Effect of hypophysectomy on sex transformation in frog tadpoles

Development of an in vitro diagnostic method to determine the genotypic sex of Xenopus laevis

A genotypic sex determination assay provides accurate gender information of individuals with well-developed phenotypic characters as well as those with poorly developed or absent of phenotypic characters. Determination of genetic sex for Xenopus laevis can be used to validate the outcomes of Tier 2 amphibian assays, and is a requirement for conducting the larval amphibian growth and development assay (LAGDA), in the endocrine disruptor screening program (EDSP), test guidelines. The assay we developed uses a dual-labeled TaqMan probe-based real-time polymerase chain reaction (real-time PCR) method to determine the genotypic sex. The reliability of the assay was tested on 37 adult specimens of X. laevis collected from in-house cultures in Eurofins EAG Agroscience, Easton. The newly designed X. laevis-specific primer pair and probe targets the DM domain gene linked-chromosome W as a master female-determining gene. Accuracy of the molecular method was assessed by comparing with phenotypic sex, determined by necropsy and histological examination of gonads for all examined specimens. Genotypic sex assignments were strongly concordant with observed phenotypic sex, confirming that the 19 specimens were male and 18 were female. The results indicate that the TaqMan® assay could be practically used to determine the genetic sex of animals with poorly developed or no phenotypic sex characteristics with 100% precision. Therefore, the TaqMan® assay is confirmed as an efficient and feasible method, providing a diagnostic molecular sex determination approach to be used in the amphibian endocrine disrupting screening programs conducted by regulatory industries. The strength of an EDSP is dependent on a reliable method to determine genetic sex in order to identify reversals of phenotypic sex in animals exposed to endocrine active compounds.

Gonadal differentiation and secretions of estradiol and testosterone of the ovaries of Rana catesbeiana tadpoles treated with 4-hydroxyandrostenedione

Laparotomized female tadpoles of Rana catesbeiana at TK stages X-XII, about 9 months old, were implanted intraperitoneally with empty capsules or capsules containing 4-hydroxyandrostenedione (4-OHA), known as an aromatase inhibitor in vertebrates. Histology, gonosomatic index, and secretions of estradiol (E2) and testosterone (T) of the ovaries were investigated. Three months after the treatment, histological examination revealed various degrees of sex reversal in the ovaries treated with 4-OHA and 79% (57 in 72) were transformed into testes. The ovaries of control tadpoles, however, displayed normal histological appearance. Radioimmunoassay showed that secretion of E2 was decreased while that of T was increased in 4-OHA treated ovaries. The gonosomatic index displayed a decline tendency from control females through experimental animals to untreated control males. These results indicated that activity of aromatase in the ovaries was inhibited by 4-OHA, resulting in accumulation of T which induced transformation of the ovaries into testes.

Estradiol secretion by the ovarian tissue, in response to hypophyseal stimulation, during ontogenesis of the bullfrog

Ovaries of tadpoles, froglets, young frogs, and mature frogs of Rana catesbeiana were cut into small pieces. They were incubated for 6 hr in Krebs-Ringer bicarbonate buffer as controls. Another series of ovaries of the same developmental stages were incubated with pituitary extracts in the buffer as experimentals. Media were then analyzed for estradiol secretion by radioimmunoassay. The results showed that estradiol secretion by tadpole ovaries during development was not affected by the addition of pituitary extracts of mature frogs in the media at any stage while those of young and mature frogs with pituitary extracts secreted more estradiol than those without. These findings indicate that tadpole ovaries are unresponsive to pituitary agents to produce estradiol while frog ovaries are dependent on some pituitary hormones to synthesize estradiol. Thus frog ovaries acquire dependence on the pituitary agent only after metamorphosis.

In vitro estradiol synthesis and secretion by tadpole ovaries of different developmental stages

The tadpole ovaries at TK stage XIX can synthesize and secrete estradiol (E2), yet it is unknown when this ovarian function starts and how it develops. To this end, the present work has been carried out. The ovaries of different developmental stages of tadpoles and young frogs of Rana catesbeiana were taken, cut into small pieces, and incubated for 6 hr at 20 degrees in Krebs-Ringer bicarbonate buffer. The media were then analyzed for E2 by radioimmunoassay. The results indicate that the tadpoles synthesize and release E2 as early as stage X, that E2 increases slowly and gradually through stage XXV, and that E2 increases rapidly in juvenile frogs. This trend of growth of the ovarian function in estradiol secretion in vitro is in accordance with that reported for female chick embryos. There exists the possibility that the growth of E2 secretion could be biphasic, one with slow increment of autonomous E2 secretion at early stages and the other with quick increase due to pituitary stimulation at the stage of metamorphic climax.