Identification of Wnt4 as the ovary pathway gene and temporal disparity of its expression vis-a-vis testis genes in the garden lizard, Calotes versicolor

Conservation of Ovary-Specific Genes, Foxl2, Aromatase, and Rspo1, in the Common Indian Garden Lizard, Calotes versicolor, That Lacks Chromosomal or Temperature-Dependent Sex Determination

Foxl2,Rspo1, and Aromatase are genes important in the ovary developmental pathway in mammals and birds. Here, we show their presence in the lizard, Calotes versicolor, which is known to lack a chromosomal as well as a temperature-dependent mode of sex determination and has an indeterminate, bipotential gonad throughout embryonic development. The expression of the 3 genes, as well as that of CvSox9 and Wnt4 - the known testis and ovary pathway genes - was studied by RT-PCR and whole tissue RNA in situ hybridization (WRISH) on the developing mesonephros gonadal complex (MGC). The expression of all 3 genes was initiated in the gonad shortly after its evagination from the mesonephros (day 5 onwards). CvFoxl2 generally was expressed in those MGCs in which CvSox9 was either not expressed or lowly expressed and vice versa. On the other hand, CvArom was expressed rather sporadically and randomly, showing no association with CvFoxl2, CvRspo1, or CvSox9, though in later stages WRISH preparations showed its coincidence with CvWnt4. CvRspo1 was expressed in almost all embryos right from day 5. Immunofluorescence localization of Rspo1 and Foxl2 proteins showed their presence in the gonads from day 10 onwards, and by day 25 it was primarily confined to the cortex but away from the coelomic epithelium of the gonadal cortex. Apparently both proteins were localized in the pregranulosa cells, Rspo1 in the cytoplasm and Foxl2 in the nucleus. Thus, it is clear that both CvFoxl2 and CvRspo1 are active in ovary formation, but whether they are expressed in the same or different cells is unknown. Though the transcription pattern of CvArom remains circumspect for its role in differentiation of the ovary, earlier evidence on aromatase inhibitor-induced reversal to the male sex indicates its importance in ovary function.

MeDIP-seq and nCpG analyses illuminate sexually dimorphic methylation of gonadal development genes with high historic methylation in turtle hatchlings with temperature-dependent sex determination

Background

DNA methylation alters gene expression but not DNA sequence and mediates some cases of phenotypic plasticity. Temperature-dependent sex determination (TSD) epitomizes phenotypic plasticity where environmental temperature drives embryonic sexual fate, as occurs commonly in turtles. Importantly, the temperature-specific transcription of two genes underlying gonadal differentiation is known to be induced by differential methylation in TSD fish, turtle and alligator. Yet, how extensive is the link between DNA methylation and TSD remains unclear. Here we test for broad differences in genome-wide DNA methylation between male and female hatchling gonads of the TSD painted turtle Chrysemys picta using methyl DNA immunoprecipitation sequencing, to identify differentially methylated candidates for future study. We also examine the genome-wide nCpG distribution (which affects DNA methylation) in painted turtles and test for historic methylation in genes regulating vertebrate gonadogenesis.

Results

Turtle global methylation was consistent with other vertebrates (57% of the genome, 78% of all CpG dinucleotides). Numerous genes predicted to regulate turtle gonadogenesis exhibited sex-specific methylation and were proximal to methylated repeats. nCpG distribution predicted actual turtle DNA methylation and was bimodal in gene promoters (as other vertebrates) and introns (unlike other vertebrates). Differentially methylated genes, including regulators of sexual development, had lower nCpG content indicative of higher historic methylation.

Conclusions

Ours is the first evidence suggesting that sexually dimorphic DNA methylation is pervasive in turtle gonads (perhaps mediated by repeat methylation) and that it targets numerous regulators of gonadal development, consistent with the hypothesis that it may regulate thermosensitive transcription in TSD vertebrates. However, further research during embryogenesis will help test this hypothesis and the alternative that instead, most differential methylation observed in hatchlings is the by-product of sexual differentiation and not its cause.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-017-0136-2) contains supplementary material, which is available to authorized users.

Cloning and characterization of wnt4a gene and evidence for positive selection in half-smooth tongue sole (Cynoglossus semilaevis)

Wnt4 gene plays a role in developmental processes in mammals. However, little is known regarding its function in teleosts. We cloned and characterized the full-length half-smooth tongue sole (Cynoglossus semilaevis) wnt4a gene (CS-wnt4a). CS-wnt4a cDNA was 1746 bp in length encoding 353aa. CS-wnt4a expression level was highest in the testis, and gradually increased in the developing gonads until 1 year of age. In situ hybridization revealed that CS-wnt4a expression level was highest in stage II oocytes and sperm in the adult ovary and testis, respectively. CS-wnt4a expression level was significantly up-regulated in the gonads after exposure to high temperature. The level of methylation of the CS-wnt4a first exon was negatively correlated with the expression of CS-wnt4a. The branch-site model suggested that vertebrate wnt4a differed significantly from that of wnt4b, and that the selective pressures differed between ancestral aquatic and terrestrial organisms. Two positively selected sites were found in the ancestral lineages of teleost fish, but none in the ancestral lineages of mammals. One positively selected site was located on the α-helices of the 3D structure, the other on the random coil. Our results are of value for further study of the function of wnt4 and the mechanism of selection.

Conserved action of β-catenin during female fate determination in the red-eared slider turtle

In reptiles such as the red-eared slider turtle Trachemys scripta, development of an ovary from the bipotential gonad requires a coordinated expansion of the cortical domain and regression of the medulla. While estrogen, which is necessary and sufficient for ovarian development in non-mammalian vertebrates, is thought to feminize both compartments, it remains unclear whether there is a signaling relationship between the two domains that coordinates their fates. We show that aromatase, the estrogen-synthesizing enzyme, is localized to the medulla of the differentiating turtle ovary and that differentiation of the medulla precedes and is independent of cortical expansion. Coordinated feminization of the two domains may therefore rely on an estrogenic signal from the differentiating medulla. In eutherian mammals, where estrogen is dispensable for early ovary development, the canonical Wnt signaling pathway is critical for female fate determination. Whether this function is conserved among vertebrates and how it is potentially integrated with estrogen signaling are uncertain. Using a novel in vitro turtle gonad culture system, we demonstrate that ectopic activation of the canonical Wnt signaling pathway in presumptive male gonads induced a partial sex-reversal of the medulla, but inhibition of the pathway was not sufficient to sex-reverse differentiating ovaries. These patterns are similar to those previously observed in mice. Wnt signaling appears to function downstream of estrogen, as ectopic activation of the pathway rescued female development when estrogen synthesis was inhibited. Our findings therefore suggest that the ovary-promoting effects of the Wnt signaling pathway may be functionally conserved between mammals and reptiles.