Temperature Sex Determination Research Articles

TEMPERATURE-DEPENDENT TURNOVERS IN SEX-DETERMINATION MECHANISMS: A QUANTITATIVE MODEL

Sex determination is often seen as a dichotomous process: individual sex is assumed to be determined either by genetic (genotypic sex determination, GSD) or by environmental factors (environmental sex determination, ESD), most often temperature (temperature sex determination, TSD). We endorse an alternative view, which sees GSD and TSD as the ends of a continuum. Both effects interact a priori, because temperature can affect gene expression at any step along the sex-determination cascade. We propose to define sex-determination systems at the population- (rather than individual) level, via the proportion of variance in phenotypic sex stemming from genetic versus environmental factors, and we formalize this concept in a quantitative-genetics framework. Sex is seen as a threshold trait underlain by a liability factor, and reaction norms allow modeling interactions between genotypic and temperature effects (seen as the necessary consequences of thermodynamic constraints on the underlying physiological processes). As this formalization shows, temperature changes (due to e.g., climatic changes or range expansions) are expected to provoke turnovers in sex-determination mechanisms, by inducing large-scale sex reversal and thereby sex-ratio selection for alternative sex-determining genes. The frequency of turnovers and prevalence of homomorphic sex chromosomes in cold-blooded vertebrates might thus directly relate to the temperature dependence in sex-determination mechanisms.

Genetic determination and temperature effects on turbot Scophthalmusmaximus sex differentiation: An investigation using steroid sex-inverted males and females

The relative roles of genetic (GSD) and temperature sex determination (TSD) were investigated in turbot, Scophthalmusmaximus, with the objective of producing all-female stocks for aquaculture. Androgen treated males (ATM) and oestrogen treated females (OTF) were produced after testing various 17α-methyltestosterone or 17β-œstradiol treatments delivered orally (3 or 5 mg steroids/kg food) from 35 days post-fertilization (dpf) during either 500 or 700 degree-days respectively. GSD was analyzed by checking the sex ratio of families (n=33) obtained by crossing control parents with other control parents, either ATM or OTF. TSD was investigated on 8 families reared at three temperatures (15 °C, 17 °C or 24 °C) from weaning to 100 dpf.All the steroid treatments were efficient in producing either 100% male or 100% female populations demonstrating the production of androgen sex-inversed males and oestrogen sex-inversed females. The results of the GSD analysis suggested that most of the data fit to a ZZ/ZW model of sex determination. However, some crosses deviated from this model and other minor factors may be involved. Interestingly, minor effects of temperature and a limited but significant interaction between family and temperature have been observed. In conclusion, the present data suggest that a major ZZ/ZW genetic model is involved in turbot sex determination. Unexpected sex ratios deviating from this model are discussed, including a possible minor effect of environmental factors, such as temperature.

Protracted rainfall decreases temperature within leatherback turtle ( Dermochelys coriacea) clutches in Grenada, West Indies: Ecological implications for a species displaying temperature dependent sex determination

Protracted or intense rainfall may affect the reproductive success of reptilian species on a number of levels ranging from the availability of prey, the integrity of the nesting site and the subsequent survivability of offspring. For sea turtles (a species displaying temperature sex determination) nesting throughout the tropics and subtropics, rainfall has previously been shown to influence the development environment of clutches; in its extreme resulting in high levels of egg or hatchling mortality. Yet when compared to other abiotic variables affecting clutch success, rainfall has received relatively little attention. We therefore examined how fluctuations in local rainfall at a tropical nesting site for leatherback turtles ( Dermochelys coriacea) affected the nest environment. Temperature data loggers placed within clutches ( n = 8) revealed that protracted rainfall had a marked cooling effect on nests, so that seasonally improbable male-producing temperatures (< 29.75 °C) were produced. We use these data to explore how rainfall may ultimately influence the sex ratios of sea turtle hatchlings both within and between nesting seasons, and discuss the importance of robust estimates of rainfall for future demographic models.

Genetic and temperature sex determination in the turbot Scophthalmus maximus

Genetic (GSD) and temperature sex determination (TSD) have been investigated in the turbot, Scophthalmus maximus using sex inversed males (androgen treated males=ATM) and females (oestradiol treated female-s= OTF). ATM and OTF have been produced through feeding treatments using 17[alpha]-methyltestosterone and 17[beta]-oestradiol at a dosage of 3 or 5 mg/kg food starting from 35 days post fertilization (dpf) during periods of 500 or 700° days. 16 ATM and 6 OTF were mated with normal females or males. The 22 resulting progenies and 11 control families were reared at 17°C. For TSD evaluation, parts of 8 of these 33 families (2 from ATM and 6 from controls) have been reared at 15°C, 17°C or 24°C from 35 dpf to 100 dpf. Then all progenies (GSD and TSD) have been reared at 17°C until 11 months old (100 g body weight) for the sex ratio assessment. For 27 progenies reared at 17°C, sex ratios perfectly fit in with the hypothesis of a male homogamety ZZ/ZW system (X-square test; P<0.01). However, 2 ATM, 2 OTF and 2 control progenies presented skewed sex ratios with an excess of males (65-80%). Finally, 24 C treatments significantly affected 3 out of the 8 progenies evaluated for TSD in decreasing the rate of males. It is concluded that in turbot sex is mainly determined by a pair of sex chromosome with a female heterogamety (ZZ/ZW) but sex ratios may also be modulated by parental factors, high temperature and possible temperature/genotype interactions. This project was partly supported by France Turbot and the EU in the PL 97-3796 FAIR Project. (Resume d'auteur)

Female philopatry in a heterogeneous environment: ordinary conditions leading to extraordinary ESS sex ratios

BackgroundWe use a simulation-based model to study the impact of female philopatry and heterogeneity of habitat quality on the evolution of primary sex ratio.ResultsWe show that these conditions may lead to strongly biased ESS habitat-dependent sex ratios, under two kinds of density-dependent population regulation. ESS sex ratios are always biased towards females in good habitats, towards males in poor habitats, and are generally equilibrated considering the whole population. Noticeably, the predicted bias of sex ratio usually increases with decreasing female philopatry.ConclusionThe selection forces responsible for these results are fully described. This study provides a new perspective on the evolutionary significance of temperature sex determination. We discuss the case of turtles by comparing our theoretical results with field observations.

Gonadotropin-releasing hormone neuronal development during the sensitive period of temperature sex determination in the pejerrey fish, Odontesthes bonariensis

The development of gonadotroping-releasing hormone (GnRH) neurons was studied in relation to the sensitive period of thermolabile sex determination in the pejerrey Odontesthes bonariensis, an atherinid fish from South America. Fish were raised from hatching at three different temperatures: 17 °C (100% females), 24 °C (70% females), and 29 °C (100% males). Three groups of immunoreactive GnRH (ir-GnRH) neurons were identified at the terminal nerve ganglion (TNG), the midbrain tegmentum (MT), and the preoptic area (POA). Immnoreactive GnRH (ir-GnRH) neurons were identified in the TNG at hatching (day 0) and in the MT at day 3 at all the experimental temperatures. In the POA ir-GnRH neurons were identified in the nucleus preopticus periventricularis simultaneously with the first appearance of ir-GnRH fibers in the pituitary on days 11, 14, and 17 for larvae kept at 29, 24, and 17 °C, respectively. The number of ir-GnRH neurons in the TNG did not show any statistical difference between temperatures. The number of ir-GnRH neurons in the MT increased in number during the experiment for larvae kept at 17 and 24 °C but decreased between days 17 and 31 in larvae kept at 29 °C. The number of ir-GnRH neurons in the POA increased during development with a peak at day 28 for all temperatures studied and the magnitude of this peak showed a correlation with incubation temperature. These results reinforce the notion that the hypothalamus–pituitary–gonadal axis is active during sex determination in pejerrey suggesting a possible role of the central nervous system and GnRH in this process. It is also suggested that GnRH neurons located in the preoptic area might be the physiological transducers of temperature during the temperature sensitive period in this species.

Expression profiles of Dax1, Dmrt1, and Sox9 during temperature sex determination in gonads of the sea turtle Lepidochelys olivacea

Sex determination is controlled either by genetic or environmental factors. In mammals Sry initiates determination but no homologue of this gene exists in non-mammalian species. Other genes of the mammalian sex-determining pathway have been identified in gonads of different vertebrates. Sox9, Dax1, and Dmrt1 are expressed at the onset of gonadal development in birds and reptiles. In the sea turtle Lepidochelys olivacea, a species with temperature sex determination (TSD), Sox9 is expressed in undifferentiated gonads at male- (MPT) or female-promoting temperatures (FPT). At MPT, Sox9 remains expressed in male gonads, but at FPT it is downregulated coinciding with the onset of the ovarian morphologic differentiation and female sex determination. At MPT however, male sex is determined early than at FPT in still undifferentiated gonads suggesting that other genes maintain Sox9 expression in testis. Here we used RT-PCR to study the expression profiles of Dax1, Dmrt1, and Sox9 in gonads of embryos of L. olivacea incubated at MPT or at FPT. The profiles were correlated with sex determination during and after the temperature-sensitive period (TSP). Dax1 maintained similar levels at both temperatures during the TSP. The Dax1 expression level increased significantly in ovaries compared to testes at stage 27, once they were morphologically distinct. The expression levels of Dmrt1 were higher at MPT than at FPT at all stages, in contrast with Sox9 levels which were similar at both temperatures at stages 23–25. Together, current results suggest that, whereas Dax1 is not involved in TSD in L. olivacea, upregulation of Dmrt1 and downregulation of Sox9 may play a role in male and female sex determination, respectively.

Temperature sex determination in the European sea bass, Dicentrarchus labrax (L., 1758) (Teleostei, Perciformes, Moronidae): critical sensitive ontogenetic phase.

The temperature sex determination (TSD) mechanism in the European sea bass (Dicentrarchus labrax L.) was studied in respect to: a) the TSD sensitivity during the different developmental stages; and b) the intrapopulation correlation of sex determination with the growth rate up to the end of the TSD-sensitive period. At the stage of half-epiboly, eggs from the same batch were divided into four groups and subjected to different thermal treatments: a) 15 degrees C (G15 group) and b) 20 degrees C (G20 group) up to the middle of metamorphosis stage; c) 15 degrees C up to the end of yolk-sac larval stage and subsequently to 20 degrees C (G15-5 group); and d) 15 degrees C up to the end of the preflexion stage and then to 20 degrees C (G15-10 group). At the end of the treatments, size grading was applied and four additional populations were established from the upper (L) and lower (S) size portions of the G15 and G20 populations: G15L, G15S, G20L, and G20S. During the following growing phase, all populations were subjected to common rearing conditions. The sex ratios of each population were macroscopically determined at 190-210 mm mean total length. Female incidence was significantly affected (P < 0.05) by the different thermal treatments: 66.1% in the G15, 47.1% in the G15-10, 37.6% in the G15-5, and 18.1% in the G20 group. In addition, sex ratio was correlated with the growth rate of the fish up to the end of the TSD-sensitive period, with the larger fish presenting a significantly higher (P < 0.01) female incidence than the smaller fish in both thermal regimes tested: 73.1% in G15L vs. 57% in G15S, and 36.6% in G20L vs. 22.5% in G20S group. Results provide, for the first time, clear evidence that the sea bass is sensitive to TSD during all different ontogenetic stages up to metamorphosis, and that sex ratio is correlated with the growth rate of the fish well before the differentiation and maturation of the gonads.

A large sample of Leposoma (Squamata, Gymnophthalmidae) from the atlantic forests of Bahia, the status of Leposoma annectans Ruibal, 1952, and notes on conservation

Leposoma annectans Ruibal, 1952 is resurrected from synonymy with Leposoma scincoides (Spix, 1825). Both were obtained syntopically, along with new specimens of Leposoma nanodactylus, from remaining Atlantic forests of the State of Bahia. Leposoma annectans differs from Leposoma scincoides by having a posteriorly enlarged interparietal with slightly concave and divergent lateral margins; parietals as long as large and shorter than interparietal; by the presence of a small scale separating third pair of chinshiels from infralabials; by presenting a large and almost squarish third supraocular; by the presence of sharp striations in the ventral part of head; and, by its shorter body size and relative tail length. Additionally it differs from L. scincoides by having a lower number of dorsal and ventral scales, fewer scales around body, fewer fourth finger and toe infradigital lamellae and, a lower total number of pores. The highly skewed sex-ratio towards males in large samples of Leposoma annectans and L. scincoides suggests that a temperature sex determination mechanism (TSD) could be the mode of sex determination in these species. Populations of reptiles with TSD mechanisms may be more prone to extinction due to skewed sex ratios caused by increased temperatures in disturbed tropical forests.

Timing of SOX9 downregulation and female sex determination in gonads of the sea turtle Lepidochelys olivacea.

The SRY-related gene SOX9 is involved in the differentiation of Sertoli cells in male gonads of vertebrates with different kinds of sex determination. In the olive ridley Lepidochelys olivacea, a species with temperature sex determination (TSD), the SOX9 protein is expressed at stages 21-24 in medullary cells in gonads of embryos incubated at both male-(MPT) or female-promoting temperatures (FPT). However, at FPT the expression of SOX9 protein decreases at stage 25 and disappears at stage 26, suggesting this as the critical period for SOX9 regulation by temperature. Here, we used reverse transcriptase polymerase chain reaction (RT-PCR) to detect SOX9 transcripts in gonads of embryos switched from MPT to FPT at stage 23 and sampled at days 6-14. Simultaneously, groups of embryos were switched back to MPT and gonadal sex was established. SOX9 transcripts were detected at days 6-12 of switching, when embryos reached stage 25 and were no longer detected at day 14, when the embryos were at stage 26. Embryos switched back to MPT at days 6 or 8 formed testes, whereas embryos switched at days 10 or 14 developed ovaries. Results suggest that at MPT the male sex-determining pathway that maintains SOX9 expression in male gonads is established at stage 24. In contrast, at FPT, the female sex-determining pathway involved in downregulation of SOX9 in female gonads occurs within two days at stage 25. J. Exp. Zool. 290:498-503, 2001.

Temperature Sex Determination in Reptiles: The Third Strategy.

Considering gamete distribution of sex heteromorphic chromosomes, three groups of vertebrates can be distinguished: i. Heterogametic males with X:Y chromosome pairs; ii. Heterogametic females with Z:W chromosome pairs and, iii. Species without heteromorphic sex chromosomes. Whereas in species with heteromorphic sex chromosomes, sex is determined at fertilization, in the other species sex determination occurs later. Among this group, there are species where temperature may act as sex-determining factor. In turtles, higher temperatures are female-promoting whereas in crocodiles they are male-promoting. Regulation of steroid hormones levels by temperature is suggested as the physiological mechanism of sex determination. Several genes related to sex determination in mammals and birds are also expressed in gonads of reptiles with temperature sex determination (TSD). However, the diverse sequence and timing of their expression indicates that different genetic pathways of sex determination have evolved. Determination of Sertoli cells is a critical stage in developing gonads of vertebrates for sex determination. Sox9 is a Sry-related gene implicated in differentiation of Sertoli cells in reptiles, birds and mammalian embryos. In the present review, Sox9 expression in the sea turtle Lepidochelys olivacea, a species with TSD, is compared with Sox9 expression in other vertebrate species with temperature or genetic sex determination.

Temperature Regulates SOX9 Expression in Cultured Gonads of Lepidochelys olivacea, a Species with Temperature Sex Determination

Although sex determination starts in the gonads, this may not be the case for species with temperature sex determination (TSD). Since temperature affects the whole embryo, extragonadal thermosensitive cells may produce factors that induce gonadal sex determination as a secondary event. To establish if gonads of a species with TSD respond directly to temperature, pairs of gonads were cultured, one at female-promoting temperature (FPT) and the contralateral at male-promoting temperature (MPT). Histological and immunohistochemical detection of SOX9 revealed that the response to temperature of isolated gonads was similar to that of the gonads of whole embryos. While gonads cultured at MPT maintained SOX9 expression, it was downregulated in gonads at FPT. Downregulation of SOX9 took longer in gonads cultured at stage 23 than in gonads cultured at stage 24, suggesting that a developmental clock was already established at the onset of culture. To find out if sex commitment occurs in vitro, gonads were switched from FPT to MPT at different days. Results showed that the ovarian pathway was established after 4 days of culture. The present demonstration that gonads have an autonomous temperature detector that regulates SOX9 expression provides a useful starting point from which the molecular pathways underlying TSD can be elucidated.

Environment and sex determination in farmed fish

A certain plasticity of gonadal sex differentiation was reported in the 1930s following exogenous steroid treatments in fish, but demonstration that some environmental factors (temperature, pH, density and social interactions) can influence the sex ratio in gonochoristic species, is relatively recent. In fish, as in reptiles and amphibians displaying environmental sex determination the main environmental factor influencing sex seems to be temperature (TSD = Temperature Sex Determination). In most of the thermosensitive species (some Atherinids, Poecilids, Cichlids, the tilapias, the goldfish, a Siluriform, a flatfish,...) the male to female ratio increases with temperature, and/or ovarian differentiation is induced by low temperatures. Conversely, in some rare species (Dicentrarchus labrax, Ictalurus punctatus), high temperatures may produce female-biased sex ratios and/or low temperatures promote male-biased sex ratios. In the case of the hirame Paralichthys olivaceus, both high and low temperatures induce monosex inale populations while intermediate temperatures yield a 1 :I sex ratio (U-shape curve). Fish show particularities in their TSD patterns since monosex populations are generally not produced at extreme temperatures, suggesting the existence of strong temperature/genotype interactions. In reptiles, amphibians as well as fish displaying TSD, temperature treatments must be applied at a critical sensitive period which is relatively similar to the hormone-sensitive period. In gonochoristic fish, steroid hormones with estrogens in females and II-oxygenated androgens in males, are probably key physiological steps in the regulation of the gonadal sex differentiation. Cytochrome P450-aromatase, an enzyme which catalyses the conversion of androgens to estrogens, seems to be a critical enzyme for ovarian differentiation. The molecular mechanisms of thermosensitivity have been addressed in only two species. In both the tilapia, Oreochromis niloticus and the hirame, aromatase gene expression is down-regulated by masculinizing-temperature treatments. Furthermore, in tilapia the gene expression of II [beta]-hydroxylase (a key enzyme involved in the synthesis of II-oxygenated androgens) does not appear to be affected by temperature treatments. (Texte integral)

Response of diencephalon but not the gonad to female-promoting temperature with elevated estradiol levels in the sea turtle Lepidochelys olivacea.

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 female-promoting 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.

An experimental demonstration of Fisher's principle: evolution of sexual proportion by natural selection.

Most sexually reproducing species have sexual proportions around 1:1. This major biological phenomenon remained unexplained until 1930, when FISHER proposed that it results from a mechanism of natural selection. Here we report the first experimental test of his model that obeys all its assumptions. We used a naturally occurring X-Y meiotic drive system--the sex-ratio trait of Drosophila mediopunctat--to generate female-biased experimental populations. As predicted by FISHER, these populations evolved toward equal sex proportions due to natural selection, by accumulation of autosomal alleles that direct the parental reproductive effort toward the rare sex. Classical Fisherian evolution is a rather slow mechanism: despite a very large amount of genetic variability, the experimental populations evolved from 16% of males to 32% of males in 49 generations and would take 330 generations (29 years) to reach 49%. This slowness has important implications for species potentially endangered by skewed sexual proportions, such as reptiles with temperature sex determination.

Correlation among Thermosensitive Period, Estradiol Response, and Gonad Differentiation in the Sea TurtleLepidochelys olivacea

Reptile embryos with temperature sex determination have a thermosensitive period (TSP). The finding that exogenous estradiol (E2) overcomes the effect of male-promoting temperature led to the idea that temperature may regulate estrogen concentration in the gonad during TSP. Since interspecific variations in TSP and in the effect of exogenous E2 exist, we undertook a study in the olive ridleyLepidochelys olivacea.Four parameters were correlated: the TSP (time dimension), the thermosensitive stages (rate of development), gonad development (histological aspect), and the estradiol response. Two kinds of experiments were performed: (1) Eggs were shifted once, at different stages of development, from a male-promoting temperature to a female-temperature (or vice versa) for the remainder of development. (2) Eggs at male-promoting temperature were treated once with 6 or 12 μg of estradiol (E2) at various times of incubation. Sex ratio was established around hatching in each experimental series. We found that the temporal dimension of the TSP was around 7 days (Days 20–27 of incubation) at a male-promoting or a female-promoting temperature. The rate of development of the whole embryo and gonadal growth was faster at female-promoting temperature than at male-promoting temperature. Formation of the genital ridge began at stage 21–22 and histological differentiation of the gonads occurred around stage 26–27. Although these stages coincided with the TSP, at male-promoting temperature the thermosensitive stages occurred earlier (from stages 20–21 to stages 23–24) than at female-promoting temperature (from stages 23–24 to stages 26–27). Thus, at male promoting-temperature, sex was determined in embryos with incipient or undifferentiated gonads. In contrast, E2 treatment continued to feminize the gonads of embryos at a male-promoting temperature beyond the TSP up to stage 25–26, but the E2-induced ovaries were significantly smaller than temperature-induced ovaries. It is suggested that the doses of E2 used were higher than the concentration of endogenous E2 required for normal sex determination. The lack of correlation between sex determination and gonad differentiation suggests that irreversible molecular processes underlying sex determination occur earlier at male- than at female-promoting temperature. Results suggest that the male sex may be the default state and that the female condition must be imposed upon it.

Environmental sex determination in reptiles

In recent years, representatives of more than 40 families of reptiles have been studied to understand how environmental parameters affect sex determination. In this review, we summarise the distribution and taxonomic pattern of sex determining mechanisms, outline the main hypotheses of the adaptive significance of temperature sex determination (TSD), and of skewed population sex ratios. We also examine the competing hypotheses of the physiological and molecular mechanisms involved in TSD.

Psychophysiological and neuropsychological distinction between familial and sporadic schizophrenia

Bipotential gonads represent the structural framework from which alternative molecular sex determination networks have evolved. Maintenance of Sox9 expression in Sertoli cells is required for the structural and functional integrity of male gonads in mammals and probably in most amniote vertebrates. However, spatial and temporal patterns of Sox9 expression have diversified along evolution. Species with temperature sex determination are an interesting predictive model since one of two alternative developmental outcomes, either ovary or testis occurs under controlled laboratory conditions. In the sea turtle Lepidochelys olivacea, Sox9 is expressed in the medullary cords of bipotential gonads when incubated at both female- or male-promoting temperature (FT or MT). Sox9 is then turned off in presumptive ovaries, while it remains turned on in testes. In the current study, Sox9 was used as a marker of the medullary cell lineage to investigate if the medullary cords originate from mesothelial cells at the genital ridges where Sox9 is upregulated, or, if they derive from a cell population specified at an earlier developmental stage, which maintains Sox9 expression. Using immunofluorescence and in situ hybridization, embryos were analyzed prior to, during and after gonadal sex determination. A T-shaped domain (T-Dom) formed by cytokeratin (CK), N-cadherin (Ncad) and SOX9-expressing cells was found at the upper part of the hindgut dorsal mesentery. The arms of the T-Dom were extended to both sides towards the ventromedial mesonephric ridge before the thickening of the genital ridges, indicating that they contained gonadal epithelial cell precursors. Thereafter, expression of Sox9 was maintained in medullary cords while it was downregulated at the surface epithelium of bipotential gonads in both FT and MT. This result contrasts with observations in mammals and birds, in which Sox9 upregulation starts at a later stage in the inner cells underlying the Sox9-negative surface epithelium, suggesting that the establishment of a self-regulatory Sox9 loop required for Sertoli cell determination has evolved. The T-shaped domain at the upper part of the hindgut dorsal mesentery found in the current study may represent the earliest precursor of the genital ridges, previously unnoticed in amniote vertebrates.

Studies on the phylogenetic conservation of the SRY gene

A probe from a conserved motif of the SRY gene (sex-determining region Y), a prime candidate for the human testis-determinant, was hybridized to DNA from 23 species representing 5 vertebrate classes. Hybridization occurred in species with male or female heterogamety, in species with and without sex chromosomes and in those with temperature sex determination. Sex-specific signals were observed only in mammals. Conservation of sequences homologous with SRY through 400 million years of vertebrate evolution would indicate persistence of function. However, if SRY is the primary sex determinant in mammals, it is not clear that it has a similar function, or even one that is sex-related, in nonmammals.

Sex‐specific DNA in reptiles with temperature sex determination

Banded krait minor ("Bkm") satellite DNA, originating in the W-chromosome of the snake Bungarus fasciatus, has been found in the genome of diverse eukaryotic species including fruit fly, quail, and horse. Concentrations of Bkm have been found in the presumptive W-chromosome of snakes with isomorphic sex chromosomes and in the male-determining region of the Y-chromosome in mouse and man. We therefore asked whether Bkm-related DNA might be present in quantitative excess in DNA from males or females in two related species of sea turtle, Chelonia mydas, in which sex is determined by the temperature of the incubating egg, and Lepidochelys kempi, in which the critical sex-determining temperature has recently been described. Filter hybridization with the Bkm 2(8) probe revealed male-specific fragments in both species; female-specific fragments were also revealed in C. mydas. Sex-specific DNA sequences in temperature-sex-determined species such as Kemp's ridley and the green turtle were unexpected, but could be explained if there were an underlying genetic mode of sex determination in these animals, or alternatively, if temperature-influenced sex determination involved structural modifications in DNA adjacent to, or directly concerned with, the sex-determining genes. If these results are confirmed across a broader sample of sea turtles, the techniques described in this paper might be used routinely to identify gener in the young of these endangered animals, in which male and female are grossly indistinguishable.