Insights into Emydid Turtle Cytogenetics: The European Pond Turtle as a Model Species

Our knowledge of Testudines evolution is limited by the lack of modern cytogenetic data. Compared to other reptiles, there is little information even on chromosome banding, let alone molecular cytogenetic data. Here, we provide detailed information on the karyotype of the European pond turtle Emys orbicularis, a model Emydidae, employing both chromosome banding and molecular cytogenetics. We provide a high-resolution G-banded karyotype and a map of rDNA genes and telomeric sequences using fluorescence in situ hybridization. We test hypotheses of sex-determining mechanisms in Emys by comparative genomic hybridization to determine if Emys has a cryptic sex-specific region. Our results provide valuable data to guide future efforts on genome sequencing and anchoring in Emydidae and for understanding karyotype evolution in Testudines.

Genetic contribution to sex determination in turtles with environmental sex determination

In many reptiles, sex determination is temperature-sensitive. This phenomenon has been shown to take place in the laboratory as well as in nature, but its effect on natural populations remains questionable. In the turtle Emys orbicularis, the effects of temperature override a weak mechanism of genetic sex determination which is revealed in incubation at pivotal temperature. At this temperature, the sexual phenotype is concordant with the expression of the serologically defined H-Y antigen (H-Ys) in non-gonadal tissues; males are H-Ys negative (H-Y−) whereas females are H-Ys positive (H-Y+). To estimate the importance of sexual inversion (sexual phenotype and H-Ys expression discordant) in populations of Brenne (France), the frequencies of male and female sexual phenotypes among H-Ys phenotypes were determined. The frequencies of sex reversed individuals are low, only 6 % of phenotypic females being H-Y− and 11 % of phenotypic males being H-Y+. According to these data, two theoretical models have been constructed to estimate the contribution to sex determination of individuals in relation to their genotype. The first model excludes any influence of incubation temperature and sexual phenotype on the fitness of individuals. The second one considers that these parameters influence fitness because this model has been previously shown to favour environmental sex determination. In both models, it appears that sex determination can be viewed as genotypic and monogenic with some individuals sexually inverted by theaction of temperature. One category of homozygous animals differentiates mainly into one sex, and the heterozygous animals differentiate mainly into the other sex. The second category of homozygotes has a low frequency in the populations and can differentiate as male or female without high constraint. Then it is estimated that in Brenne approximately 83% of the eggs are incubated in conditions allowing the genetic component to influence sex determination.

Molecular mechanisms of TSD in reptiles: a search for the magic bullet

Significant progress has been made in understanding mechanisms of genetic sex determination. The ZFY gene encodes a zinc finger protein but is not the primary signal in sex determination. The SRY gene is the testis determining gene in man, mouse, rabbit, and probably marsupial mouse and wallaby. Temperature dependent sex determination probably involves a modification of development of the indifferent gonad due to differential expression of one or more specific DNA sequences whose behavior is controlled by some temperature sensitive process or to differential action of a gene product such as a protein. There are ZFY and SRY-like genes in reptiles. We cloned and sequenced a portion of the ZFY gene (Zft) from snapping turtle (Chelydra serpentina) that is found in both sexes. We cloned and sequenced portions of SRY-like genes (Sra for SRY-related-autosomal) from snapping turtle. Similar genes are found in alligator (Alligator mississippiensis) and lizards. Cladistic analysis suggests that there are two or three major families of SRY-like genes in vertebrates in addition to sex specific SRY genes located on the Y chromosome of eutherian and marsupial mammals. When placed on a phylogenetic tree these data indicate that Sras were present in early tetrapods. Sequestering of the SRY gene on the Y chromosome probably happened only once and this may have been the defining moment that set the mammalian line of Therapsid reptiles apart from other reptilian groups.

Environmental regulation of sex determination in reptiles

The various patterns of environmental sex determination in squamates, chelonians and crocodilians are described. High temperatures produce males in lizards and crocodiles but females in chelonians. Original experiments on the effects of incubation at 30 degrees C (100% females) or 33 degrees C (100% males) on development in Alligator mississippiensis are described. These include an investigation of the effect of exposing embryos briefly to a different incubation temperature on the sex ratio at hatching, and a study of the effects of 30 degrees C and 33 degrees C on growth and development of alligator embryos and gonads. A 7-day pulse of one temperature on the background of another was insufficient to alter the sex ratio dramatically. Incubation at 33 degrees C increased the rate of growth and development of alligator embryos. In particular, differentiation of the gonad at 33 degrees C was enhanced compared with 30 degrees C. A hypothesis is developed to explain the mechanism of temperature-dependent sex determination (TSD) in crocodilians. The processes of primary sex differentiation are considered to involve exposure to a dose of some male-determining factor during a specific quantum of developmental time during early incubation. The gene that encodes for the male-determining factor is considered to have an optimum temperature (33 degrees C). Any change in the temperature affects the expression of this gene and affects the dose or quantum embryos are exposed to. In these cases there is production of females by default. The phylogenetic implications of TSD for crocodilians, and reptiles in particular, are related to the life history of the animal from conception to sexual maturity. Those animals that develop under optimal conditions grow fastest and largest and become male. A general association between the size of an animal and its sex is proposed for several types of vertebrate.

Temperature-dependent gonadal differentiation in the turtle Emys orbicularis: concordance between sexual phenotype and serological H-Y antigen expression at threshold temperature

As in many other turtles, the sexual differentiation of gonads in embryos of Emys orbicularis is temperature-sensitive, 100% phenotypic males being obtained below 27.5 degrees C and 100% phenotypic females above 29.5 degrees C. The expression of the serologically defined H-Y (SD-H-Y) antigen at both low and high temperatures has been shown to be different in gonads and in blood : in gonads, it is closely associated with ovarian structure, whereas in blood it is independent of sexual phenotype and appears to indicate sexual genotype. Both sexes differentiate at 28.5 degrees C, suggesting that at this intermediate (threshold) temperature, sexual differentiation of gonads conforms with sexual genotype. To test this hypothesis, the expression of SD-H-Y antigen has been carried out in blood cells of Emys individuals raised from eggs incubated at the threshold temperature (28.5 degrees C). All phenotypic males typed SD-H-Y negative, whereas most phenotypic females typed SD-H-Y positive. From this concordance between sexual phenotype of gonads and SD-H-Y phenotype of blood, we postulate that a ZZ male/ZW female mechanism of genotypic sex determination is revealed at the threshold temperature for gonad differentiation in Emys.

On the evolution of sex determination

Female mice reject skin grafts from intrastrain males because of the H-Y transplantation antigen. Those females produce antibodies that recognize a male-specific cell-surface antigen in serological tests. The serological antigen has also been called 'H-Y', but there is evidence that the two antigens are distinct. We therefore refer to the transplantation antigen as H-Yt, or transplantation H-Y, and to the serological antigen as serological H-Y, or simply H-Y, without prejudice whether these are the same or related or separate antigens. In this study, sex-specific expression of serological H-Y antigen was found in 25 new vertebrate species representing each of seven major vertebrate classes. There was a strong correlation between expression of H-Y and occurrence of the heterogametic-type gonad, although unusual patterns of H-Y expression were noted in cases of temperature-influenced sex determination and in systems representing possible transition from one mode of heterogamety to the other. Male and female heterogamety are found side-by-side in certain freshwater toothed carps; and distinct sex chromosomes have been recognized in certain amphibians, even though they are not apparent in certain reptiles and primitive birds. In seven ophidian species, in which the female is the heterogametic sex, H-Y was detected in the female; and in three species of Ranidae in which the male is heterogametic, it was detected in the male. In three species of cartilaginous fish and in one of the cyclostomes, in which heterogamety has not been ascertained, H-Y was detected in the male, suggesting that those primitive fishes are male-heterogametic. Evidently, then, heterogamety and sex-chromosome heteromorphism are polyphyletic, although certain sex-determining genes may be held in common among the diverse taxonomic groups.

The H-Y antigen and its functions: a review and a hypothesis

Having reviewed the status of H-Y as the sex-determining antigen concerned with the differentiation of the dominant gonad, we consider some of the problems deriving from the tests for this antigen, and from their application to the study of natural experiments. To reconcile the results of these studies with the alleged influence of H-Y on gonadal development, we propose and discuss a hypothesis on the genetic control of the synthesis of this antigen. This states that an autosomally-coded, positively cross-reacting precursor is rendered biologically active by a Y-chromosomal gene, and transformed (in a dose-dependent manner) into a biologically inactive, antigenically negative substance under the influence of an X-chromosomal gene.

H-Y antigen expression in temperature sex-reversed turtles (Emys orbicularis)

H-Y antigen has been used as a marker for the heterogametic sex and is assumed to be an organizing factor for the heterogametic gonad. In the turtle Emys orbicularis, H-Y antigen is restricted to the female cells, indicating a female heterogamety (ZZ/ZW) sex-determining mechanism. Moreover, the sexual differentiation of the gonads is temperature sensitive, and complete sex reversal can be obtained at will. In this framework the relationships between H-Y antigen, temperature, and gonadal phenotype were studied. Mouse H-Y antiserum was absorbed with blood and gonadal cells of control wild male and female adults, and with blood and gonadal cells from three lots of young turtles from eggs incubated at 25-26 degrees C (100% phenotypic males), at 30-30.5 degrees C (100% phenotypic females), or at 28.5-29 degrees C (majority of females with some males and intersexes). The residual activity of H-Y antiserum was then estimated using an immunobacterial rosette technique. In adults, both blood cells and gonadal cells were typed as H-Y negative in males and as H-Y positive in females. In each of the three lots of young, blood cells were H-Y negative in some individuals and H-Y positive in others. The proposed interpretation is that the H-Y negative individuals were genotypic males (ZZ) and the H-Y positive were genotypic females (ZW). The gonads of these animals were then pooled in different sets according to their sexual phenotype and to the presumed genotypic sex (i.e., blood H-Y phenotype). Testicular cells were typed as H-Y negative in genotypic males as well as in the presumed sex-reversed genotypic females; likewise, ovarian cells were typed as H-Y positive in genotypic females as well as in the presumed sex-reversed genotypic males. These results provide additional evidence that H-Y antigen expression is closely associated with ovarian structure in vertebrates displaying a ZZ/ZW sex-determining mechanism.