A laboratory investigation into features of morphology and physiology for their potential to predict reproductive success in male frogs

Effects of mifepristone, a model compound with anti-progestogenic activity, on the reproduction of African clawed frog (Xenopus laevis)

This is the first study on how a substance with anti-progestogenic activity affects amphibian reproduction. Mifepristone, a synthetic anti-progestin used in abortion pills, was chosen as model compound. African clawed frog (Xenopus laevis) females were exposed to four mifepristone concentrations (0.7, 9, 120, and 1380 ng∙L-1) for 30 days. A control group was also included. Egg-laying during the experiment was significantly less at the highest concentration. At the experiment's end, mifepristone-exposed and control females were randomly mated with sexually mature males. Breeding rate for females exposed to 1380 ng∙L-1 mifepristone was only 50 %. Histology revealed no significant changes in gonads, thyroid, or liver. Females exposed to 1380 ng∙L-1 mifepristone had lower estradiol levels in plasma, lower mRNA expression of lh and fsh in brain-pituitary complex, and p450scc in ovaries. In liver, mRNA level of npr was significantly increased in females exposed to 120 ng∙L-1 mifepristone. mRNA expression of mpr, erβ, dio2, and dio3 were upregulated in animals exposed to 9 ng∙L-1 and 120 ng∙L-1 mifepristone, whereas vtg expression was significantly downregulated in females exposed to 1380 ng∙L-1 mifepristone. All these findings show that exposure to compounds with anti-progestogenic activity affects the hypothalamus-pituitary-gonad axis and decreases reproductive success.

Impaired spermatogenesis and associated endocrine effects of azole fungicides in peripubertal Xenopus tropicalis

Early life exposure to endocrine disrupting chemicals (EDCs) has been suggested to adversely affect reproductive health in humans and wildlife. Here, we characterize endocrine and adverse effects on the reproductive system after juvenile exposure to propiconazole (PROP) or imazalil (IMZ), two common azole fungicides with complex endocrine modes of action. Using the frog Xenopus tropicalis, two short-term (2-weeks) studies were conducted. I: Juveniles (2 weeks post metamorphosis (PM)) were exposed to 0, 17 or 178 µg PROP/L. II: Juveniles (6 weeks PM) were exposed to 0, 1, 12 or 154 µg IMZ/L. Histological analysis of the gonads revealed an increase in the number of dark spermatogonial stem cells (SSCs)/testis area, and in the ratio secondary spermatogonia: dark SSCs were increased in all IMZ groups compared to control. Key genes in gametogenesis, retinoic acid and sex steroid pathways were also analysed in the gonads. Testicular levels of 3β-hsd, ddx4 were increased and cyp19 and id4 levels were decreased in the IMZ groups. In PROP exposed males, increased testicular aldh1a2 levels were detected, but no histological effects observed. Although no effects on ovarian histology were detected, ovarian levels of esr1, rsbn1 were increased in PROP groups, and esr1 levels were decreased in IMZ groups. In conclusion, juvenile azole exposure disrupted testicular expression of key genes in retinoic acid (PROP) and sex steroid pathways and in gametogenesis (IMZ). Our results further show that exposure to environmental concentrations of IMZ disrupted spermatogenesis in the juvenile testis, which is a cause for concern as it may lead to impaired fertility. Testicular levels of id4, ddx4 and the id4:ddx4 ratio were associated with the number of dark SSCs and secondary spermatogonia suggesting that they may serve as a molecular markers for disrupted spermatogenesis.

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.

Non-destructive methods to assess health of wild tropical frogs (túngara frogs: Engystomops pustulosus) in Trinidad reveal negative impacts of agricultural land

Amphibians are threatened globally with at least 43% of species declining and the most important stressor being habitat loss or degradation. Amphibians inhabiting highly biodiverse tropical regions are disproportionately threatened; however, the effects of landscape alterations on amphibian health are virtually unknown. In this study, we utilised non-destructive techniques to compare size (weight, snout-vent length [SVL]), body condition, male secondary sexual features (forelimb width, nuptial pad length) and breeding success (egg number, fertility [percentage fertilised eggs], hatching success) in túngara frogs (Engystomops pustulosus) collected from reference (n = 5), suburban (n = 6) and agricultural (n = 4) sites in Trinidad; characterised by presence/absence of crops/houses. All measured endpoints were negatively impacted in frogs collected from agricultural sites. The largest effect was observed for hatching success (2.77-fold lower) and egg number (2.5-fold lower). Less pronounced effects were observed on male frogs (weight: 1.77-fold lower; SVL: 1.18-fold lower; forelimb width: 1.33-fold lower; nuptial pad length: 1.15-fold lower). Our findings demonstrate negative impacts of agricultural sites on túngara frog health, with the number of viable offspring reduced by almost one third. The methods outlined here are technically simple and low-cost and thereby have potential for application to other species in order to investigate the potential impacts of habitat degradation on amphibian health. Furthermore, as these methods are non-destructive, they could be used to investigate the potential contribution of frog size and/or reproductive capability as a causative factor contributing to population declines in threatened species, which is particularly pressing in tropical regions.

Immunoreactivities of AR, ERα, ERβ and aromatase in the nuptial pad of Chinese brown frog (<em>Rana dybowskii</em>) during pre-hibernation and the breeding period

There is a prominent local raised pad called nuptial pad on the forelimb of Chinese brown frog (Rana dybowskii), which is hypothetically concluded as an enhancement of the grip and a spreader of pheromone during the amplexus. In this study, we investigated the immunolocalization and protein expression levels of androgen receptors (AR), estrogen receptor α (ERα), ERβ and aromatase in the nuptial pad of R. dybowskii during pre-hibernation and the breeding period. Histologically, the annual development of the nuptial pad in R. dybowskii is manifested as the larger area of specialized mucous gland and the longer length of papillary epidermal projection during the breeding period. AR, ERα, ERβ and aromatase are present in the stratum granulosum, stratum spinosum, stratum basale and the secretory portion of specialized mucous glands during both periods. Western blotting results confirmed that AR, ERα and ERβ protein levels are higher during pre-hibernation than those during the breeding season. These results suggest that nuptial pad is the direct target organ of androgen and estrogen. Androgen may participate in the regulation of annual development and glandular function of nuptial pad, and estrogen may play an endocrine, autocrine or paracrine role during pre-hibernation and the breeding period.