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

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

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


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 in zebrafish. We are currently investigating cell-type-specific functions of dmrt1 in sex determination and differentiation as well as the role of other members of the dmrt gene family in these processes.


Telomere Regulation of Meiosis

The telomeres are well-known for their role in maintaining the ends of chromosomes, however they also have an essential role in meiosis. Homologue pairing is unique to meiotic prophase I and is critical for normal chromosome segregation in meiosis. The telomeres help to mediate homologue pairing by interacting with a protein complex that spans the nuclear envelope to connect with motor proteins that mediate dynamic chromosome movements. These movements are thought to facilitate homologue pairing. We are exploring the role of telomeres in zebrafish meiosis by investigating meiotic defects in zebrafish with shortened telomeres. Furthermore, we’ve identified a mutant that restores fertility in fish with shortened telomeres. Our investigation of this suppressor mutation will shed light on mechanisms underlying how germ cells, as well as other cells, respond to shortened telomeres.

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Germ Line Maintenance

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. We found that the cell cycle regulator, cdk21, is necessary for robust and continuous germ cell production, particularly in fish participating in frequent breeding. We also found that the adad1 gene functions in both stem cell maintenance and meiosis in the testis. We are continuing to study the mechanisms that these two genes regulate to maintain the germ line stem cell population.

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