CIRB - Research team

Evolution and Development of Germ Cells

Principal Investigator: Jean-René Huynh, DR2 CNRS

Presentation

Germ cells are the only cells which are transmitted from one generation to the next and can be considered immortal. Germ cells produce highly specialized cells, called gametes, which carry the genetic and cytoplasmic information defining a given species and which can initiate the formation of an entire organism.

Understanding how germ cells develop is not only of paramount medical interest for reproductive medicine, but is also crucial to comprehend how animal shapes and forms evolve through generations. Drosophila adult females present several key advantages as a model system to study germ cell development. In each ovary, there are germline stem cells (GSCs), which produce eggs (the female gamete) throughout the female life. It is thus possible to follow the entire development of germ cells from stem cell to fertilized egg in a single fly.

In females, GSCs are located at the anterior apex of a specialized structure called the germarium. GSCs divide asymmetrically leading to the formation of a self-renewing GSC and a differentiating cystoblast. The cystoblast then undergoes four rounds of asymmetric divisions with incomplete cytokinesis, leading to the formation of a cyst of 16 germline cells interconnected by cytoplasmic bridges called ring canals. Only one cell becomes the egg and goes through meiosis, while the 15 remaining cells become polyploidy nurse cells. Despite representing an excellent model system to study stem cell biology, cell cycle control, meiosis, cell fate determination or cell polarity, the germarium remains poorly explored.

Our lab address the following questions:

How is the duration of cytokinesis regulated in germ cells?
How do homologue chromosomes find each other during meiosis?
How is the germline genetic material protected from DNA damages?
How are the germline and somatic tissues coordinated for the morphogenesis of the egg chamber?

Developmental biology aims to study how cells divide and grow to form organs and entire organisms. Germ cells give rise to highly specialized cells called gametes used during sexual reproduction. During their differentiation, germ cells acquire all the genetic, cytoplasmic and epigenetic information to be transmitted to the next generation. In most species, this information is set up during the very first stages of germ cell development.

For example, it is at this time that germ cells undergo several rounds of mitotic amplification to establish stocks of oocytes in female vertebrates. It is also when meiosis starts, and homolog chromosomes have to pair and recombine. These early stages are the most conserved of gametogenesis among species ranging from jellyfish to Drosophila and vertebrates. Simple model and non-model organisms can thus be used to study many causes of sterility in humans happening during these initial steps of germ cell development. A better characterization of these stages will thus benefit our understanding of both the developmental biology of the germline, and the causes of many reproductive dysfunctions in humans.

Exploring the first stages of germ cell development in any species is a challenge. These precursor cells are either transient or rare or hidden in complex tissues. The Drosophila ovary is an excellent model system to take up this challenge. It is a stem cell-based system, which can produce egg chambers throughout the life of the adult female. All the early stages of development are thus present at any time and can be studied throughout adulthood. Two types of stem cells, germline (GSC) and somatic (FSC), are located in a tiny specialized structure called the germarium at the anterior of each ovary.
The germarium is an organized structure divided into four functional regions. GSCs reside in the most anterior part of the mitotic zone, also called region 1. GSCs divide asymmetrically to self-renew and to generate a precursor cell called a cystoblast. The cytoblast then undergoes exactly four rounds of mitosis to produce a germline cyst made of 16 cells. During these mitoses, cytokinesis is not completed and abscission, the last step of cytokinesis, is aborted. All 16 cells thus remain connected through ring canals and share the same cytoplasm. After the last mitosis, cysts enter region 2a, which is the meiotic zone, and all 16 cells begin meiosis. However, during differentiation in region 2, only one cell per cyst remains in meiosis, while the 15 others exit meiosis and endoreplicate their DNA. Each phase in the germarium, region 1, 2a/2b and 3, lasts about 24 hours. It thus takes approximately 3 days to go from stem cell to egg chamber in the germarium.

Our lab has pioneered the use of live imaging of the germarium, and has set up for the first time conditions allowing for the monitoring of single stem cells in their intact microenvironment. This allowed us to uncover the mechanisms underlying the asymmetric growth of GSCs and, more recently, to study the regulation of abscission duration in these cells.

How is the duration of cytokinesis regulated in germ cells? We identified mutations in Drosophila Aurora-B and Cyclin-B genes, which regulate complete abscission in germline stem cells and incomplete abscission in differentiating germ cells.
How do homologue chromosomes find each other during meiosis? We are studying the potential role of the cytoplasmic cytoskeleton in regulating chromosome organization during the early steps of meiosis in the germarium)
How is the germline genetic material protected from DNA damages? The genetic information contained in the female gamete needs to be safely transmitted to the next generation. We are studying how mobile DNA elements such as transposons are silenced in germline cells.
How are the germline and somatic tissues coordinated for the morphogenesis of the egg chamber? We recently discovered that mechanical coordination between the germline and the somatic tissues is essential for proper encapsulation.