Cellular fate of intersex differentiation

Gonadal and cellular dynamics during protogynous sex change in the harlequin sandsmelt Parapercis pulchella

Some teleost fishes change their sex, and some of these fishes have specific gonads known as "ovotestes," that is, gonads containing both ovarian and testicular tissues. In this study, we revealed the gonadal transformation process and cell dynamics during the female-to-male sex change in the harlequin sandsmelt, Parapercis pulchella (Pinguipetidae), in which females possess ovotestes. Histological observations revealed that although female ovotestes were composed of oocytes, a few cysts of male germ cells were observed among them. At the initial phase of sex change, male germ cells increased, and spermatogenesis proceeded. After that, oocytes decreased and finally disappeared, and the gonads became functional testes. Immunohistochemistry using antibodies against Pcna (proliferating cell nuclear antigen) as a cell proliferation marker revealed that spermatogonia were Pcna positive, whereas spermatocytes were negative, in female ovotestes. This suggests that spermatogenesis is arrested at the spermatocyte stage. In addition, some somatic cells surrounding oocytes, which were thought to be the female follicle cells, were Pcna positive during sex change, indicating that these cells proliferate during sex change and are reused in male testes after sex change. Also, immunostaining using antibodies against active cleaved-Caspase3a as an apoptosis marker demonstrated that oocytes degenerated through apoptotic cell death at the late transition stage. Together with previous findings in other fishes, these findings suggested that the histological processes in gonads during sex change, such as the order of developmental events, developmental fates of ovarian cavities, and ovotestis structures, are diversified among fish species. In contrast, cellular dynamics of female germ and somatic cells during sex change are common among protogynous species.

Single-Cell Transcriptomics Analysis Reveals a Cell Atlas and Cell Communication in Yak Ovary

Yaks (Bos grunniens) are the only bovine species that adapt well to the harsh high-altitude environment in the Qinghai-Tibetan plateau. However, the reproductive adaptation to the climate of the high elevation remains to be elucidated. Cell composition and molecular characteristics are the foundation of normal ovary function which determines reproductive performance. So, delineating ovarian characteristics at a cellular molecular level is conducive to elucidating the mechanism underlying the reproductive adaption of yaks. Here, the single-cell RNA-sequencing (scRNA-seq) was employed to depict an atlas containing different cell types with specific molecular signatures in the yak ovary. The cell types were identified on the basis of their specifically expressed genes and biological functions. As a result, a cellular atlas of yak ovary was established successfully containing theca cells, stromal cells, endothelial cells, smooth muscle cells, natural killer cells, macrophages, and proliferating cells. A cell-to-cell communication network between the distinct cell types was constructed. The theca cells were clustered into five subtypes based on their biological functions. Further, CYP11A1 was confirmed as a marker gene for the theca cells by immunofluorescence staining. Our work reveals an ovarian atlas at the cellular molecular level and contributes to providing insights into reproductive adaption in yaks.

A single-cell transcriptomic atlas characterizes cell types and their molecular features in yak ovarian cortex

The ovary as one of the most dynamic organs produces steroids to orchestrate female secondary sexual characteristics, harbors ovarian reserve for oocytes, releases mature oocytes for fertilization, and maintains pregnancy. Yak (Bos grunniens) is the only bovid animal that can adapt to the harsh climatic conditions on the Qinghai-Tibetan Plateau (altitudes of over 3000 m above sea level). However, the cellular atlas is composed of oocytes and other somatic cells, and their individual molecular characteristics remain to be elucidated in the yak ovary. Here, single-cell RNA sequencing (scRNA-seq) was performed to delineate the molecular signature of various cell types in the yak ovarian cortex. A cellular atlas of yak ovarian cortex was constructed successfully on the basis of the differentially expressed genes (DEGs) from the distinct cell types and their functional enrichment analysis, comprising endothelial cells, nature kill cells, stromal cells, smooth muscle cells, oocytes, macrophages, epithelial cells, and granulosa cells. Meanwhile, the signature genes were determined based on their expression specificity in each cell type. A cell-to-cell communication network was built in light of the differentially overexpressed ligand and receptor genes from each cell type. Further, the oocytes were subdivided into four subtypes based on their individual DEGs and the functional enrichment of the DEGs. FST and TOP2A were identified as maker genes for oocytes by immunostaining in the yak ovarian cortex. The cellular atlas reveals the biological characteristics of the ovarian cortex at the cellular molecular level and provides insights into female reproductive biology via cellular communications in the yak.

Gonadal bacterial community composition is associated with sex-specific differences in swamp eels (Monopterus albus)

Organisms are colonized by microorganism communities and play a pivotal role in host function by influencing physiology and development. In mammals, bacterial community may alter gonadal maturation and drive sex-specific differences in gene expression and metabolism. However, bacterial microbiota diversity in the gonads of early vertebrates has not been fully elucidated. Here, we focused on the swamp eel (Monopterus albus), which naturally undergoes sex reversal, and systematically analyzed the bacterial microbiota profiles between females and males using 16S rRNA gene sequences. Specifically, the microbial abundance and community diversity of gonads in males were higher than in females. Although Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria were characterized as the dominating phyla in ovary and testis, the relative abundance of Firmicutes was significantly higher in males than females. Detailed analysis of the microbial community revealed that Bacilli were the dominant bacteria in ovaries and Clostridium in testes of M. albus. More importantly, we proposed that differences in the microbial composition and distribution between ovaries and testes may be linked to functional categories in M. albus, especially metabolism. These findings represent a unique resource of bacterial community in gonads to facilitate future research about the mechanism of how microbiota influence sex-specific differences and sex reversal in vertebrates.

Isolation and Characterization of Germline Stem Cells in Protogynous Hermaphroditic Monopterus albus

Germline stem cells (GSCs) are a group of unique adult stem cells in gonads that act as important transmitters for genetic information. Donor GSCs have been used to produce offspring by transplantation in fisheries. In this study, we successfully isolated and enriched GSCs from the ovary, ovotestis, and testis of Monopterus albus, one of the most important breeding freshwater fishes in China. Transcriptome comparison assay suggests that a distinct molecular signature exists in each type of GSC, and that different signaling activities are required for the maintenance of distinct GSCs. Functional analysis shows that fGSCs can successfully colonize and contribute to the germline cell lineage of a host zebrafish gonad after transplantation. Finally, we describe a simple feeder-free method for the isolation and enrichment of GSCs that can contribute to the germline cell lineage of zebrafish embryos and generate the germline chimeras after transplantation.

Decoding genome recombination and sex reversal

Over the past 440 years since the discovery of the medicinal value of swamp eels, much progress has been made in the study of their biology. The fish is emerging as an important model animal in sexual development, in addition to economic and pharmaceutical implications. Tracing genomic history that shapes speciation of the fish has led to discovery of the whole genome-wide chromosome fission/fusion events. Natural intersex differentiation is a compelling feature for sexual development research. Notably, identification of progenitors of germline stem cells that have bipotential to differentiate into either male or female germline stem cells provides new insight into sex reversal. Here, we review these advances that have propelled the field forwards and present unsolved issues that will guide future investigations to finally elucidate vertebrate sexual development using the new model.