Sex determination and primary sex differentiation in amphibians: Genetic and developmental mechanisms

Abstract

Most amphibians lack morphologically distinguishable sex chromosomes, but a number of experimental techniques have shown that amphibian sex determination is controlled genetically. The few studies suggesting that environment influences sex determination in amphibians have all been conducted at temperatures outside of the range normally experienced by the species under study, and these effects probably do not occur under natural conditions. No sexdetermining genes have been described in amphibians, and sex differentiation can be altered by treatment with exogenous steroid hormones. The effects of sex steroids vary extensively between species, and a variety of steroids can alter the sex ratios of treated larvae. The role of endogenous sex steroids in gonadal differentiation has not been fully explored; thus the natural role of steroids in amphibian gonadal differentiation is unknown. Sex steroid receptors have not been examined in amphibian gonads, and the mechanism of steroid action on the gonad is unclear. In addition to steroids, the thyroid hormones may play a role in gonadal differentiation. Pituitary gonadotrop(h)ins affect gonadal growth, but not differentiation or maturation of gonads.In addition to the issue of resolving the mechanisms underlying hormone action in gonadal differentiation, other debates concerning interactions between the developing gonads and the invading germ cells, and even the origin of the medullary and cortical portions of the developing gonads, remain unresolved. Studies examining links between sex determination and gonadal differentiation are needed. In addition, examinations of variation in steroidal effects on gonadal development in a phylogenetic context are lacking.

Characterization of Atrazine-Induced Gonadal Malformations in African Clawed Frogs (Xenopus laevis) and Comparisons with Effects of an Androgen Antagonist (Cyproterone Acetate) and Exogenous Estrogen (17β-Estradiol): Support for the Demasculinization/Feminization Hypothesis

“…Oocytes are not observed in females even at NF stage 66 under the conditions used in our laboratory. In addition to the animals examined here, this characterization is based on more than 10,000 observations over 10 years of study in our laboratory and consistent with multiple reports from the literature dating back to 1953 (Gallien 1953; Hayes 1998, 2005b). In the present study, analysis of untreated controls and all solvent-only controls revealed that none differed from the expected sex ratio 50:50:0, male:female:malformed gonads ( p > 0.5 in all cases).…”

“…As described previously (Hayes 1998), “primary sex differentiation” is the development of the undifferentiated (sometimes called “indifferent” or “bipotential”) gonads into testes (male) or ovaries (female). Although used to mean many different things in common language, a true hermaphrodite (after the Greek “ Hermaphroditus,” the child of the mythical Hermes and Aphrodite who bore both male and female sex organs) is a single organism bearing both testicular and ovarian tissue.…”

“…Exposure to estrogens throughout the larval period results in 100% females in X. laevis (produces ovaries in genetic males) (Chang and Witschi 1955), but female sex differentiation is not altered by exogenous sex steroids in X. laevis : exogenous androgens induce testicular development in genetic females of other anuran species (Hayes 1998), but androgens do not affect primary sex differentiation in genetic female X. laevis larvae (Hayes 1998). Given the immutability of ovarian differentiation in female X. laevis larvae, we suggest that the hermaphrodites represent males that have been partially feminized rather than females that have been masculinized.…”

“…Although it is well known that estrogens induce 100% females in X. laevis (Chang and Witschi 1955; Gallien 1953; Hayes 1998), the concentration and timing of exposure are important for complete sex reversal of all exposed individuals. We treated larvae with E 2 (100 μg/L) for 7 days (NF stages 50–53), 14 days (NF stages 50–55), or 49 days (NF stages 50–66) (Figure 2).…”

“…Estrogens feminize exposed male amphibians in multiple species (Chang and Witschi, 1955; Hayes 1998; Richards and Nace 1978; Villapando and Merchant-Larios 1990), including the species feminized by atrazine in the laboratory studies described above ( X. laevis and R. pipiens ). Although atrazine has a negligible affinity for the estrogen receptor (Roberge et al 2004), many studies have shown that atrazine exposure elevates estrogens in every species (and class of vertebrate) examined (see “Discussion”).…”