Novel components of germline sex determination acting downstream of foxl3 in medaka

foxl2l is a germ cell-intrinsic gatekeeper of oogenesis in zebrafish

Zebrafish serve as a valuable model organism for studying germ cell biology and reproductive processes. The AB strain of zebrafish is proposed to exhibit a polygenic sex determination system, where most males initially develop juvenile ovaries before committing to male fate. In species with chromosomal sex determination, gonadal somatic cells are recognized as key determinants of germ cell fate. Notably, the loss of germ cells in zebrafish leads to masculinization, implying that germ cells harbor an intrinsic feminization signal. However, the specific signal triggering oogenesis in zebrafish remains unclear. In the present study, we identified foxl2l as an oocyte progenitor-specific gene essential for initiating oogenesis in germ cells. Results showed that foxl2l-knockout zebrafish bypassed the juvenile ovary stage and exclusively developed into fertile males. Further analysis revealed that loss of foxl2l hindered the initiation of oocyte-specific meiosis and prevented entry into oogenesis, leading to premature spermatogenesis during early gonadal development. Furthermore, while mutation of the pro-male gene dmrt1 led to fertile female differentiation, simultaneous disruption of foxl2l in dmrt1 mutants completely blocked oogenesis, with a large proportion of germ cells arrested as germline stem cells, highlighting the crucial role of foxl2l in oogenesis. Overall, this study highlights the unique function of foxl2l as a germ cell-intrinsic gatekeeper of oogenesis in zebrafish.

Medaka Terb1 Mutant Displays Defects of Synaptonemal Complex Formation and Sexual Difference in Gametogenesis

Formation of the synaptonemal complex (SC) is a prerequisite for proper recombination and chromosomal segregation during meiotic prophase I. One mechanism that ensures SC formation is chromosomal movement, which is driven by the force derived from cytoskeletal motors. Here, we report the phenotype of medaka mutants lacking the telomere repeat binding bouquet formation protein 1 (TERB1), which, in combination with the SUN/KASH protein, mediates chromosomal movement by connecting telomeres and cytoskeletal motors. Mutations in the terb1 gene exhibit defects in SC formation in medaka. Although SC formation was initiated, as seen by the punctate lateral elements and fragmented transverse filaments, it was not completed in the terb1 mutant meiocytes. The mutant phenotype further revealed that the introduction of double strand breaks was independent of synapsis completion. In association with these phenotypes, meiocytes in both the ovaries and testes exhibited an aberrant arrangement of homologous chromosomes. Interestingly, although oogenesis halted at the zygotene-like stage in terb1 mutant, testes continued to produce sperm-like cells with aberrant DNA content. This indicates that the mechanism of meiotic checkpoint is sexually different in medaka, similar to the mammalian checkpoint in which oogenesis proceeds while spermatogenesis is arrested. Moreover, our results suggest that spermatogenesis is mechanistically dissociable from meiosis.

Comparative Studies on Duplicated foxl2 Paralogs in Spotted Knifejaw Oplegnathus punctatus Show Functional Diversification

As a member of the forkhead box L gene family, foxl2 plays a significant role in gonadal development and the regulation of reproduction. During the evolution of deuterostome, whole genome duplication (WGD)-enriched lineage diversifications and regulation mechanisms occurs. However, only limited research exists on foxl2 duplication in teleost or other vertebrate species. In this study, two foxl2 paralogs, foxl2 and foxl2l, were identified in the transcriptome of spotted knifejaw (Oplegnathus punctatus), which had varying expressions in the gonads. The foxl2 was expressed higher in the ovary, while foxl2l was expressed higher in the testis. Phylogenetic reconstruction, synteny analysis, and the molecular evolution test confirmed that foxl2 and foxl2l likely originated from the first two WGD. The expression patterns test using qRT-PCR and ISH as well as motif scan analysis revealed evidence of potentially functional divergence between the foxl2 and foxl2l paralogs in spotted knifejaw. Our results indicate that foxl2 and foxl2l may originate from the first two WGD, be active in transcription, and have undergone functional divergence. These results shed new light on the evolutionary trajectories of foxl2 and foxl2l and highlights the need for further detailed functional analysis of these two duplicated paralogs.

Functional Modules in Gametogenesis

Gametogenesis, the production of eggs and sperm, is a fundamental process in sexually reproducing animals. Following gametogenesis commitment and sexual fate decision, germ cells undergo several developmental processes to halve their genomic size and acquire sex-specific characteristics of gametes, including cellular size, motility, and cell polarity. However, it remains unclear how different gametogenesis processes are initially integrated. With the advantages of the teleost fish medaka (Oryzias latipes), in which germline stem cells continuously produce eggs and sperm in mature gonads and a sexual switch gene in germ cells is identified, we found that distinct pathways initiate gametogenesis cooperatively after commitment to gametogenesis. This evokes the concept of functional modules, in which functionally interlocked genes are grouped to yield distinct gamete characteristics. The various combinations of modules may allow us to explain the evolution of diverse reproductive systems, such as parthenogenesis and hermaphroditism.

Germline sexual fate is determined by the antagonistic action of dmrt1 and foxl3/foxl2 in tilapia

Germline sexual fate has long been believed to be determined by the somatic environment, but this idea is challenged by recent studies of foxl3 mutants in medaka. Here, we demonstrate that the sexual fate of tilapia germline is determined by the antagonistic interaction of dmrt1 and foxl3, which are transcriptionally repressed in male and female germ cells, respectively. Loss of dmrt1 rescued the germ cell sex reversal in foxl3Δ7/Δ7 XX fish, and loss of foxl3 partially rescued germ cell sex reversal but not somatic cell fate in dmrt1Δ5/Δ5 XY fish. Interestingly, germ cells lost sexual plasticity in dmrt1Δ5/Δ5 XY and foxl3Δ7/Δ7 XX single mutants, as aromatase inhibitor (AI) and estrogen treatment failed to rescue the respective phenotypes. However, recovery of germ cell sexual plasticity was observed in dmrt1/foxl3 double mutants. Importantly, mutation of somatic cell-specific foxl2 resulted in testicular development in foxl3Δ7/Δ7 or dmrt1Δ5/Δ5 mutants. Our findings demonstrate that sexual plasticity of germ cells relies on the presence of both dmrt1 and foxl3. The existence of dmrt1 and foxl3 allows environmental factors to influence the sex fate decision in vertebrates.

Cystic proliferation of germline stem cells is necessary to reproductive success and normal mating behavior in medaka

The production of an adequate number of gametes is necessary for normal reproduction, for which the regulation of proliferation from early gonadal development to adulthood is key in both sexes. Cystic proliferation of germline stem cells is an especially important step prior to the beginning of meiosis; however, the molecular regulators of this proliferation remain elusive in vertebrates. Here, we report that ndrg1b is an important regulator of cystic proliferation in medaka. We generated mutants of ndrg1b that led to a disruption of cystic proliferation of germ cells. This loss of cystic proliferation was observed from embryogenic to adult stages, impacting the success of gamete production and reproductive parameters such as spawning and fertilization. Interestingly, the depletion of cystic proliferation also impacted male sexual behavior, with a decrease of mating vigor. These data illustrate why it is also necessary to consider gamete production capacity in order to analyze reproductive behavior.

Observation of Medaka Larval Gonads by Immunohistochemistry and Confocal Laser Microscopy

The combination of immunohistochemistry and confocal laser microscopy enables the observation of cellular structures and protein localization within cells using whole-mount tissues. However, such high-resolution imaging requires several steps, such as proper dissection before fixation and antibody staining, and the appropriate positioning of tissues on a glass slide for observation. Here, we describe the method developed by our laboratory for the immunohistochemistry of medaka embryonic and larval gonads, focusing on the dissection and mounting of tissues for confocal laser microscopy. Positioning the gonad just beneath the coverslips is essential to obtain high-resolution images at a level where cellular components of germ cells, such as germ plasm and nuclear structures, can be clearly observed using an oil immersion objective lens.

Differential expression patterns of the two paralogous Rec8 from Nile tilapia and their responsiveness to retinoic acid signaling

REC8 (meiotic recombination protein 8) is an essential component of meiotic cohesion complexes. Interestingly, two paralogous rec8 genes happen to exist in the stra8 (stimulated by retinoic acid gene 8)-absent fishes but not in stra8-existing fishes. Stra8 is usually considered as the prerequirement during RA (retinoic acid)-mediated meiosis initiation in mammals. However, how RA triggers meiosis in the stra8-absent fishes just like Nile tilapia (Oreochromis niloticus) remains elusive. Here we characterized the two paralogous rec8 genes in Nile tilapia (Onrec8a and Onrec8b), and investigated their expression patterns and responsiveness to RA signaling by treatment of ex vivo testicular culture and promoter luciferase reporter assay. OnRec8a and OnRec8b share 36% identity to each other and are true orthologs of REC8. Their expression was predominantly restricted to meiotic germline cells with differential spatiotemporal patterns. During spermatogenesis, OnRec8b predominantly exhibited nuclear expression in spermatocytes from 60 dah (days after hatching), while OnRec8a exhibited cytoplasmic expression from 90 dah. During oogenesis, OnRec8a was expressed from 30 dah, while OnRec8b from 90 dah. Further study shows that RA signaling could upregulate the expression of both Onrec8a and Onrec8b. Collectively, our data implies that OnRec8a and OnRec8b might have differential function during meiosis and be involved in RA-mediated meiosis program.

In vitro effects of androgen on testicular development by the AR-foxl3-rec8/fbxo47 axis in orange-spotted grouper (Epinephelus coioides)

In orange-spotted grouper, androgen can promote the development of testis and spermatogenesis, but the effect of androgen on testis development is unclear. Forkhead box L 3 (Foxl3) is important in the development of fish testis. Rec8 and fbxo47 are involved in meiosis, which impacts spermatogenesis. The present study investigated the plausible role of testis development through the Foxl3 transcriptional regulation of rec8 and fbxo47. The results of tissue distribution showed that rec8 and fbxo47 are highly expressed in gonad. In addition, the highest expression of foxl3, rec8, and fbxo47 was in the testis and intersex compared with the other stages of gonadal development, suggesting that foxl3, rec8, and fbxo47 are important in testis development. In addition, by using dual-luciferase assays, we found that the androgen can increase foxl3 promoter activity and Foxl3 can upregulate rec8 and fbxo47 promoter activity. Furthermore, the addition of β-testosterone significantly increased foxl3, rec8, and fbxo47 promoter activity. Together, these results suggest that foxl3 plays a decisive role in testis development by regulating the expression of rec8 or fbxo47 and imply that AR-foxl3-rec8/fbxo47 affects the testis development pathway.

foxl3, a sexual switch in germ cells, initiates two independent molecular pathways for commitment to oogenesis in medaka

Germ cells have the ability to differentiate into eggs and sperm and must determine their sexual fate. In vertebrates, the mechanism of commitment to oogenesis following the sexual fate decision in germ cells remains unknown. Forkhead-box protein L3 (foxl3) is a switch gene involved in the germline sexual fate decision in the teleost fish medaka (Oryzias latipes). Here, we show that foxl3 organizes two independent pathways of oogenesis regulated by REC8 meiotic recombination protein a (rec8a), a cohesin component, and F-box protein (FBP) 47 (fbxo47), a subunit of E3 ubiquitin ligase. In mutants of either gene, germ cells failed to undergo oogenesis but developed normally into sperm in testes. Disruption of rec8a resulted in arrest at a meiotic pachytenelike stage specifically in females, revealing a sexual difference in meiotic progression. Analyses of fbxo47 mutants showed that this gene regulates transcription factors that facilitate folliculogenesis: LIM homeobox 8 (lhx8b), factor in the germline α (figla), and newborn ovary homeobox (nobox). Interestingly, we found that the fbxo47 pathway ensures that germ cells do not deviate from an oogenic pathway until they reach diplotene stage. The mutant phenotypes together with the timing of their expression imply that germline feminization is established during early meiotic prophase I.

Regulation of germ cell sex identity in medaka

Germline stem cells are sexually indifferent or flexible even in the mature ovary and testis. Acquiring sex identity consistent with the sex of the body is a critical issue in germline stem cells for producing eggs or sperm. However, the molecular mechanism of the sexual fate decision in germ cells is unclear. Medaka is the first vertebrate in which germline stem cells were found in the mature ovary (Nakamura, Kobayashi, Nishimura, Higashijima, & Tanaka, 2010), and a germ cell autonomous switch gene involved in the sexual fate decision, foxl3, was identified (Nishimura et al., 2015) in vertebrates. Here, the mechanism underlying the sex identity of germ cells is described based on the current understanding of germ cell behavior during the sexual fate decision. The control of foxl3 expression in germ cells and components acting downstream of foxl3 are also described.