaders: from 101 individuals brought to Australia in 1935, poisonous toads now cover an area >1.2 million km² with adverse effects on native fauna. Despite extensive research on the role of macroparasites in cane toad invasion, viral research is lagging. We compared viral prevalence and diversity between toads in their native range (French Guiana, n=25) and two introduced ranges: Australia (n=151) and Hawai'i (n=10) with a metatranscriptomic and metagenomic approach combined with PCR screening. Australian toads almost exclusively harbor one of seven viruses detected globally. Rhimavirus-A (Picornaviridae) exhibited low genetic diversity and likely actively infected 9% of sampled Australian toads extending across ~2,000km of Northern Australia and up to the current invasion front. In native range cane toads, we identified multiple phylogenetically distinct viruses (Iridoviridae, Picornaviridae, Papillomaviridae, and Nackedna-like virus). None of the same viruses was detected in both ranges, suggesting that Australian cane toads have largely escaped the viral infection experienced by their native range counterparts. The novel native range viruses described here are potential biocontrol agents, as Australian toads likely lack prior immunological exposure to these viruses. Overall, our evidence suggests that there may be differences between viruses infecting cane toads in their native vs. introduced ranges, which lays the groundwork for further studies on how these viruses have influenced the toads' invasion history.

Optimal cooling rates for sperm cryopreservation in a threatened lizard conform to two-factor hypothesis of cryo-injury

Assisted reproductive technologies provide important tools for wildlife conservation but have rarely been developed for reptiles. A critical step in developing cryopreservation protocols is establishing optimal cooling rates for cell survival. The two-factor hypothesis explaining cryoinjury to cells originates from an inverted 'U' shape of recovery curves generated in many cell types thawed after cryopreservation, due to cell recovery declining at cooling rates either side of a single optimum. We generated such a curve for the yellow-spotted monitor lizard Varanus panoptes, the first for any reptile. We cryopreserved sperm using two cooling devices (LN₂ dry shipper; LN₂ bath vapour) and two sperm-holding vessels (Cassou sperm straws; Nunc CryoTubes) to generate four different cooling-rate curves during freezing. Sperm motility and viability (47.3% and 76.5% respectively) were highest when frozen in straws suspended in a LN₂ bath at an intermediate cooling rate of 73 ^°C/min between 0 and -50 °C, whereas sperm frozen in straws suspended in a dry shipper at the fastest cooling rate (231 ^°C/min between 0 and -50 °C) produced the lowest recovery (10.4% and 36.4% motility and viability, respectively). Sperm frozen in cryotubes at the lowest cooling rates in either LN₂ bath vapour or dry shipper produced intermediate recovery. The shape of the optimal cooling curve conformed to the two-factor hypothesis of cryoinjury, the first such evidence in reptile sperm. This in turn led to the identification of simple cryopreservation setups (LN₂ vapour with straws and cryotubes; dry shipper with cryotubes but not straws) suitable for cryopreserving lizard sperm in the field.

Effects of learning and adaptation on population viability

Cultural adaptation is one means by which conservationists may help populations adapt to threats. A learned behavior may protect an individual from a threat, and the behavior can be transmitted horizontally (within generations) and vertically (between generations), rapidly conferring population-level protection. Although possible in theory, it remains unclear whether such manipulations work in a conservation setting; what conditions are required for them to work; and how they might affect the evolutionary process. We examined models in which a population can adapt through both genetic and cultural mechanisms. Our work was motivated by the invasion of highly toxic cane toads (Rhinella marina) across northern Australia and the resultant declines of endangered northern quolls (Dasyurus hallucatus), which attack and are fatally poisoned by the toxic toads. We examined whether a novel management strategy in which wild quolls are trained to avoid toads can reduce extinction probability. We used a simulation model tailored to quoll life history. Within simulations, individuals were trained and a continuous evolving trait determined innate tendency to attack toads. We applied this model in a population viability setting. The strategy reduced extinction probability only when heritability of innate aversion was low (<20%) and when trained mothers trained >70% of their young to avoid toads. When these conditions were met, genetic adaptation was slower, but rapid cultural adaptation kept the population extant while genetic adaptation was completed. To gain insight into the evolutionary dynamics (in which we saw a transitory peak in cultural adaptation over time), we also developed a simple analytical model of evolutionary dynamics. This model showed that the strength of natural selection declined as the cultural transmission rate increased and that adaptation proceeded only when the rate of cultural transmission was below a critical value determined by the relative levels of protection conferred by genetic versus cultural mechanisms. Together, our models showed that cultural adaptation can play a powerful role in preventing extinction, but that rates of cultural transmission need to be high for this to occur.

Chemical cues that attract cannibalistic cane toad (Rhinella marina) larvae to vulnerable embryos

Chemical cues produced by late-stage embryos of the cane toad (Rhinella marina) attract older conspecific larvae, which are highly cannibalistic and can consume an entire clutch. To clarify the molecular basis of this attraction response, we presented captive tadpoles with components present in toad eggs. As previously reported, attractivity arises from the distinctive toxins (bufadienolides) produced by cane toads, with some toxins (e.g., bufagenins) much stronger attractants than others (e.g., bufotoxins). Extracts of frozen toad parotoid glands (rich in bufagenins) were more attractive than were fresh MeOH extracts of the parotoid secretion (rich in bufotoxins), and purified marinobufagin was more effective than marinobufotoxin. Cardenolide aglycones (e.g., digitoxigenin) were active attractors, whereas C-3 glycosides (e.g., digoxin, oubain) were far less effective. A structure–activity relationship study revealed that tadpole attractant potency strongly correlated with Na⁺/K⁺ ATPase inhibitory activity, suggesting that tadpoles monitor and rapidly react to perturbations to Na⁺/K⁺ ATPase activity.

A review of the introduced herpetofauna of Mexico and Central America, with comments on the effects of invasive species and biosecurity methodology

Among the principal causes producing detrimental effects on global biodiversity are introductions of alien species. Very few attempts to control introduced amphibians and reptiles in Middle America (Mexico and Central America) can be identified, so listings are provided for 24 exotic species, 16 translocated species, and 11 species that were removed from the introduced species listing because of lack of substantiating evidence that they are from established populations. Biosecurity methods are also identified that can be applied for preventing, controlling, and managing introduced and especially invasive species.

Efficacy of short-term cold storage prior to cryopreservation of spermatozoa in a threatened lizard

Assisted reproductive technologies (ARTs) have a significant role to play in reptile conservation, yet are severely lacking. Previous attempts to cryopreserve spermatozoa in the threatened lizard Varanus panoptes achieved approximately 48% motile sperm post-thaw for samples frozen immediately after collection. However, the feasibility of extended cold storage before cryopreservation has not been tested. We held V. panoptes spermatozoa at either 25°C or 4°C for 8 days, assessing sperm motility at days 1, 2, 4 and 8. Subsamples were cryopreserved on days 1 and 4 following the previously reported protocol for this species. Percentage motility decreased rapidly at 25°C, but did not decrease significantly until 4 days after collection at 4°C, with >30% motility maintained after 8 days. There was no significant difference in post-thaw motility or viability of samples cryopreserved after 1 or 4 days storage at 4°C, yielding substantial results for both parameters (mean motility 23.8% and 28.1% and mean viability 50.1% and 57.5% after 1 and 4 days respectively). We demonstrate the capacity to extend sperm viability for up to 8 days in unfrozen samples and to produce acceptable post-thaw motility in samples frozen after 4 days of storage, contributing to the development of valuable ARTs for lizards and other reptiles.

Got Glycogen?: Development and Multispecies Validation of the Novel Preserve, Precipitate, Lyse, Precipitate, Purify (PPLPP) Workflow for Environmental DNA Extraction from Longmire's Preserved Water Samples

Unfiltered and filtered water samples can be used to collect environmental DNA (eDNA). We developed the novel "Preserve, Precipitate, Lyse, Precipitate, Purify" (PPLPP) workflow to efficiently extract eDNA from Longmire's preserved unfiltered and filtered water samples (44-100% recovery). The PPLPP workflow includes initial glycogen-aided isopropanol precipitation, guanidium hypochlorite and Triton X-100-based lysis, terminal glycogen-aided polyethylene glycol precipitation, and inhibitor purification. Three novel eDNA assays that exclusively target species invasive to Australia were also developed: Tilapia_v2_16S concurrently targets Oreochromis mossambicus (Mozambique tilapia) and Tilapia mariae (spotted tilapia) while R.marina_16S and C.caroliniana_matK discretely target Rhinella marina (cane toad) and Cabomba caroliniana (fanwort), respectively. All 3 assays were validated in silico before in vitro and in situ validations using PPLPP workflow extracted samples. PPLPP workflow was concurrently validated in vitro and in situ using all 3 assays. In vitro validations demonstrated that 1) glycogen inclusion increased extracellular DNA recovery by ∼48-fold compared with glycogen exclusion, 2) swinging-bucket centrifugation for 90 min at 3270 g is equivalent to fixed-angle centrifugation for 5-20 min at 6750 g, and 3) Zymo OneStep Inhibitor Removal Kit, Qiagen DNeasy PowerClean Pro Cleanup Kit, and silica-Zymo double purification provide effective inhibitor removal. In situ validation demonstrated 95.8 ± 2.8% (mean ± SEM) detectability across all 3 target species in Longmire's preserved unfiltered and filtered water samples extracted using the PPLPP workflow (without phenol:chloroform:isoamyl alcohol purification) after 39 d of incubation at room temperature and 50°C. PPLPP workflow is recommended for future temperate and tropical eDNA studies that use Longmire's to preserve unfiltered or filtered water samples.

Estimating the benefit of quarantine: eradicating invasive cane toads from islands

Islands are increasingly used to protect endangered populations from the negative impacts of invasive species. Quarantine efforts on islands are likely to be undervalued in circumstances in which a failure incurs non-economic costs. One approach to ascribe monetary value to such efforts is by modeling the expense of restoring a system to its former state. Using field-based removal experiments on two different islands off northern Australia separated by &gt; 400 km, we estimate cane toad densities, detection probabilities, and the resulting effort needed to eradicate toads from an island. We use these estimates to conservatively evaluate the financial benefit of cane toad quarantine across offshore islands prioritized for conservation management by the Australian federal government. We calculate density as animals per km of freshwater shoreline, and find striking concordance of density estimates across our two island study sites: a mean density of 352 [289, 466] adult toads per kilometre on one island, and a density of 341 [298, 390] on the second. Detection probability differed between our two study islands (Horan Island: 0.1 [0.07, 0.13]; Indian Island: 0.27 [0.22, 0.33]). Using a removal model and the financial costs incurred during toad removal, we estimate that eradicating cane toads would, on average, cost between $22 487 [$14 691, $34 480] (based on Horan Island) and $39 724 [$22 069, $64 001] AUD (Indian Island) per km of available freshwater shoreline. We estimate the remaining value of toad quarantine across islands that have been prioritized for conservation benefit within the toads’ predicted range, and find the net value of quarantine efforts to be $43.4 [28.4–66.6] – $76.7 [42.6–123.6] M depending on which island dataset is used to calibrate the model. We conservatively estimate the potential value of a mainland cane toad containment strategy – to prevent the spread of toads into the Pilbara Bioregion – to be $80 [52.6–123.4] – $142 [79.0–229.0] M. We present a modeling framework that can be used to estimate the value of preventative management, via estimating the length and cost of an eradication program. Our analyses suggest that there is substantial economic value in cane toad quarantine efforts across Australian offshore islands and in a proposed mainland containment strategy.

Field trials of chemical suppression of embryonic cane toads (Rhinella marina) by older conspecifics

Laboratory experiments have shown that the viability of embryos of the invasive cane toad (Rhinella marina) can be reduced by exposure to chemical cues from older conspecific larvae. These effects (very strong in laboratory trials) may offer an exciting new approach to controlling this problematic invasive species in Australia. However, the degree to which the method works in natural environments has yet to be assessed.Our experiments in the laboratory and in seminatural outdoor waterbodies show that chemical cues from tadpoles do indeed suppress the growth, development, and survival of conspecific larvae that are exposed as embryos and do so in a dose-dependent manner; higher tadpole densities cause greater suppression of embryos.In seminatural outdoor waterbodies, suppressor-exposed tadpoles were less than half as likely to survive to metamorphosis as were controls, and were much smaller when they did so and hence, less likely to survive the metamorph stage. Additionally, female cane toads were less likely to oviposit in a waterbody containing free-ranging (but not cage-enclosed) tadpoles, suggesting that the presence of tadpoles (rather than the chemical cues they produce) may discourage oviposition.Broadly, our results suggest that the suppression effect documented in laboratory studies does indeed occur in the field also, and hence that we may be able to translate that approach to develop new and more effective ways to reduce rates of recruitment of peri-urban populations of cane toads in their invasive range.

Exposure of cane toad hatchlings to older conspecifics suppresses chemosensory food tracking behaviour and increases risk of predation post-exposure

Attempts to control invasive species using species-specific pheromones need to incorporate an understanding of interactive effects among those pathways. The larvae of invasive cane toads (Rhinella marina) utilise chemical cues to repulse, attract or suppress conspecific larvae. We can exploit these effects to reduce toad abundance, but the effects of each cue may not be additive. That is, exposure to one type of cue may lessen the impact of exposure to another cue. To assess this possibility, we exposed toad larvae to combinations of cues. Tadpoles that had been exposed to the suppression cue during larval development exhibited no response to the attraction cue, resulting in lower capture rates in attractant-baited traps. Suppression, however, did not affect a tadpole’s response to the alarm cue, and exposure to the alarm cue during tadpole development did not affect response to the attraction cue. Tadpoles exposed to the suppression cue were smaller than control tadpoles at 10 days post-exposure, and consequently were more vulnerable to gape-limited invertebrate predators. Our results demonstrate that the responses by toad tadpoles to chemical cues interact in important ways, and are not simply additive when combined. Control efforts need to incorporate an understanding of such interactions if we are to most effectively use chemical-communication pathways to control invasive amphibians.

Evolutionary principles guiding amphibian conservation

The Anthropocene has witnessed catastrophic amphibian declines across the globe. A multitude of new, primarily human-induced drivers of decline may lead to extinction, but can also push species onto novel evolutionary trajectories. If these are recognized by amphibian biologists, they can be engaged in conservation actions. Here, we summarize how principles stemming from evolutionary concepts have been applied for conservation purposes, and address emerging ideas at the vanguard of amphibian conservation science. In particular, we examine the consequences of increased drift and inbreeding in small populations and their implications for practical conservation. We then review studies of connectivity between populations at the landscape level, which have emphasized the limiting influence of anthropogenic structures and degraded habitat on genetic cohesion. The rapid pace of environmental changes leads to the central question of whether amphibian populations can cope either by adapting to new conditions or by shifting their ranges. We gloomily conclude that extinction seems far more likely than adaptation or range shifts for most species. That said, conservation strategies employing evolutionary principles, such as selective breeding, introduction of adaptive variants through translocations, ecosystem interventions aimed at decreasing phenotype-environment mismatch, or genetic engineering, may effectively counter amphibian decline in some areas or for some species. The spread of invasive species and infectious diseases has often had disastrous consequences, but has also provided some premier examples of rapid evolution with conservation implications. Much can be done in terms of setting aside valuable amphibian habitat that should encompass both natural and agricultural areas, as well as designing protected areas to maximize the phylogenetic and functional diversity of the amphibian community. We conclude that an explicit consideration and application of evolutionary principles, although certainly not a silver bullet, should increase effectiveness of amphibian conservation in both the short and long term.

A comparison of nonlethal sampling methods for amphibian gut microbiome analyses

Noninvasive sampling methods for studying intestinal microbiomes are widely applied in studies of endangered species and in those conducting temporal monitoring during manipulative experiments. Although existing studies show that noninvasive sampling methods among different taxa vary in their accuracy, no studies have yet been published comparing nonlethal sampling methods in adult amphibians. In this study, we compare microbiomes from two noninvasive sample types (faeces and cloacal swabs) to that of the large intestine in adult cane toads, Rhinella marina. We use 16S rRNA gene sequencing to investigate how microbial communities change along the digestive tract and which nonlethal sampling method better represents large intestinal microbiota. We found that cane toads' intestinal microbiota was dominated by Bacteroidetes, Proteobacteria and Firmicutes and, interestingly, we also saw a high proportion of Fusobacteria, which has previously been associated with marine species and changes in frog immunity. The large and small intestine of cane toads had a similar microbial composition, but the large intestine showed higher diversity. Our results indicate that cloacal swabs were more similar to large intestine samples than were faecal samples, and small intestine samples were significantly different from both nonlethal sample types. Our study provides valuable information for future investigations of the cane toad gut microbiome and validates the use of cloacal swabs as a nonlethal method to study changes in the large intestine microbiome. These data provide insights for future studies requiring nonlethal sampling of amphibian gut microbiota.

Paradoxical population resilience of a keystone predator to a toxic invasive species

Abstract ContextThe invasive cane toad (Rhinella marina) has decimated populations of a keystone predator, the yellow-spotted monitor (Varanus panoptes), causing trophic cascades in Australian animal communities. Paradoxically, some V. panoptes populations coexist with toads. Demonstrating patterns in heterogeneous population-level impacts could reveal mechanisms that mediate individual effects, and provide managers with the ability to predict future impacts and assist in population recovery. AimsThe aim of the present study was to search for spatial patterns of population resilience of V. panoptes to invasive cane toads. MethodsPublished literature, unpublished data, reports and anecdotal information from trained herpetologists were used to test the emerging hypothesis that resilient predator populations are mainly coastal, whereas non-resilient populations are mostly inland. Key resultsPost-toad invasion data from 23 V. panoptes populations supported the idea that toad impacts on V. panoptes were heterogeneous; roughly half the populations could be designated as resilient (n=13) and half as non-resilient (n=10). Resilient populations had longer times since toad invasion than did non-resilient populations (39 versus 9 years respectively), supporting the idea that some recovery can occur. Non-resilient populations were exclusively inland (n=10), whereas resilient populations were split between inland (n=5) and coastal (n=8) populations. Resilient inland populations, however, were mainly confined to areas in which decades had passed since toad invasion. ConclusionsThe findings suggest that coastal V. panoptes populations fare much better than inland populations when it comes to surviving invading cane toads. ImplicationsUnambiguous recovery of monitor populations remains undemonstrated and will require long-term population monitoring before and after toad invasion.

Immune and environment-driven gene expression during invasion: An eco-immunological application of RNA-Seq

Host-pathogen associations change rapidly during a biological invasion and are predicted to impose strong selection on immune function. It has been proposed that the invader may experience an abrupt reduction in pathogen-mediated selection ("enemy release"), thereby favoring decreased investment into "costly" immune responses. Across plants and animals, there is mixed support for this prediction. Pathogens are not the only form of selection imposed on invaders; differences in abiotic environmental conditions between native and introduced ranges are also expected to drive rapid evolution. Here, we use RNA-Seq to assess the expression patterns of immune and environmentally associated genes in the cane toad (Rhinella marina) across its invasive Australian range. Transcripts encoding mediators of costly immune responses (inflammation, cytotoxicity) showed a curvilinear relationship with invasion history, with highest expression in toads from oldest and newest colonized areas. This pattern is surprising given theoretical expectations of density dynamics in invasive species and may be because density influences both intraspecific competition and parasite transmission, generating conflicting effects on the strength of immune responses. Alternatively, this expression pattern may be the result of other evolutionary forces, such as spatial sorting and genetic drift, working simultaneously with natural selection. Our findings do not support predictions about immune function based on the enemy release hypothesis and suggest instead that the effects of enemy release are difficult to isolate in wild populations, especially in the absence of information regarding parasite and pathogen infection. Additionally, expression patterns of genes underlying putatively environmentally associated traits are consistent with previous genetic studies, providing further support that Australian cane toads have adapted to novel abiotic challenges.

Targeted gene flow and rapid adaptation in an endangered marsupial

Targeted gene flow is an emerging conservation strategy. It involves translocating individuals with favorable genes to areas where they will have a conservation benefit. The applications for targeted gene flow are wide-ranging but include preadapting native species to the arrival of invasive species. The endangered carnivorous marsupial, the northern quoll (Dasyurus hallucatus), has declined rapidly since the introduction of the cane toad (Rhinella marina), which fatally poisons quolls that attack them. There are, however, a few remaining toad-invaded quoll populations in which the quolls survive because they know not to eat cane toads. It is this toad-smart behavior we hope to promote through targeted gene flow. For targeted gene flow to be feasible, however, toad-smart behavior must have a genetic basis. To assess this, we used a common garden experiment, comparing offspring from toad-exposed and toad-naïve parents raised in identical environments, to determine whether toad-smart behavior is heritable. Offspring from toad-exposed populations were substantially less likely to eat toads than those with toad-naïve parents. Hybrid offspring showed similar responses to quolls with 2 toad-exposed parents, indicating the trait may be dominant. Together, these results suggest a heritable trait and rapid adaptive response in a small number of toad-exposed populations. Although questions remain about outbreeding depression, our results are encouraging for targeted gene flow. It should be possible to introduce toad-smart behavior into soon to be affected quoll populations.

How many and when? Optimising targeted gene flow for a step change in the environment

Targeted gene flow is an emerging conservation strategy that involves introducing individuals with particular traits to places where these traits are of benefit. One obvious application is to adapt a recipient population to a known threat, but questions remain as to how best to achieve this. Here, we vary timing and size of the introduction to maximise our objective - survival of the recipient population's genome. We explore a generic population model as well as a specific example - the northern quoll, an Australian marsupial predator threatened by the toxic cane toad. We reveal a trade-off between preserving the recipient genome and reducing population extinction risk, but key management levers can often optimise this so that nearly 100% of the recipient population's genome is preserved. Any action was better than none but the size of the benefit was sensitive to outbreeding depression, recombination rate, and the timing and size of the introduction.

Differential success in obtaining gametes between male and female Australian temperate frogs by hormonal induction: A review

Most Australian frogs fall into two deeply split lineages, conveniently referred to as ground frogs (Myobatrachidae and Limnodynastidae) and tree frogs (Pelodryadidae). Species of both lineages are endangered because of the global chytrid pandemic, and there is increasing interest and research on the endocrine manipulation of reproduction to support the use of assisted reproductive technologies in conservation. Hormonal induction of gamete release in males and females is one such manipulation of the reproductive process. This paper reviews progress in temperate ground and tree frogs towards developing simple and efficient hormonal protocols for induction of spermiation and ovulation, and presents some new data, that together build towards an understanding of advances and obstacles towards progress in this area. We report that protocols for the non-invasive induction of sperm release, relying on single doses of gonadotropin-releasing hormone (GnRH) or human chorionic gonadotropin are very effective in both ground and tree frog species investigated to date. However, we find that, while protocols based on GnRH, and GnRH and dopamine antagonists, are moderately efficient in inducing ovulation in ground frogs, the same cannot be said for the use of such protocols in tree frogs. Although induced ovulation in the pelodryadid tree frogs has not been successfully implemented, and is difficult to explain in terms of the underlying endocrinology, we propose future avenues of investigation to address this problem, particularly the need for a source of purified or recombinant follicle-stimulating hormone and luteinising hormone for species from this group.

Viral Discovery in the Invasive Australian Cane Toad (Rhinella marina) Using Metatranscriptomic and Genomic Approaches

Cane toads are poisonous amphibians that were introduced to Australia in 1935 for insect control. Since then, their population has increased dramatically, and they now threaten many native Australian species. One potential method to control the population is to release a cane toad virus with high mortality rates, yet few cane toad viruses have been characterized. This study samples cane toads from different Australian locations and uses an RNA sequencing and computational approach to find new viruses. We report novel complete picornavirus and retrovirus sequences that were genetically similar to viruses infecting frogs, reptiles, and fish. Using data generated in other studies, we show that these viral sequences are present in cane toads from distinct Australian locations. Three sequences related to circoviruses were also found in the toad genome. The identification of new viral sequences will aid future studies that investigate their prevalence and potential as agents for biocontrol.

Cane toads are a notorious invasive species, inhabiting over 1.2 million km² of Australia and threatening native biodiversity. The release of pathogenic cane toad viruses is one possible biocontrol strategy yet is currently hindered by the poorly described cane toad virome. Metatranscriptomic analysis of 16 cane toad livers revealed the presence of a novel and full-length picornavirus, Rhimavirus A (RhiV-A), a member of a reptile- and amphibian-specific cluster of the Picornaviridae basal to the Kobuvirus-like group. In the combined liver transcriptome, we also identified a complete genome sequence of a distinct epsilonretrovirus, Rhinella marina endogenous retrovirus (RMERV). The recently sequenced cane toad genome contains 8 complete RMERV proviruses as well as 21 additional truncated insertions. The oldest full-length RMERV provirus was estimated to have inserted 1.9 million years ago (MYA). To screen for these viral sequences in additional toads, we analyzed publicly available transcriptomes from six diverse Australian locations. RhiV-A transcripts were identified in toads sampled from three locations across 1,000 km of Australia, stretching to the current Western Australia (WA) invasion front, while RMERV transcripts were observed at all six sites. Finally, we scanned the cane toad genome for nonretroviral endogenous viral elements, finding three sequences related to small DNA viruses in the family Circoviridae. This shows ancestral circoviral infection with subsequent genomic integration. The identification of these current and past viral infections enriches our knowledge of the cane toad virome, an understanding of which will facilitate future work on infection and disease in this important invasive species.

IMPORTANCE Cane toads are poisonous amphibians that were introduced to Australia in 1935 for insect control. Since then, their population has increased dramatically, and they now threaten many native Australian species. One potential method to control the population is to release a cane toad virus with high mortality rates, yet few cane toad viruses have been characterized. This study samples cane toads from different Australian locations and uses an RNA sequencing and computational approach to find new viruses. We report novel complete picornavirus and retrovirus sequences that were genetically similar to viruses infecting frogs, reptiles, and fish. Using data generated in other studies, we show that these viral sequences are present in cane toads from distinct Australian locations. Three sequences related to circoviruses were also found in the toad genome. The identification of new viral sequences will aid future studies that investigate their prevalence and potential as agents for biocontrol.

Methods for invasive species control are transferable across invaded areas

Cane Toads (Rhinella marina) are invasive pests in many parts of the world, including the Japanese island of Ishigaki. Extensive research in Australia has identified promising new methods for control, but also has shown that toads exhibit geographic variation in many traits (suggesting that methods developed in one location may not work in another). Can the approaches developed in Australia play a useful role for controlling this invasive species in Japan? Our experimental trials on Ishigaki Island suggest that these new methods can be successfully applied to Japan. First, Cane Toad embryos exposed to chemical cues of conspecific tadpoles exhibited a reduction in viability (subsequent growth and development). This response appears to be species-specific, with native frog embryos not being affected by exposure to cues from toad tadpoles, and Cane Toad embryos not being affected by exposure to cues from native frog tadpoles. Second, Cane Toad tadpoles were attracted to traps containing water from conspecific eggs, and toxin from adult conspecifics. Third, adult Cane Toads were attracted to acoustic cues of calling males, with sex differences in rates of attraction to specific versions of a synthetic call (males were attracted to choruses whereas females were attracted to low-frequency calls). Our results suggest that the methods developed by Australian researchers are applicable to controlling invasive Cane Toads in Japan.

Success of capture of toads improved by manipulating acoustic characteristics of lures

BACKGROUND

Management of invasive vertebrates is a crucial component of conservation. Trapping reproductive adults is often effective for control, and modification of traps may greatly increase their attractiveness to such individuals. Cane toads (Rhinella marina) are invasive, and males use advertisement vocalisations to attract reproductive females. In amphibians, including toads, specific structural parameters of calls (e.g. dominant frequency and pulse rate) may be attractive to females. Some cane toad traps use an artificial advertisement vocalisation to attract toads. We determined whether variation of the call's parameters (volume, dominant frequency and pulse rate) could increase the capture rate of gravid females.

RESULTS

Overall, traps equipped with loud calls (80 dB at 1 m) caught significantly more toads, and proportionally more gravid females, than traps with quiet calls (60 dB at 1 m), and traps with low dominant frequency calls caught more gravid females than traps with median frequency calls. Traps with high pulse rate calls attracted more females than traps with low pulse rate calls. Approximately 91% of the females trapped using a low frequency and high pulse rate combination call were gravid, whereas in traps using a call with population median parameters only approximately 75% of captured females were gravid.

CONCLUSION

Calls that indicated large-bodied males (low frequency) with high energy reserves (high pulse rate) are often attractive to female anurans and were effective lures for female toads in our study. The design of future trapping regimes should account for behavioural preferences of the target sex. © 2017 Society of Chemical Industry.