Chronology of Gonadal Development in the Malayan Snail-eating Turtle Malayemys macrocephala

Ovotestes suggest cryptic genetic influence in a reptile model for temperature-dependent sex determination

Sex determination and differentiation in reptiles is complex. Temperature-dependent sex determination (TSD), genetic sex determination (GSD) and the interaction of both environmental and genetic cues (sex reversal) can drive the development of sexual phenotypes. The jacky dragon (Amphibolurus muricatus) is an attractive model species for the study of gene-environment interactions because it displays a form of Type II TSD, where female-biased sex ratios are observed at extreme incubation temperatures and approximately 50 : 50 sex ratios occur at intermediate temperatures. This response to temperature has been proposed to occur due to underlying sex determining loci, the influence of which is overridden at extreme temperatures. Thus, sex reversal at extreme temperatures is predicted to produce the female-biased sex ratios observed in A. muricatus. The occurrence of ovotestes during development is a cellular marker of temperature sex reversal in a closely related species Pogona vitticeps. Here, we present the first developmental data for A. muricatus, and show that ovotestes occur at frequencies consistent with a mode of sex determination that is intermediate between GSD and TSD. This is the first evidence suggestive of underlying unidentified sex determining loci in a species that has long been used as a model for TSD.

Pattern of Gonadal Sex Differentiation in the Rice Field Frog Hoplobatrachus rugulosus (Anura: Dicroglossidae)

Sex differentiation during gonadal development is diversified among anuran amphibian species. In this study, the anuran experimental species Hoplobatrachus rugulosus was examined. The pattern of gonadal sex differentiation was observed by morphological and histological approaches. The gonad was observed morphologically at Gosner stage 33, while distinct testis and ovary were evident from 3-4 weeks after metamorphosis ended. Histological analysis showed that genital ridge formation began at stage 25 and ovarian differentiation began at stage 36. The developing ovary appeared with numerous primary oogonia, which developed into oocytes, while the medulla regressed to form an ovarian cavity. During metamorphosis, only an ovary was observed. Testicular differentiation seemed to begin later, during the first week after metamorphosis, and occurred via an intersex condition. The intersex gonads contained developing testicular tissue with both normal and atretic oocytes. The fully developed testis was first identified at 6 weeks after metamorphosis. Comparing the times of gonadal differentiation and somatic development revealed that the ovary exhibited a basic rate of differentiation while the testis exhibited a retarded one. These results establish that males of this species develop later than do females, and the testis develops through an intersex gonad, as is evident from its seminiferous cord formation, the presence of testis-ova, and atretic oocytes in the tissue. Thus, the pattern of gonadal sex differentiation in H. rugulosus is an undifferentiated type, in which only female gonads are observed during metamorphosis and intersex and male gonads are observed later. These results are crucial for further research on the sexual development of anurans.

Estrogen Receptor 1 (ESR1) Agonist Induces Ovarian Differentiation and Aberrant Müllerian Duct Development in the Chinese Soft-shelled Turtle, Pelodiscus sinensi

Estrogens play critical roles in ovarian and reproductive organ development, but the molecular signaling pathways in non-mammalian vertebrates are not well understood. Studies of reptiles have indicated that administration of exogenous estrogens during embryonic development causes ovarian differentiation and presumptive male to female sex-reversal. The Chinese soft-shelled turtle, Pelodiscus sinensis, belongs to the family Trionychidae and exhibits genotypic sex determination system with ZZ/ZW sex chromosomes. In order to assess the role of estrogens and their signaling pathway on sex determination and differentiation, P. sinensis eggs were given a single administration of endogenous estrogen,17β-estradiol (E2) or a synthetic estrogen receptor 1 (ESR1) agonist, 4,4',4"-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol (PPT) in ovo during gonadal differentiation, and the subsequent effects were examined during a final developmental stage prior to hatching. The administration of both E2 and PPT induced ovarian differentiation in genetic male embryos. Intriguingly, PPT but not E2 induced the Müllerian duct enlargement and aberrant glandular development. These data suggest that ovarian differentiation and reproductive tract anomalies induced by the exogenous estrogen exposure act through ESR1 in the Chinese soft-shelled turtles.