Biology session

For ectothermic vertebrates, a potentially harmful effect of the anthropogenic environment could be sex reversal. The process of sex reversal can happen at the embryonic or larval phase during ontogenesis, due to extreme environmental factors (for example: high temperature, dehydration, chemicals, starvation). Sex reversal means that individuals develop a phenotypic sex that does not match their genetic sex determined by their sex chromosomes. It is essential to study this phenomenon and also its causes in wild populations, as it can lead to a number of consequences, including unbalanced sex ratio and even the extinction of a population. According to some experiments with fish, sex reversal may be caused by stress-induced elevation of the secretion of glucocorticoid hormones, which inhibit the aromatase enzyme that would convert male hormones into female hormones. However, for reptiles, such experiments gave conflicting results, and for amphibians, this hypothesis has not been tested yet. In this study, we tested how glucocorticoid hormones affect the sex ratio of agile frogs (Rana dalmatina). We used one of the glucocorticoid hormones called corticosterone that is considered the main ”stress hormone” of amphibians. The prediction we tested was that an increase in corticosterone levels would cause sex reversal, therefore resulting in male-dominated phenotypic sex ratios in the treated groups. The tadpoles were divided into 5 treatment groups. To the water of the solvent control group 0.1 ml/l ethanol was added, and for the other 4 groups, different corticosterone concentrations (0.01, 10, 100 and 1000 nM) were applied using the same amount of ethanol as the control group had. The hormone treatments lasted for 6 days during the phase when the sexual development of agile frogs is sensitive to environmental conditions (in the third week after reaching the free-swimming larval stage). Approximately two months after metamorphosis, when the gonads had already differentiated, the phenotypic sex of the individuals was determined based on the macroscopic appearance of their gonads upon dissection. As for the phenotypic sex ratio, no significant difference was found between the control and the treatment groups. However, the sample size decreased because of the unexpectedly high mortality rate, regardless of treatment group. In the groups treated with 0.01 or 10 nM corticosterone, the proportion of males increased to 53-61% compared to the 44% of the control group. Also, significantly more abnormalities of gonadal morphology were detected in these two groups. In the next steps of our study, we will examine genetic sex and gonadal histology to identify the sex-reversed individuals, to ascertain whether corticosterone affects sex development in amphibians and thus whether anthropogenic stressors can distort the sex ratio of the populations.