Dranow DB, Tucker RP and Draper BW. Germ cells are required to maintain a stable sexual phenotype in adult zebrafish. Developmental Biology 376: 43-50.
Adult sex-reversal – the change of primary sex (gonadal sex) and secondary sex characteristics and to another sex during adulthood, occurs in many fish species and is triggered by social or environmental conditions. This is an extreme example of phenotypic plasticity – the ability of animal to change its form due to a cue from the environment. Sex reversal requires considerable changes to both the reproductive system (testis or ovary) and changes to secondary sex characteristics (such as pigment, body shape).
One example of sex-reversal occurring in nature is observed in some species of goby fish. These fish get around as a group of females (harem) with a single dominant male. With the loss of the male from group, one of the adult female fish (usually the largest) undergoes sex reversal to become the male of the group .
The prized fish model for studying development and modeling of human disease is zebrafish. Adult zebrafish can also undergo sex-reversal induced by certain environmental conditions such as water temperature or the presence of aromatase inhibitors. Despite being used for decades as a developmental and genetic model, the primary sex determination mechanism is unknown. Recent research indicates that the primitive germ cells have a key role. The developing gonad is made up of somatic cells that form either the supporting cells or hormone producing cells of the gonad and the primitive germs cells which form the future sperm or oocytes. In zebrafish gonad development goes through a transient stage where – no matter what the ultimate sex will be – both future male and female junvenile fish gonads contain primary oocytes. In the male these early oocytes die off through programmed cell death and the gonad (and the final morphological phenotype) develops as a male. In females, the primary oocytes are maintained and the gonads differentiate into ovaries and the female secondary characteristics develop. (for more information see)
In a new article by Dranow et al., they examined the role of oocytes in maintaining the female sex phenotype in zebrafish. To determine if loss of oocytes from adult female zebrafish could cause sex-reversal – they used two methods:
One method was to make use of a mutant animal that undergoes germ cell loss later in adulthood – nanos 3 null mutants. Nanos proteins are a evolutionary conserved proteins required for survival of germ cells thus mutation or deletion of nanos results loss of germ cells and infertility. Zebrafish nanos3 is required to maintain the germ line – therefore nanos3 mutants are initially fertile, but by 5 months old females are now infertile due to the loss of oocytes during maturation.
Dranow et al., examined the sex characteristics of nanos3 mutant adult fish at 2.5 and 5 months. Initially in nanos3 homozygotes (ie. have no functional copy of the nanos3 gene) the females had typical female phenotypic characteristics (at 2.5 months). However by 5 months their overall body shape and pigmentation was now similar to that of a adult male zebrafish. Indicating that the later life loss of oocytes (due to lack of nanos3) in the females adult mutants, resulted in female to male sex-reversal.
The second method they used was a transgenic approach to induce germ cell loss after chemical treatment. They used transgenics to produce a line of zebrafish that expresses an enzyme in specifically in germ cells which induces cells to undergo cell death when exposed to a certain drug, metronidazole (Mtz). One week following Mtz treatment, the loss of oocytes from transgenic gonads was already evident and by two months the females were showing increased yellow pigmentation and body shape changes – ie. they were now more similar to a male phenotype. Gonad histology and detection of sex-specific markers confirmed female to male sex reversal had occurred in the adult fish. They were even able get 4 sex-reversed fish to induce females to spawn and produce viable embryos. This indicates that sex-reversal is complete – initially adult females, following the loss of oocytes, now become sperm producing males. This may give some clues as to the mechanism of natural forms of adult sex-reversal in other fish species.
What about mammals? Evidence using mouse models suggest that somatic sex needs to be maintained/reinforced throughout adulthood. Loss of FOXL2 (a gene required for ovarian development) expression in the adult ovary results in reprogramming of ovarian cell types (eg grandulosa cell) into cells types typically found in the testis, expressing male marker genes such as Sox9. Like-wise deletion a gene required for male development in adult testes, DMRT1, results in loss of Sertoli cells (testis cell type) and new expression of female marker genes (ie. genes that are normally only expressed in the ovary).
Key summary points and remaining questions (from this paper and previous publications in this area)
- Gonadal sex is surprisingly labile; the gender phenotype needs to be reinforced throughout adulthood.
- Oocytes are required for development of female zebrafish (both gonadal and secondary sex characteristics) in juveniles and maintain the phenotype in the adults.
- Somatic cells in the juvenile appear to be bipotenial – induced to produce Sertoli or grandulosa cells and can be reprogrammed in the adult.
- How can these cells be reprogrammed in an adult? ie. What are the molecular mechanisms underlying sex-reversal of specialized differentiated cell types?
- The primary signal of sex determination in zebrafish still remains a mystery but clearly the germ-somatic cell communication plays an important role in determining sex.