The chromosome 11 region from strain 129 provides protection from sex reversal in XYPOS mice

Testis Determination Requires a Specific FGFR2 Isoform to Repress FOXL2

Male sex determination in mammals relies on sex determining region Y-mediated upregulation of sex determining region-box 9 (SOX9) expression in XY gonads, whereas Wnt family member (WNT)/R-spondin 1 signaling and forkhead box L2 (FOXL2) drive female sex determination in XX gonads. Fibroblast growth factor (FGF) 9 signaling ensures sustained SOX9 expression through repression of one of the ovarian pathways (WNT signaling), whereas the significance of FGF-mediated repression of the FOXL2 pathway has not been studied. Previously, we demonstrated that FGFR2 is the receptor for FGF9 in the XY gonad. Whether a specific isoform (FGFR2b or FGFR2c) is required was puzzling. Here, we show that FGFR2c is required for male sex determination. Initially, in developing mouse embryos at 12.5 to 13.5 days postcoitum (dpc), XY Fgfr2c-/- gonads appear as ovotestes, with SOX9 and FOXL2 expression predominantly localized to the posterior and anterior gonadal poles, respectively. However, by 15.5 dpc, XY Fgfr2c-/- gonads show complete male-to-female sex reversal, evident by the lack of SOX9 and ectopic expression of FOXL2 throughout the gonads. Furthermore, ablation of the Foxl2 gene leads to partial or complete rescue of gonadal sex reversal in XY Fgfr2c-/- mice. Together with previous findings, our data suggest that testis determination involves FGFR2c-mediated repression of both the WNT4- and FOXL2-driven ovarian-determining pathways.

Regulation of sex determination in mice by a non-coding genomic region

To identify novel genomic regions that regulate sex determination, we utilized the powerful C57BL/6J-Y(POS) (B6-Y(POS)) model of XY sex reversal where mice with autosomes from the B6 strain and a Y chromosome from a wild-derived strain, Mus domesticus poschiavinus (Y(POS)), show complete sex reversal. In B6-Y(POS), the presence of a 55-Mb congenic region on chromosome 11 protects from sex reversal in a dose-dependent manner. Using mouse genetic backcross designs and high-density SNP arrays, we narrowed the congenic region to a 1.62-Mb genomic region on chromosome 11 that confers 80% protection from B6-Y(POS) sex reversal when one copy is present and complete protection when two copies are present. It was previously believed that the protective congenic region originated from the 129S1/SviMJ (129) strain. However, genomic analysis revealed that this region is not derived from 129 and most likely is derived from the semi-inbred strain POSA. We show that the small 1.62-Mb congenic region that protects against B6-Y(POS) sex reversal is located within the Sox9 promoter and promotes the expression of Sox9, thereby driving testis development within the B6-Y(POS) background. Through 30 years of backcrossing, this congenic region was maintained, as it promoted male sex determination and fertility despite the female-promoting B6-Y(POS) genetic background. Our findings demonstrate that long-range enhancer regions are critical to developmental processes and can be used to identify the complex interplay between genome variants, epigenetics, and developmental gene regulation.

Fine time course expression analysis identifies cascades of activation and repression and maps a putative regulator of mammalian sex determination

In vertebrates, primary sex determination refers to the decision within a bipotential organ precursor to differentiate as a testis or ovary. Bifurcation of organ fate begins between embryonic day (E) 11.0-E12.0 in mice and likely involves a dynamic transcription network that is poorly understood. To elucidate the first steps of sexual fate specification, we profiled the XX and XY gonad transcriptomes at fine granularity during this period and resolved cascades of gene activation and repression. C57BL/6J (B6) XY gonads showed a consistent ~5-hour delay in the activation of most male pathway genes and repression of female pathway genes relative to 129S1/SvImJ, which likely explains the sensitivity of the B6 strain to male-to-female sex reversal. Using this fine time course data, we predicted novel regulatory genes underlying expression QTLs (eQTLs) mapped in a previous study. To test predictions, we developed an in vitro gonad primary cell assay and optimized a lentivirus-based shRNA delivery method to silence candidate genes and quantify effects on putative targets. We provide strong evidence that Lmo4 (Lim-domain only 4) is a novel regulator of sex determination upstream of SF1 (Nr5a1), Sox9, Fgf9, and Col9a3. This approach can be readily applied to identify regulatory interactions in other systems.

Sex and the circuitry: progress toward a systems-level understanding of vertebrate sex determination

In vertebrates, the gonad arises as a bipotential primordium that can differentiate as a testis or ovary. Cells are initially primed to adopt either fate by balanced antagonistic signaling pathways and transcription networks. Sexual fate is determined by activating the testis or ovarian pathway and repressing the alternative pathway. A complex, dynamic transcription network underlies this process, as approximately half the genome is being transcribed during this period, and many genes are expressed in a sexually dimorphic manner. This network is highly plastic; however, multiple lines of evidence suggest that many elements of the pathway converge on the stabilization or disruption of Sox9 expression. The single gene mutational approach has led to the identification of ∼30 additional genes involved in vertebrate sex determination. However, >50% of human disorders of sexual development (DSDs) are not explained by any of these genes, suggesting many critical elements of the system await discovery. Emerging technologies and genetic resources enable the investigation of the sex determination network on a global scale in the context of a variable genetic background or environmental influences. Using these new tools we can investigate how cells establish a bipotential state that is poised to adopt either sexual fate, and how they integrate multiple signaling and transcriptional inputs to drive a cell fate decision. Elucidating the genetic architecture underlying sex determination in model systems can lead to the identification of conserved modules correlated with phenotypic outcomes, and critical pressure points in the network that predict genes involved in DSDs in humans.

An autosomal locus controls sex reversal in interspecific XY hybrids of the medaka fishes

Although the two medaka species Oryzias latipes and O. curvinotus share the sex-determining gene Dmy, XY sex reversal occurs in interspecific hybridization between O. latipes females of the Hd-rR inbred strain and O. curvinotus males. In this Hd-rR-curvinotus mating, all XX and XY hybrids developed as females. In this study, we used another O. latipes inbred strain (HNI) for the mating, and found that 23% of XY hybrids developed as males, although all XX and the remaining XY hybrids developed as females. Linkage analysis using 236 XY hybrid males obtained from (Hd-rR × HNI) F(1) females showed that a single major locus, Hybrid maleless (Hml), on autosomal linkage group 17, contributed to the strain difference in the XY sex reversal. Furthermore, we found that crossing females of a different O. latipes inbred strain, HO4C, did not cause XY sex reversal in the interspecific hybrids, and that the XY hybrids from (Hd-rR × HO4C) F(1) females showed a 1:1 sex ratio. XY hybrid males had the HO4C allele at sequence-tagged site loci around the Hml locus whereas XY females had the Hd-rR allele, confirming the strong contribution of this locus to XY sex reversal. Reverse transcriptase PCR analysis showed a reduced expression of Dmy(curvinotus) in XY fry of the Hd-rR-curvinotus hybrids at hatching. These results suggest that the Hd-rR allele at the Hml locus interfere with the function of Dmy(curvinotus) on a hybrid background, thus resulting in XY sex reversal.

Elucidation of the transcription network governing mammalian sex determination by exploiting strain-specific susceptibility to sex reversal

Despite the identification of some key genes that regulate sex determination, most cases of disorders of sexual development remain unexplained. Evidence suggests that the sexual fate decision in the developing gonad depends on a complex network of interacting factors that converge on a critical threshold. To elucidate the transcriptional network underlying sex determination, we took the first expression quantitative trait loci (eQTL) approach in a developing organ. We identified reproducible differences in the transcriptome of the embryonic day 11.5 (E11.5) XY gonad between C57BL/6J (B6) and 129S1/SvImJ (129S1), indicating that the reported sensitivity of B6 to sex reversal is consistent with a higher expression of a female-like transcriptome in B6. Gene expression is highly variable in F2 XY gonads from B6 and 129S1 intercrosses, yet strong correlations emerged. We estimated the F2 coexpression network and predicted roles for genes of unknown function based on their connectivity and position within the network. A genetic analysis of the F2 population detected autosomal regions that control the expression of many sex-related genes, including Sry (sex-determining region of the Y chromosome) and Sox9 (Sry-box containing gene 9), the key regulators of male sex determination. Our results reveal the complex transcription architecture underlying sex determination, and provide a mechanism by which individuals may be sensitized for sex reversal.

Hermaphrodism and sex reversal associated with the dominant hemimelia mutation in XY mice

After two generations of backcrossing six different Y-consomic strains onto a C57BL/6J inbred mouse strain with a dominant hemimelia (Dh) mutation, a small percentage of Dh/+ males containing a Y chromosome from the AKR/J or RF/J strain showed hermaphrodism; they lacked the right testis and had an ovary and a uterus instead. Approximately 15% of Dh/+ females considered to be phenotypically normal had the Sry gene from the AKR/J or RF/J strain; they were actually sex-reversed XY females. Backcrossing of Y chromosomes from BALB/cA, C3H/HeJ, C57BL/6J, DH/Sgn, and DDD/Sgn onto the C57BL/6J strain with Dh did not result in hermaphrodism or sex reversal in adult mice. Subsequent linkage mapping analysis revealed that at least one C57BL/6J-derived homozygous allele at a locus on chromosome 13 was required for hermaphrodism and sex reversal. This condition was genetically distinct from known inherited sex-reversal conditions. It therefore offers a novel opportunity to investigate the genetic basis of sex determination in mammals.