Embryonic brain-gonadal axis in temperature-dependent sex determination of reptiles: A role for P450 aromatase (CYP19)

Aromatase activity during embryogenesis in the brain and adrenal-kidney-gonad of the red-eared slider turtle, a species with temperature-dependent sex determination

Gonadal sex in the red-eared slider turtle is determined by the incubation temperature that the embryo experiences during the mid-trimester of development. High temperatures result in female-biased sex ratios, and low temperatures produce male-biased sex ratios. The physiological equivalent of temperature appears to be a combination of the nature and abundance of steroidogenic enzymes and their products-including estradiol and its precursor, testosterone-and aromatase, the enzyme that converts testosterone to estradiol. Aromatase has been hypothesized to play a major role in the female developmental pathway in this species, and research in other species with temperature-dependent sex determination points to the brain as an organ that transduces the temperature signal into an aromatase response. In this study, we used a tritiated water assay to compare the pattern of estradiol biosynthesis at male- and female-producing temperatures in the brain and adrenal-kidney-gonad (AKG) through development. The pattern for both sexes in the AKG was one of increased activity after the temperature-sensitive period (TSP), but with no significant difference between sexes. In the brain, however, putative females exhibited a significantly higher level of aromatase activity than putative males at the beginning of the TSP, after which activity in both male and female brains decreased, dropping below detection in females before hatch. These results point to the brain as a site of aromatase response to temperature in this species, and they suggest that the product of aromatase activity, estradiol, may induce alterations in the neuroendocrine axis controlling gonadal sex steroid hormone production.

Evidence of temperature-dependent sex determination in the European sea bass (Dicentrarchus labrax L.)

To test the hypothesis that sex determination in the European sea bass (Dicentrarchus labrax L.) Can be affected by the incubating temperature during the very early developmental stages, eggs from the same batch of spontaneously spawned broodstock were divided at the stage of half-epiboly into three groups according to rearing temperature: G13 = 13 degrees C, G15 = 15 degrees C, and G20 = 20 degrees C. Temperature treatment lasted until the middle of metamorphosis (17-18 mm total length, [TL]), and, with the exclusion of water temperature, all biotic and abiotic conditions were identical for the three experimental groups. The on-growing phase was performed under ambient photoperiod and temperature conditions for all groups. Sex proportions were determined by histological examination of the gonads of fish at 308, 467, and 568 days posthatch (DPH). At 308 DPH (TL: 135-201 mm), 100% of the specimens had differentiated into males and females. A significantly higher (P < 0.01) proportion of females was found in groups G13 (72-74%) and G15 (67-73%) than in group G20 (24-28%). At the final sampling there was no statistically significant difference in body weight between the experimental groups. However, in all groups, female fish were larger than males (P < 0.001). Results provide for the first time clear evidence that temperature during the very early developmental stages is the crucial factor affecting the process of sex differentiation of the sea bass, with low rearing temperatures (13 or 15 degrees C) resulting in sex proportions consistently skewed in favor of females.

Developmental expression of steroidogenic factor 1 in a turtle with temperature-dependent sex determination

A variety of reptiles possess temperature-dependent sex determination (TSD) in which the incubation temperature of a developing egg determines the gonadal sex. Current evidence suggests that temperature signals may be transduced into steroid hormone signals with estrogens directing ovarian differentiation. Steroidogenic factor 1 (SF-1) is one component of interest because it regulates the expression of steroidogenic enzymes in mammals and is differentially expressed during development of testis and ovary. Northern blot analysis of SF-1 in developing tissues of the red-eared slider turtle (Trachemys scripta), a TSD species, detected a single primary SF-1 transcript of approximately 5.8 kb across all stages of development examined. Analysis by in situ hybridization indicated nearly equivalent SF-1 expression in early, bipotential gonads at male (26 degrees C)- and female (31 degrees C)-producing incubation temperatures. In subsequent stages, as gonadal sex first becomes histologically distinguishable during the temperature-sensitive period, SF-1 expression increased in gonads at a male-producing temperature and decreased at a female-producing temperature, suggesting a role for SF-1 in the sex differentiation pathway. SF-1 message was also found in adrenal and in the periventricular region of the preoptic area and diencephalon, but there was no apparent sex bias in these tissues at any stage examined. The overall developmental pattern of SF-1 mRNA expression in T. scripta appears to parallel that found in mammals, indicating possible homologous functions.

Temperature-dependent sex determination in the red-eared slider turtle, Trachemys scripta

Temperature-dependent sex determination (TSD) in the red-eared slider turtle, Trachemys scripta, has been the subject of a variety of past studies. Incubation temperature appears to affect sex determination in a dose-dependent fashion. This suggests that temperature could be affecting a dosage-sensitive element in the sex-determination cascade. Sex determination in T. scripta is sensitive to estrogen, and data from many studies support the hypothesis that endogenous estrogen production may be involved in female sex determination. However, this hypothesis has not yet been evaluated through aromatase expression studies in this species. Several recent studies have cloned cDNAs for genes that could be involved in sex determination and/or sex differentiation. The cDNAs for SF-1 and MIS have been cloned in T. scripta, indicating that these may represent conserved elements in the sex-determination/sex-differentiation cascade of reptiles. The SOX9 cDNA also has been cloned in T. scripta (Spotila et al., '98), and it shows a sex-specific expression pattern. Future studies targeted at aromatase expression as well as the expression of factors such as SOX9, SF-1, and MIS will begin to provide a more comprehensive picture of the events involved in TSD in T. scripta. Further, such studies could help pinpoint the temperature-sensitive element(s).