Live-imaging analysis of germ cell proliferation in the C. elegans adult supports a stochastic model for stem cell proliferation

• Photoconversion of histone H2B is used to determine germ cell proliferation rates and movement in live C.elegans adults.
• The distal-most 6–8 rows of germ cells have similar proliferation rates and are likely stem cells.
• Cells beyond rows 6–8 exit the proliferative state without transit-amplification.
• All cells beyond row 2 will be displaced over time and exit the stem cell niche.

The C. elegans adult hermaphrodite contains a renewable pool of mitotically dividing germ cells that are contained within the progenitor zone (PZ), at the distal region of the germline. From the PZ, cells enter meiosis and differentiate, ensuring the continued production of oocytes. In this study, we investigated the proliferation strategy used to maintain the PZ pool by using a photoconvertible marker to follow the fate of selected germ cells and their descendants in live worms. We found that the most distal pool of 6–8 rows of cells in the PZ (the distal third) behave similarly, with a fold expansion corresponding to one cell division every 6 h on average. Proximal to this region, proliferation decreases, and by the proximal third of the PZ, most cells have stopped dividing. In addition, we show that all the descendants of cells in rows 3 and above move proximally and leave the PZ over time. Combining our data with previous studies, we propose a stochastic model for the C. elegans PZ proliferation, where a pool of proliferating stem cells divide symmetrically within the distal most 6–8 rows of the germline and exit from this stem cell niche occurs by displacement due to competition for limited space.