A common environmental contaminant affects sexual behavior in the clawed frog, Xenopus tropicalis

A review of non-destructive biomonitoring techniques to assess the impacts of pollution on reproductive health in frogs and toads

In anuran amphibians (frogs and toads), evidence linking pollution to population declines is limited, in particular through impaired reproduction. Here we review the evidence for pollutant-induced alterations on reproductive endpoints in wild anurans with a particular focus on the application of non-destructive endpoints including on sex ratios, male reproductive phenotypes (data are too scarce for females) and reproductive outputs (reflective of mating success). Data evidencing alterations in sex ratio in wild anurans are scarce, however, both feminisation and masculinisation in response to pollution have been reported (seven studies). Male nuptial pad morphology and calling behaviour display high sensitivity to pollutant-exposure and are important features determining male breeding success, however there is considerable variation in these endpoints and inconsistencies in the responses of them to pollution are reported in wild anurans. Data for clutch size are insufficient to assess sensitivity to pollutants (five studies only). However, hatch success and offspring fitness (tadpole survival/development) are sensitive to pollution, with clear linkages to population stability. In conclusion, there are a wide range of non destructive measures with good potential for application to assess/monitor reproductive health in wild anurans, however, a greater understanding of pollutant effects on these endpoints is needed. There measures deserve wider application as they are relatively simple and inexpensive to implement, and as they can be applied non-destructively are widely applicable to our declining anuran populations.

Influence of octylphenol on gene expression of gonadotropins and their receptors, testicular structure and mating behavior of male Rana chensinensis

In the present study, responses of the Chinese brown frog (Rana chensinensis) to exposure to different doses and duration of Octyphenol (OP) which degraded from alkylphenol ethoxylates (APEs) were characterized during the adult periods. The effects of OP on growth, development and reproduction and the expression of genes in gonad were investigated. The expression levels of fshβ, lhβ, fshr and lhr had significant differences as the exposure time increased. The pathological and morphological changes were also observed in the OP treatments. Furthermore, the number of TUNEL positive cells and the TUNEL index was elevated after exposed to OP. Besides that, OP treatment could influence its mating behavior and reduce the fertilization rates. Taken together, these results indicated that OP disrupt sex steroid signaling, normal development of spermatogenesis, courtship behavior of male frogs and decline fertilization rate in R. chensinensis.

Pesticide exposure affects reproductive capacity of common toads (Bufo bufo) in a viticultural landscape

Amphibian populations are declining worldwide at alarming rates. Among the large variety of contributing stressors, chemical pollutants like pesticides have been identified as a major factor for this decline. Besides direct effects on aquatic and terrestrial amphibian stages, sublethal effects like impairments in reproduction can affect a population. Therefore, we investigated the reproductive capacity of common toads (Bufo bufo) in the pesticide-intensive viticultural landscape of Palatinate in Southwest Germany along a pesticide gradient. In a semi-field study, we captured reproductively active common toad pairs of five breeding ponds with different pesticide contamination level and kept them in a net cage until spawning. Toads from more contaminated ponds showed an increased fecundity (more eggs) but decreased fertilization rates (fewer hatching tadpoles) as well as lower survival rates and reduced size in Gosner stage 25, suggesting that the higher exposed populations suffer from long-term reproductive impairments. In combination with acute toxicity effects, the detected sublethal effects, which are mostly not addressed in the ecological risk assessment of pesticides, pose a serious threat on amphibian populations in agricultural landscapes.

Testicular morphometric changes in neotropical anurans from agroecosystems

One of the causes of the global decline of amphibians is agricultural activity, responsible for causing habitat fragmentation and bringing a range of agrochemicals and fertilizers in the environment, compounds with a potential disrupting effect on non-target organisms, such as frogs. Exposure to these compounds has numerous harmful effects on the testes of these animals, which can compromise reproduction and, consequently, the maintenance of their communities. In this context, we compared the morphology and morphometry of the testes of three species of neotropical anurans (Physalaemus cuvieri, Dendropsophus minutus, and Boana albopunctata) from an agricultural area and a conservation unit. Histologically, the testicular morphology of the species was similar for both environments; however, morphometrically, there was a difference in the measured testicular parameters (locular area and area of spermatogenic cysts). Physalaemus cuvieri presented higher averages of locular and spermatogonial area in the agricultural environment, whereas the area occupied by the spermatozoa was smaller. Additionally, the testicular pigmentation, which is only present in this species, was greater in animal from the agricultural area. In D. minutus, the locular, spermatogonial, and sperm areas showed lower values in the agricultural area, whereas in B. albopunctata, the opposite pattern was found, with the area of the locule, spermatocytes, and spermatozoids being higher. Agricultural activities influence the testicular metric parameters in different species, and our results suggest that D. minutus is most sensitive to anthropic pressures. The least sensitive species is B. albopunctata. We highlight the importance of evaluating different species, since each species responds differently to agricultural activities.

Agrochemicals disrupt multiple endocrine axes in amphibians

Concern over global amphibian declines and possible links to agrochemical use has led to research on the endocrine disrupting actions of agrochemicals, such as fertilizers, fungicides, insecticides, acaricides, herbicides, metals, and mixtures. Amphibians, like other species, have to partition resources for body maintenance, growth, and reproduction. Recent studies suggest that metabolic impairments induced by endocrine disrupting chemicals, and more particularly agrichemicals, may disrupt physiological constraints associated with these limited resources and could cause deleterious effects on growth and reproduction. Metabolic disruption has hardly been considered for amphibian species following agrichemical exposure. As for metamorphosis, the key thyroid hormone-dependent developmental phase for amphibians, it can either be advanced or delayed by agrichemicals with consequences for juvenile and adult health and survival. While numerous agrichemicals affect anuran sexual development, including sex reversal and intersex in several species, little is known about the mechanisms involved in dysregulation of the sex differentiation processes. Adult anurans display stereotypical male mating calls and female phonotaxis responses leading to successful amplexus and spawning. These are hormone-dependent behaviours at the foundation of reproductive success. Therefore, male vocalizations are highly ecologically-relevant and may be a non-invasive low-cost method for the assessment of endocrine disruption at the population level. While it is clear that agrochemicals disrupt multiple endocrine systems in frogs, very little has been uncovered regarding the molecular and cellular mechanisms at the basis of these actions. This is surprising, given the importance of the frog models to our deep understanding of developmental biology and thyroid hormone action to understand human health. Several agrochemicals were found to have multiple endocrine effects at once (e.g., targeting both the thyroid and gonadal axes); therefore, the assessment of agrochemicals that alter cross-talk between hormonal systems must be further addressed. Given the diversity of life-history traits in Anura, Caudata, and the Gymnophiona, it is essential that studies on endocrine disruption expand to include the lesser known taxa. Research under ecologically-relevant conditions will also be paramount. Closer collaboration between molecular and cellular endocrinologists and ecotoxicologists and ecologists is thus recommended.

Metabolic Costs of Exposure to Wastewater Effluent Lead to Compensatory Adjustments in Respiratory Physiology in Bluegill Sunfish

Municipal wastewater effluent is a major source of aquatic pollution and has potential to impact cellular energy metabolism. However, it is poorly understood whether wastewater exposure impacts whole-animal metabolism and whether this can be accommodated with adjustments in respiratory physiology. We caged bluegill sunfish (Lepomis macrochirus) for 21 days at two sites downstream (either 50 or 830 m) from a wastewater treatment plant (WWTP). Survival was reduced in fish caged at both downstream sites compared to an uncontaminated reference site. Standard rates of O₂ consumption increased in fish at contaminated sites, reflecting a metabolic cost of wastewater exposure. Several physiological adjustments accompanied this metabolic cost, including an expansion of the gill surface area available for gas exchange (reduced interlamellar cell mass), a decreased blood-O₂ affinity (which likely facilitates O₂ unloading at respiring tissues), increased respiratory capacities for oxidative phosphorylation in isolated liver mitochondria (supported by increased succinate dehydrogenase, but not citrate synthase, activity), and decreased mitochondrial emission of reactive oxygen species (ROS). We conclude that exposure to wastewater effluent invokes a metabolic cost that leads to compensatory respiratory improvements in O₂ uptake, delivery, and utilization.

Effects of octylphenol on the expression of StAR, CYP17 and CYP19 in testis of Rana chensinensis

It has been proposed that a decline in sperm quality is associated with exposure to environmental chemicals with estrogenic activity. Seeking possible explanations for this effect, this study investigated the effects of octylphenol (OP) on the synthesis of steroid hormones in amphibian. Rana chensinensis were exposed to 10^-8, 10^-7 and 10^-6mol/L OP after 10, 20, 30 and 40 days. The cDNA fragments of StAR (274bp), CYP17 (303bp) and CYP19 (322bp) were cloned. In situ hybridization and immunohistochemistry revealed that positive signals of StAR, CYP17, CYP19 mRNA and proteins mainly in the Leydig cells of testes. Real-time PCR showed that up-regulation of StAR and CYP19, and down-regulation of CYP17 after exposure to 10^-8, 10^-7 and 10^-6mol/L OP. The results suggest that OP can alter transcriptions of StAR, CYP17 and CYP19, thus disturb the expressions of StAR, P450c17 and P450arom, thereby adversely affect steroid synthesis.

Octylphenol induced gene expression in testes of Frog, Rana chensinensis

Octylphenol (OP) is an endocrine-disrupting chemical (EDC), which can disrupt the reproductive system. To understand the effect of OP, a subtractive cDNA library was constructed using suppression subtractive hybridization (SSH) to identify alterations of gene transcription in the testes of the frog Rana chensinensis after OP exposure. Two hundred positive clones were selected and 134 sequences of gene fragments were produced from the subtractive library randomly. These genes were identified to be involved in metabolic process, cellular process, biological regulation, stimulus, immune system and female pregnancy process. In order to verify the efficiency of the subtractive cDNA library, PSG9 and PAPP-A were analyzed further as two representatives of differentially expressed transcription genes using semi-quantitative RT-PCR. Our result was the first successful construction of the subtractive cDNA library in frog testes after OP treatment. Based on this cDNA library, OP was shown to affect multiple physiological processes including inducing immune response, disrupting the steroid hormone synthesis and influencing spermatogenesis in the testis by up-regulation of specific genes.

Effects of the environmental estrogenic contaminants bisphenol A and 17α-ethinyl estradiol on sexual development and adult behaviors in aquatic wildlife species

Endocrine disrupting chemicals (EDCs), including the mass-produced component of plastics, bisphenol A (BPA) are widely prevalent in aquatic and terrestrial habitats. Many aquatic species, such as fish, amphibians, aquatic reptiles and mammals, are exposed daily to high concentrations of BPA and ethinyl estradiol (EE2), estrogen in birth control pills. In this review, we will predominantly focus on BPA and EE2, well-described estrogenic EDCs. First, the evidence that BPA and EE2 are detectable in almost all bodies of water will be discussed. We will consider how BPA affects sexual and neural development in these species, as these effects have been the best characterized across taxa. For instance, such chemicals have been in many cases reported to cause sex-reversal of males to females. Even if these chemicals do not overtly alter the gonadal sex, there are indications that several EDCs might demasculinize male-specific behaviors that are essential for attracting a mate. In so doing, these chemicals may reduce the likelihood that these males reproduce. If exposed males do reproduce, the concern is that they will then be passing on compromised genetic fitness to their offspring and transmitting potential transgenerational effects through their sperm epigenome. We will thus consider how diverse epigenetic changes might be a unifying mechanism of how BPA and EE2 disrupt several processes across species. Such changes might also serve as universal species diagnostic biomarkers of BPA and other EDCs exposure. Lastly, the evidence that estrogenic EDCs-induced effects in aquatic species might translate to humans will be considered.

The influence of long-term cadmium exposure on phonotaxis in male Pelophylax nigromaculata

Cadmium (Cd) is a common industrial and agricultural heavy metal found in the natural environment that disrupts the endocrine systems of vertebrates. Amphibians are particularly vulnerable to endocrine disruptors because of their aquatic habitats and permeable skin. Endocrine disruptors are known to negatively affect amphibian acoustic behavior, but whether and how the ubiquitous pollutant Cd impacts this crucial amphibian signaling system remains unknown. Male black-spotted frogs (Pelophylax nigromaculata) show phonotactic responses to female receptive calls by emitting advertisement calls and moving towards females during the mating season, essential for reproductive success. To study whether long-term (60 d) exposure to low Cd concentrations (10(-8), 10(-7) and 10(-6) M) affects male phonotaxis, we recorded male responses to female calls following Cd exposure during the breeding season. We found that Cd adversely affected advertisement call characteristics (call latency, call duration and call rate), the proportion of individuals that responded and the time to first movement of the male. These results indicate that long-term exposure to Cd at environmentally relevant concentrations alters phonotaxis in male P. nigromaculata.

Impaired liver function in Xenopus tropicalis exposed to benzo[a]pyrene: transcriptomic and metabolic evidence

Background

Despite numerous studies suggesting that amphibians are highly sensitive to cumulative anthropogenic stresses, the role pollutants play in the decline of amphibian populations remains unclear. Amongst the most common aquatic contaminants, polycyclic aromatic hydrocarbons (PAHs) have been shown to induce several adverse effects on amphibian species in the larval stages. Conversely, adults exposed to high concentrations of the ubiquitous PAH, benzo[a]pyrene (BaP), tolerate the compound thanks to their highly efficient hepatic detoxification mechanisms. Due to this apparent lack of toxic effect on adults, no studies have examined in depth the potential toxicological impact of PAH on the physiology of adult amphibian livers. This study sheds light on the hepatic responses of Xenopus tropicalis when exposed to high environmentally relevant concentrations of BaP, by combining a high throughput transcriptomic approach (mRNA deep sequencing) and a characterization of cellular and physiological modifications to the amphibian liver.

Results

Transcriptomic changes observed in BaP-exposed Xenopus were further characterized using a time-dependent enrichment analysis, which revealed the pollutant-dependent gene regulation of important biochemical pathways, such as cholesterol biosynthesis, insulin signaling, adipocytokines signaling, glycolysis/gluconeogenesis and MAPK signaling. These results were substantiated at the physiological level with the detection of a pronounced metabolic disorder resulting in a possible insulin resistance-like syndrome phenotype. Hepatotoxicity induced by lipid and cholesterol metabolism impairments was clearly identified in BaP-exposed individuals.

Conclusions

Our data suggested that BaP may disrupt overall liver physiology, and carbohydrate and cholesterol metabolism in particular, even after short-term exposure. These results are further discussed in terms of how this deregulation of liver physiology can lead to general metabolic impairment in amphibians chronically exposed to contaminants, thereby illustrating the role xenobiotics might play in the global decline in amphibian populations.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-666) contains supplementary material, which is available to authorized users.