Toxic effects of long-term dual or single exposure to oxytetracycline and arsenic on Xenopus tropicalis living in duck wastewater

Direct discharge of aquaculture wastewater may have toxic effects,due to the presence of heavy metals,antibiotics,and even resistant pathogens,but little attention has been given.Here,tanks simulating a wild ecosystem were built to study the effects of long-term exposure to duckwastewater containing oxytetracycline(OTC)and/or arsenic(As)on the growth,physiological function,and gut microbiota evolution of Xenopus tropicalis.The results showed that duck wastewater had no apparent impact on X.tropicalis,but the impact increased significantly(P<0.05)with exposure to OTC and/or As,especially the impact on body weight and growth rate.Biochemical indicators revealed varying degrees of oxidative stress damage,hepatotoxicity(inflammation,necrosis,and sinusoids),and collagen fibrosis of X.tropicalis in all treated groups after 72 days of exposure,which indirectly inhibited X.tropicalis growth.Moreover,16S rDNA amplicon sequencing results showed that the gut microbiota structure and metabolic function were perturbed after chronic exposure,which might be the leading cause of growth inhibition.Interestingly,the abundance of intestinal resistance genes(RGs)increased with exposure time owing to the combined direct and indirect effects of stress factors in duck wastewater.Moreover,once the RGs were expressed,the resistance persisted for at least 24 days,especially that conferred by tetA.These results provide evidence of the toxic effects of DW containing OTC(0.1–4.0 mg/L)and/or As(0.3–3.5μg/L)on amphibians and indicate that it is vital to limit the usage of heavy metals and antibiotics on farms to control the biotoxicity of wastewater.

Histological observation on unique phenotypes of malformation induced in Xenopus tropicalis larvae by tributyltin

Tributyltin（TBT）,a biocide used in antifouling paints,has shown strong teratogenic effects on Xenopus tropicalis embryos at environmentally relevant concentrations.X.tropicalis embryos were exposed to 50,100 and 200 ng/L tributyltin chloride for 72 hr.The histological changes were further observed on abnormal eyes,enlarged trunks,enlarged proctodaeums and absence of fins induced by TBT.The lens and the retinal layers of abnormal eyes were slightly or barely differentiated,and that the pigment epithelium was neither continuous nor smooth.The abdomens were full of undifferentiated gut tissue with yolk-rich inclusions in the tadpoles with enlarged trunks.The proctodaeums formed a bump-like or columnar structure.The mass of yolk-rich cells occupied the lumen,blocked the opening and even turned inside out of the proctodaeum.Both the ventral and dorsal fins in trunks and tails became narrow or even disappeared totally.Our results suggest that great changes of histology took place corresponding to the unique phenotypes.The gut tissue was poorly differentiated,which led to the failed elongation of the guts and subsequently the enlarged trunks.The enlarged proctodaeums were due to the undifferentiation of inner layer,the expansion of outer epidermal part and the absence of fins around them.In brief,the histological observations provided insights into the reason of the unique external malformations in some degree.

Ambient temperature alters body size and gut microbiota of Xenopus tropicalis

Temperature is important to determine physiological status of ectotherms. However, it is still not fully understood how amphibians and their symbiotic microbiota acclimate to ambient temperature. In this study, we investigated the changes of gut microbiota of Xenopus tropicalis at different temperatures under controlled laboratory conditions. The results showed that microbial communities were distinct and shared only a small overlap among froglet guts, culture water and food samples.Furthermore, the dominant taxa harbored in the gut exhibited low relative abundance in water and food. It indicates that bacterial taxa selected by amphibian gut were generally of low abundance in the external environment. Temperature could affect betadiversity of gut microbiota in terms of phylogenetic distance, but it did not affect alpha diversity. The composition of gut microbiota was similar in warm and cool treatments. However, signature taxa in different temperature environments were identified. The relationships between temperature, gut microbiota and morphology traits of X. tropicalis revealed in this study help us to predict the consequences of environmental changes on ectothermic animals.

Unexpected phenotypes of malformations induced in Xenopus tropicalis embryos by combined exposure to triphenyltin and 9-cis-retinoic acid

Xenopus tropicalis embryos were exposed for 48 hr to the mixtures of 5 μg Sn/L triphenyltin （TPT）, which is a well-known endocrine disruptor, and 0.25-5 μg/L 9-cis retinoic acid （9c-RA）, which is the natural ligand of retinoid X receptor. The phenotypes induced by combined exposure were more variable than those resulting from single exposure to either TPT or 9c-RA. The prominent phenotypes included underdeveloped head structures, abnormal eyes, narrow fins, enlarged proctodaeum, etc. Especially, combined exposure induced unexpected notochord malformations, which ranged from small swellings of the surface of the tails to the extension and extrusion of notochord out of the posterior tails. Compared with the 5 μg Sn/L TPT-treated group, the index of fin deficiency was not affected, and the index of axis deficiency was significantly increased with increasing RA concentrations in the mixtures. Our results suggest that combined exposure to TPT and 9c-RA induced not only more variable phenotypes of malformations than exposure to single compound but also some new and unexpected phenotypes.