Sex Determination and Gonad Differentiation

Many animals come in two morphs, male and female.  Males and females have an identical or nearly identical genetic make-up, yet have many distinguishing characters. These sexually dimorphic features may range from discrete differences such as minor size differences to striking outwardly dimorphic features such as those seen between peacocks and peahens.  What are the underlying genetic controls that result in these different sex-dependant phenotypes within a given species?  How are developmental pathways regulated differently in the two sexes to result in dimorphic characters?  We use the zebrafish as a developmental and genetic model to answer these questions.

Sex differentiation of the gonad

To better understand how the two phenotypic sexes emerge, we have chosen to study sex specification of the zebrafish gonad. In all animals the gonad develops from an undetermined “bipotential” primordium consisting of germ cells and somatic cells.  This primordium is poised to switch on one of two developmental programs in response to a sex determination signal. In zebrafish, gonad development proceeds through a juvenile ovary stage: all fish first form an immature ovary.  In those fish that develop into males, the immature oocytes are lost, probably via apoptosis, and the gonad is restructured to form a testis. We would like to understand the genetic programs that direct the gonad to develop as either testis or ovary. To accomplish this we are using both forward and reverse genetic approaches.

We have performed a forward genetic screen to identify mutations that affect sex-specific aspects of gonad development.  We have isolated several mutants with gonadogenesis defects, including mutations affecting the identity of the gonad, survival of the germ line and overall organization of the gonad.  In addition, we found mutants affecting gamete development. We are currently focusing our analysis on mutants affecting gonad fate determination.  In this class of mutants, both ovary and testis tissue is present in adult fish.  Analysis of these mutants can lead to the identification of genes involved in the transition from the juvenile ovary stage to testis formation.

In addition to a forward genetic screen, we are studying the role of genes that are homologous to known regulators of sex determination and gonad differentiation in other animals.  To this end, we are in the process of screening for mutants using the tilling technique.





Identification of primary sex determination cues

The initial sex determination cue in zebrafish has remained elusive.  Zebrafish have no apparent heteromorphic chromosomes and have variable sex ratios indicating that there is not merely one gene controlling the sex determination signal.

Our experiments suggest that there are both genetic and environmental inputs into sex determination. By raising fish at different densities, we have found a clear effect of rearing density on sex determination.  In addition, others have shown that temperature and hypoxia can influence sex determination.  Therefore multiple environmental factors influence sex determination.

In addition to environmental effects, we have found that sex determination is also genetically controlled. The genetic control of is apparently polygenic. Therefore, sex determination is controlled by multiple environmental and multiple genetic factors. Our current work is focused on identification of the genes controlling sex determination.

The role of the germ line in sex determination

Interactions between the germ line and soma are critical for germ line survival and development as well as subsequent development and morphogenesis of the gonad. The somatic gonad has a key role in proper fate specification and development of the germ line, however, the germ line has no known role in gonad fate determination, i.e. in determining testis versus ovary fate.  In other model organisms (e.g. mouse, Drosophila, and C. elegans), removal of the germ line prior to sex determination has no affect on the fate of the gonad. Therefore, somatic gonadal sex determination is generally thought to be controlled within the soma. By contrast, we have found that zebrafish without a germline develop exclusively into males.  These males have a normal, albeit small, testis devoid of germ cells.  By gene expression studies, we found that the germ line affects gonad fate determination at the precise stage when gonadal sex differentiation occurs. These data suggests that 1) sex determination can be autonomous to the gonad and 2) the germline plays a key role in determining or maintaining female/ovary fate in zebrafish.  Whether this phenomenon applies to other animals is not yet known 

 

Personnel:

• Dr. Kellee Siegfried                                        Postdoc (03/03 - 04/10)
  

• Ursula Schach                                                Technician