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Temperature-Dependent Sex Determination in Reptiles: Proximate Mechanisms, Ultimate Outcomes, and Practical Applications

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Temperature-Dependent Sex Determination in Reptiles: Proximate Mechanisms, Ultimate Outcomes, and Practical Applications DEVELOPMENTAL GENETICS 15297312 (1994) Temperature-Dependent Sex Determination in Reptiles: Proximate Mechanisms, Ultimate Outcomes, and Practical Applications DAVID CREWS, JUDITH M. BERGERON, JAMES J. BULL, DEBORAH FLORES, ALAN TOUSIGNANT, JAMES K. SKIPPER, AND THANE WIBBELS Institute of Reproductive Bio1og.v and the Department of Zoology, University of Texas at Austin, Austin ABSTRACT In many egg-laying reptiles, INTRODUCTION the incubation temperature of the egg determines Much progress has been made in our understanding the sex of the offspring, a process known as tem- of sex determination and differentiation in a variety of perature-dependent sex determination (TSD). In organisms. Indeed, until about 25 years ago, it was TSD sex determination is an “all or none” process assumed that all amniote vertebrates (mammals, and intersexes are rarely formed. How is the exter- birds, and reptiles) shared similar genotypic sex-deter- nal signal of temperature transduced into a genetic mining (GSD) mechanisms. Then in 1966 Madeline signal that determines gonadal sex and channels Charnier discovered that in the egg-laying lizard Ag- sexual development? Studies with the red-eared arna agama the incubation temperature of the egg de- slider turtle have focused on the physiological, bio- termines the sex of the hatchling. This process of tem- chemical, and molecular cascades initiated by the perature-dependent sex determination (‘ED) has now temperature signal. Both male and female devcl- been demonstrated in many turtles, some lizards, and opment are active processes-rather than the or- all crocodilians [Bull, 1980; Ewert and Nelson, 1991; ganized/defadt system characteristic of verte- Janzen and Paukstis, 19911. brates with genotypic sex determination-that In mammals, birds, and many other gonochoristic require simultaneous activation and suppression of vertebrates (separate sexes in separate individuals), testis- aqd ovary-determining cascades for normal gonadal sex is determined at fertilization by specific sex determination. It appears that temperature ac- chromosomes and results in a 1:1 sex ratio. In mam- complishes this end by acting on genes encoding mals, a gene on the Y chromosome (designated Sry) for steroidogenic enzymes and steroid hormone re- channels gonadal development to result in testes and ceptors and modifying the endocrine microenvi- the individual to develop a male-typical phenotype; the ronment in the embryo. The temperature experi- absence of this gene results in the formation of ovaries enced in development also has long-term and a female-typical phenotype (Fig. 1, top). Thus, the functional outcomes in addition to sex determina- Sry gene serves to switch the developmental cascade tion. Research with the leopard gecko indicates leading to maleness. However, Sry is part of a large that incubation temperature as well as steroid hor- family of genetic transcripts, and comparative studies mones serve as organizers in shaping the adult in a variety of species have not found other Sry-like phenotype, with temperature modulating sex hor- genes to be sex-linked in nonrnammals [Tiersch et al., mone action in sexual differentiation. Finally, prac- 1991, 19921. tical applications of this research have emerged for Unlike GSD, where it is not possible to deviate the the conservation and restoration of endangered primary sex ratio from unity [Beamer and Whitten, egg-laying reptiles as well as the embryonic devel- 19911, species with TSD do not inherit sex-specific opment of reptiles as biomarkers to monitor the chromosomes from their parents, and gonadal sex is estrogenic effects of common environmental contaminants. G 1554 Wiley-Liss, Inc. Key words: Sex determination, sexual differenti- Received for publication October 25, 1993; accepted January 12, 1994. ation, reptiles, temperature-dependent sex deter- mination, behavior, steroidogenic enzymes, aro- Address reprint requests to Dr. David Crcws, Department of Zoology, University of Texas at Austin, Austin, TX 78712. matase, reductase, estrogen, androgen, steroid hormone receptors Dr. Wibbels is now at the Department of Biology, University of Ala- bama at Birmingham, Birmingham, AL 35294. 0 1994 WILEY-LISS, INC. 298 CREWS ET AL. GENOTYPIC SEX DETERMINATION Fertilization Gonad Sexual Gonad Determining +Hormones + Differentiation -aFormation Genes of Phcnotypc TEMPERATURE-DEPENDENT SEX DETERMINATION Fertilization Temperature Gonad Ihn/.yme.! Gonad Sexual 4-b ~Orm~~s+ Determining + + Hormones + Differentiation Receptors Gcnes of Phenotype Fig. 1. In the current model of vertebrate sex determination and not irrevocably set by the genetic composition inherited at fertiliza- sexual differentiation (top), gonadal sex is fixed at fertilization by tion, but rather depends ultimately on which genes encoding for ste- specific chromosomes, a process known as genotypic sex determina- roidogenic enzymes and hormone receptor arc activated during devel- tion. Only after the gonad is formed do hormones begin to exert an opment by temperature. Incubation temperature modifies the activity influence, modifying specific structures that eventually will differ as well as the temporal and spatial sequence of enzymes and hormone between the sexes. Research on reptiles with temperature-dependent receptors such that sex-specific hormone milieus, created in the uro- sex determination indicates that sex determination in these species is genital system of the developing embryo, determine gonad type. fundamentally different in at least one way (bottom). Gonadal sex is not determined at fertilization. In TSD each individual threatened and endangered species and monitoring en- has equal potential to become male or female and there vironmental contamination. is little, or no, genetic predisposition for specific re- sponses to incubation temperature. Rather, incubation temperature serves as a switch that initiates the cas- PROXIMATE MECHANISMS OF cade that leads to the development of testes and sup- TEMPERATURE-DEPENDENT presses the cascade that leads to the development of SEX DETERMINATION ovaries (and vice versa) (Fig. 1, bottom). How is the physical stimulus of temperature trans- Reptiles with TSD may offer model systems to better duced into a physiological signal that ultimately acts understand the events that comprise sex determination on a molecular switch to determine sex? We have used and sexual differentiation in amniote vertebrates. the red-eared slider (Trachemys scriptu) to address this Mammals and birds share their most recent common question. In this species, an incubation temperature of ancestry with reptiles. It has been suggested that en- 26°C produces all males, 31°C produces all females, and vironmental sex determination may have been the pre- 29.2”C is the threshold temperature that produces ap- cursor to GSD [Ohno, 1967; Janzen and Paukstis, proximately a 1:l sex ratio (Fig. 2). The following four 19911. Before such promise can be realized, basic infor- points have been established in this and other species: mation both on how sex is determined as well as how sexuality develops in TSD species is required. Our pur- 1. Sex determination is sensitive to both the dura- pose here is to summarize a set of studies conducted in tion and magnitude of incubation temperature [Wib- this laboratory on two species, the red-eared slider and bels et al., 1991al. the leopard gecko. An obvious starting point is to de- 2. Sex determination remains labile to temperature termine how TSD is achieved. We consider the physi- changes through the early stages of gonadal differen- ological, cellular, and molecular mechanisms of TSD. A tiation and initial sex-specific changes in the gonads second issue concerns the functional outcomes of TSD are reversible [Wibbels et al., 1991a,b]. The tempera- on characteristics other than gonad phenotype. We ture-sensitive period corresponds to the middle of em- demonstrate how the temperature experienced during bryogenesis ( = sex determination period), beginning embryonic development modifies the morphology, prior to the differentiation of the gonads into recogniz- physiology, and behavior of the adult. Finally, we pro- able ovaries or testes but ending well before hatching vide new information on some practical applications of [Wibbels et al., 1991al. this research such as benefiting the conservation of 3. Temperature exerts an “all-or-none” effect on the MECHANISMS, OUTCOMES, AND APPLICATIONS OF TSD 299 '0 %%$@, %%$@, Estradiol Testosterone < ,& D i h ydrote s t 0st e r on e Fig. 3. Biosynthesis of testosterone to dihydrotestosterone (via re- ductase) and estradiol (via arornatase). of other androgens. We assessed the activity of 3a-, 3p-, and 17P-HSDH enzymes before, during, and after the temperature-sensitive period in the red-eared slider turtle. HSDH reaction product was observed in the liver, adrenal gland, and kidney before and during the 0 temperature-sensitive window at either incubation 26 27 28 29 30 31 32 temperature and in the genital ridge during the last Temperature ("C) trimester of development [Thomas et al., 19921. Similar findings have been reported for the green sea turtle Fig. 2. Relationship between incubation temperature and sex ratio (Chelonia mydas) [Merchant-Larios et al., 19891 and in the red-eared slider turtle. Depicted is the mean sex ratio obtained the European pond turtle (Emys orbicularis) [Pieau, over a 4-year period. Total number of individuals followed by the 19731. number of replicate experiments are shown in parentheses; standard
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