Webster et al., 2017

The Siegfried laboratory investigates genetic regulation underlying development of the gonad. Specifically, we examine how germ cells develop to form functional gametes and how differences between the sexes are manifested during development. These developmental processes are necessary for fertility and reproduction. We use the zebrafish model to ask several fundamental questions in reproductive development:

  1. How are two sexes determined and how does each sex differentiate distinct cell types?

  2. How do the male gametes develop from undifferentiated diploid germ cells into highly specialized haploid sperm?

  3. 3) How does the testis continuously produce a new supply of sperm in a controlled fashion to maintain fertility throughout life?

Gonadal Sex Determination and Differentiation

We are investigating the genetic regulation underlying how cells and tissues differentiate with male or female-specific characteristics. To this end, we have focused on studying a conserved regulator of sexual development across metazoan animals, the Dmrt1 (Double-sex and mab-3 related transcription factor-1) transcriptional regulator. In general, Dmrt1 regulates sex-specific gene expression through directly binding DNA and either repressing or activating gene transcription.

Through mutation analysis, we found that zebrafish dmrt1 is important for the testis versus ovary fate decision and is necessary for subsequent testis development. We showed that dmrt1 mutants typically developed as fertile females, demonstrating that dmrt1 acts to determine male fate. Although a small percentage of dmrt1 mutants were males, those males were sterile with severe gonadal dysgenesis. Further investigation of mutant males led to the discovery that key genes known to regulate sexual development were dysregulated in dmrt1 mutant testes.  This pointed to a putative role of Dmrt1 in regulating the expression of genes that control the differentiation of sex-specific cell types in the gonad. We are currently investigating direct transcriptional targets of Dmrt1 that may regulate sexual fates of gonadal tissues.

Webster et al., 2017

Germ Line Maintenance

© Kaitlyn Webster

The germ line undergoes unique developmental processes in order to produce highly specialized haploid gametes.  The Siegfried lab is focused on understanding the establishment and maintenance of fertility through investigating the genetic control of germ cell development. To identify genes that are necessary for germ cell development, we are investigating zebrafish mutant lines with defects in germ cell development, which were identified in a forward genetic screen. These mutant lines exhibit defects in renewal of the germ line stem cell population and progression through meiosis in the testis. Our analysis of these mutants will reveal mechanisms that are important for germ cell development and to promote robust fertility.

Testicular Germ Cell Tumors

The dysregulation of germ cell development leads to the formation of germ cell tumors. Testicular germ cell tumors (TGCTs) are the most common malignancy in men between the ages of 20-40 years, and the incidence of these cancers is rising. Very few mammalian TGCT models exist, and the few models available do not represent the variety of TGCTs found in patients. Zebrafish develop TGCTs spontaneously at low frequency, and two examples of mutant lines that develop TGCTs have recently been shown to be promising animal models for human cancers. We have isolated a zebrafish mutant that develops TGCTs at high frequency, called zgt, which is unique to other reported zebrafish TGCT mutant lines. We are currently characterizing the germ cell defects that lead TGCT formation in these mutants and identifying the genetic cause of this phenotype.