Correlation among Thermosensitive Period, Estradiol Response, and Gonad … (original) (raw)
Journal of Experimental Zoology Part A: Ecological Genetics and Physiology, 2009
A brief review of our current understanding (or lack of understanding) of the molecular basis of temperature-dependent sex determination (TSD) in reptiles is presented. Current theories are discussed: yolk steroids as sex determinants, the brain as the driver for TSD and the enzyme aromatase and estrogen production as the possible determinants of sex. There is little evidence to support the first two theories, but enough evidence to keep the third theory in play. As yet, however, we have no molecular understanding of how a two-degree difference in temperature during the temperature-sensitive phase of egg incubation can initiate the molecular cascade that determines whether the indifferent gonad develops as an ovary or a testis.
The Journal of Experimental Zoology, 1998
Although temperature sex determination is well known in several reptile species, the physiological mechanism underlying this process remains to be elucidated. In the current work, we analyzed the levels of testosterone (T) and estradiol (E2) in the gonads; two brain regions-telencephalon (Te) and diencephalon/mesencephalon (Di)-and the serum of developing embryos of the olive ridley Lepidochelys olivacea incubated at male-or female-promoting temperatures. Conversion of pregnenolone (P5) to T and T to E2 were studied in the gonads and brain. The analyses were performed during three periods: the thermosensitive period (TSP), histologically undifferentiated gonads (UDG), and differentiated gonads (DG). In the gonads, serum, and brain, T concentrations were higher at the female-promoting temperature during the three periods, whereas in the gonads and serum, E2 levels were similar at the female and male-promoting temperature. In Di, the concentration of E2 was significantly higher at the female-promoting temperature. Biotransformation of P5 to T in gonadal tissues were slightly higher at the femalepromoting temperature in TSP and increased during UDG and DG. Conversion of T to E2, however, was similar at the two temperatures during the three periods. In the brain, the Di showed a higher efficiency for transforming T to E2 at the female-promoting temperature. Our present results do not allow us to decide whether the diencephalon is the cause or the effect, but they conclusively demonstrate that, in L. olivacea, this region of the brain senses temperature during sex determination.
General and comparative …, 1996
triol treatment also resulted in cranially hypertrophied In many turtles the temperature during the middle of oviducts at all incubation temperatures in a dose-depenincubation determines the gonadal sex of the hatchling. dent manner, whereas animals treated with estradiol-Sex steroid hormones have been implicated in tempera-17b and estrone had normal oviducts. These results supture-dependent sex determination in the red-eared slider port the hypothesis that estrogens are involved in the turtle, Trachemys scripta; nonaromatizable androgens final common pathway of female sex determination in are involved in male sex determination and estrogens this species. ᭧ 1996 Academic Press, Inc. and aromatizable androgens in female sex determination. Administration of exogenous estradiol-17b to eggs incubating at a temperature that normally produces only In many reptiles gonadal sex is determined by the males can overcome the effect of temperature and result temperature of the incubating egg, a process known as in all offspring being female. Further, estradiol-17b and temperature-dependent sex determination (TSD). In the incubation temperature synergize to produce a greater red-eared slider turtle (Trachemys scripta), incubation of feminizing effect at intermediate incubation temperaeggs at relatively low temperatures (e.g., 20 -28.6Њ) retures that produce mixed sex ratios. This study demonsults in only male hatchlings, whereas relatively high strates that, in the red-eared slider, there is a complex temperatures (e.g., 29.6 -35Њ) results in only female interaction between incubation temperature, different hatchlings; when eggs are incubated at temperatures estrogens, and the dosage effect of each hormone. There intermediate to these, varying sex ratios are produced are changes in potency of different estrogens with incu- . Sex steroid hormones appear to be bation temperature such that estriol is more potent than the physiological equivalent of incubation temperature estrone and estradiol-17b at 26Њ (an all-male producing and both male-and female-producing incubation temincubation temperature), estrone and estriol are equipoperatures and exogenous steroids exert their effects tent to each other and more potent than estradiol-17b at during the mid-trimester of development (Crews et al., . Estrogens and aromatizable biased sex ratio), and estradiol-17b is more potent than androgens induce female sex determination, whereas estrone and estriol at 29Њ (an incubation temperature nonaromatizable androgens induce male sex determithat produced equal numbers of males and females).
Differentiation, 1993
Gonadal differentiation associated with estrogen-induced female sex determination was examined in a turtle with temperature-dependent sex determination, and was compared to ovarian differentiation at a female-producing temperature. Freshly ladi eggs of the red-eared slider, Trachemys scripta, were incubated at a male-producing temperature (26 degrees C) and were experimentally manipulated at one of three embryonic stages: stage 15, 17, or 20 (i.e. early, midway, or late in the temperature-sensitive and estrogen-sensitive periods). At those developmental stages, groups of eggs were either: (1) treated with a control solution (95% ethanol) and placed back at the male-producing temperature, (2) treated with 10 micrograms of estradiol-17 beta and placed back at the male-producing temperature, or (3) shifted to a female-producing temperature (31 degrees C). Additionally, a control group of freshly laid eggs was continually incubated at 31 degrees C throughout embryonic development. To examine morphological events occurring after the treatments, a subset of embryos from each group was examined at the time of the treatment and at 1-2 stage intervals following the treatments. The results indicate that estradiol-17 beta as well as female-producing temperature may ensure female sex determination by facilitating medullary cord regression. Further, the results reveal a chronology of differentiation in which medullary cord regression temporally precedes cortical proliferation.
Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 2006
Dmrt1 has been implicated as an important factor in sex determination in all classes of vertebrates, including reptiles with temperature-dependent sex determination (TSD). Specifically, early embryonic expression of Dmrt1 appears to be an integral part of normal testicular development in vertebrates. Recently, a number of TSD studies have placed Dmrt1 expression at the top of a short list of putative temperature-sensitive events for TSD. Dmrt1 expression has been shown to be up-regulated at male-producing temperatures during the thermosensitive period (TSP) of several TSD reptile species. An interesting finding in Dmrt1 studies of fish and amphibians has been that in species where exogenous steroids can stimulate sex reversal, Dmrt1 expression can also be manipulated by these steroid treatments. In the current study, we examine the effects of exogenous 17b-estradiol treatment on Dmrt1 expression at male-producing temperature (261C) in a reptile with TSD, the red-eared slider turtle, Trachemys scripta. Previous studies have demonstrated that exogenous estrogens can stimulate sex reversal (at male-producing temperatures) in this species of turtle. In the current study, T. scripta embryos that received estradiol treatment displayed a significant decrease in Dmrt1 expression during the TSP, while embryos in the control group (no estradiol treatment) showed significant increases in Dmrt1 expression during the TSP. Furthermore, the pattern of Dmrt1 expression for the estradiol-treated group was similar to the pattern we previously reported for Dmrt1 at female-producing temperature (311C). These findings indicate that exogenous estrogen is acting at or before the Dmrt1 event in the sex determination/sex differentiation cascade of T. scripta. Further, the results are consistent with the hypothesis that exogenous estrogen could be exerting its feminizing effect by down-regulating the expression of Dmrt1.
Seminars in Cell & Developmental Biology, 2009
The developmental processes underlying gonadal differentiation are conserved across vertebrates, but the triggers initiating these trajectories are extremely variable. The red-eared slider turtle (Trachemys scripta elegans) exhibits temperature-dependent sex determination (TSD), a system where incubation temperature during a temperature-sensitive period of development determines offspring sex. However, gonadal sex is sensitive to both temperature and hormones during this period -particularly estrogen. We present a model for temperature-based differences in aromatase expression as a critical step in ovarian determination. Localized estrogen production facilitates ovarian development while inhibiting male-specific gene expression. At male-producing temperatures aromatase is not upregulated, thereby allowing testis development.
Journal of Morphology, 1995
Temperature-dependent sex determination (TSD), in which the temperature at which an egg incubates determines the sex of the individual, occurs in egg-laying reptiles of three separate orders. Previous studies have shown that the embryonic environment can have effects lasting beyond the period of sex determination. We investigated the relative roles of incubation temperature, exogenous estradiol, and gonadal sex (testis vs. ovary) in the Address reprint requests to David Crews, Department of Zool-Alan Tousignant is now at the Trevor Zoo, Millbrook School, ogy, University of Texas, Austin, TX 78712. Millbrook. N.Y. 12545. o 1995 WILEY-LISS, INC
Ambient temperatures during embryonic development determine gonadal sex in many reptiles. The temperature sensitive period for sex determination has been defined by shifting eggs between femaleand male-producing temperatures in a few species. This phase spans 20-35% of embryogenesis in most species, which makes it difficult to define the mechanisms that transduce temperature into a signal for ovarian versus testicular development. We present an extensive set of studies that define a brief period when high temperature specifies, and then determines, ovarian fate in a northern population of snapping turtles, Chelydra serpentina. We shifted embryos from male to female temperatures, or vice versa, at various stages of development. Gonads in embryos incubated at female temperatures commit to ovarian fate earlier (by stage 18) than gonads in embryos incubated at male temperatures commit to testicular fate (by stages 19-21). In double shift studies, embryos were incubated at a female temperature, exposed to a male temperature for set times, and shifted back to the original temperature, or vice versa. The time required to induce ovarian development ( r 6 days at female temperatures) was much shorter than the time required to induce testicular formation ( 420 days at male temperatures). Differentiation of the gonads at the histological level occurred after the sex-determining period. Nevertheless, we found that a change in temperature rapidly (within 24 h) influenced expression and splicing of WT1 mRNA: the absolute abundance of WT1 mRNA, the relative abundance of þ KTS versus -KTS isoforms, as well as the ratio of þ KTS:-KTS isoforms was higher in gonads at a male versus a female temperature. In conclusion, ovarian fate is more readily determined than testicular fate in snapping turtle embryos. The short sex-determining period in this species (6-8% of embryogenesis) will facilitate studies of molecular mechanisms for specification and determination of gonad fate by temperature.
General and Comparative Endocrinology, 2016
Incubation temperatures experienced by developing embryos exert powerful influences over gonadal sex determination and differentiation in many species. However, the molecular mechanisms controlling these impacts remain largely unknown. We utilize the American alligator to investigate the sensitivity of the reproductive system to thermal signals experienced during development and ask specifically whether individuals of the same sex, yet derived from different incubation temperatures display persistent variation in the expression patterns of sex biased transcripts and plasma sex hormones. Our analysis focuses on assessments of circulating sex steroids and transcript abundance in brain and gonad, two tissues that display sexually dimorphic gene expression and directly contribute to diverse sexually dimorphic phenotypes. Whereas our results identify sexually dimorphic patterns for several target gonadal genes in postnatal alligators, sex linked variation in circulating 17b-estradiol, testosterone, and expression of two brain transcripts (aromatase and gonadotropin releasing hormone) was not observed. Regarding intrasexual variation, we found that AMH transcript abundance in hatchling testes is positively correlated with temperatures experienced during sexual differentiation. We also describe highly variable patterns of gene expression and circulating hormones within each sex that are not explained by the intensity of embryonic incubation temperatures. The magnitude of sexually dimorphic gene expression, however, is directly associated with temperature for SOX9 and AMH, two transcripts with upstream roles in Sertoli cell differentiation. Collectively, our findings regarding temperature linked variation provide new insights regarding the connections between embryonic environment and persistent impacts on sexual differentiation in a reptile species that displays temperature dependent sex determination.
Temperature-dependent sex determination: A mechanistic approach
Journal of Experimental Zoology, 1994
A variety of sex determination systems exists in reptiles, including temperaturedependent sex determination (TSD). While the precise physiological basis of TSD is unknown, numerous past studies have provided a general characterization of this phenomenon. The period of thermosensitivity encompasses the middle one third to one half of embryonic development. It has been shown that in several species of reptiles this period starts prior to (histologically defined) gonadal differentiation and extends to a time when sex specific changes are becoming evident in the gonads. During the thermosensitive period, sex determination is sensitive to both the magnitude and duration of a given incubation temperature. Further, the treatment of developing eggs with specific steroid hormones (in particular, estrogens) has been shown to override the effects of male-producing temperatures and induce female sex determination. This has led to a variety of studies investigating whether the production of estrogens may be naturally involved in TSD. The results from several studies are consistent with the hypothesis that temperature could be affecting endogenous estrogen production by controlling the production of aromatase. However, the results from other studies are not consistent with this hypothesis. Current studies by a variety of research laboratories are now addressing the estrogen hypothesis as well as TSD in general at various physiological and molecular levels. As such, these studies should provide a wealth of data on the physiological basis of TSD.